Unit 1.pdf

FUNDAMENTAL OF ELECTRONICS SEMESTER - III (COMPUTER ENGINEERING)  UNIT-I SEMICONDUCTOR DEVICES INTRODUCTON Based on the electrical conductivity all the materials in nature are classified as...

FUNDAMENTAL OF ELECTRONICS SEMESTER - III (COMPUTER ENGINEERING)  UNIT-I SEMICONDUCTOR DEVICES INTRODUCTON Based on the electrical conductivity all the materials in nature are classified as insulators, semiconductors, and conductors. Insulator: An insulator is a material that offers a very low level (or negligible) of conductivity when voltage is applied. Eg: Paper, Mica, glass, quartz. Typical resistivity level of an insulator is of the order of 1010 to 1012 Ω-cm. The energy band structure of an insulator is shown in the fig.1.1. Band structure of a material defines the band of energy levels that an electron can occupy. Valance band is the range of electron energy where the electron remain bended too the atom and do not contribute to the electric current. Conduction bend is the range of electron energies higher than valance band where electrons are free to accelerate under the influence of external voltage source resulting in the flow of charge. The energy band between the valance band and conduction band is called as forbidden band gap. It is the energy required by an electron to move from balance band to conduction band i.e. the energy required for a valance electron to become a free electron. 1 eV = 1.6 x 10-19 J For an insulator, as shown in the fig.1.1 there is a large forbidden band gap of greater than 5Ev. Because of this large gap there a very few electrons in the CB and hence the conductivity of insulator is poor. Even an increase in temperature or applied electric field is insufficient to transfer electrons from VB to CB. CB CB CB Forbidden band o Eo =≈6eV gap Eo ≈6eV VB VB VB Insulator Semiconductor Conductor FiG:1.1 Energy band diagrams insulator, semiconductor and conductor Conductors: A conductor is a material which supports a generous flow of charge when a voltage is applied across its terminals. i.e. it has very high conductivity. Eg: Copper, Aluminum, Silver, Gold. The resistivity of a conductor is in the order of 10-4 and 10-6 Ω-cm. The Valance and conduction bands overlap (fig1.1) and there is no energy gap for the electrons to move from valance band to conduction band. This implies that there are free electrons in CB even at absolute zero temperature (0K). Therefore at room temperature when electric field is applied large current flows through the conductor. Semiconductor: A semiconductor is a material that has its conductivity somewhere between the insulator and conductor. The resistivity level is in the range of 10 and 10 4 Ω-cm. Two of the most commonly used are Silicon (Si=14 atomic no.) and germanium (Ge=32 atomic no.). Both have 4 valance electrons. The forbidden band gap is in the order of 1eV. For eg., the band gap energy for Si, Ge and GaAs is 1.21, 0.785 and 1.42 eV, respectively at absolute zero temperature (0K). At 0K and at low temperatures, the valance band electrons do not have sufficient energy to move from V to CB. Thus semiconductors act a insulators at 0K. as the temperature increases, a large number of valance electrons acquire sufficient energy to leave the VB, cross the forbidden bandgap and reach CB. These are now free electrons as they can move freely under the influence of electric field. At room temperature there are sufficient electrons in the CB and hence the semiconductor is capable of conducting some current at room temperature. Inversely related to the conductivity of a material is its resistance to the flow of charge or current. Typical resistivity values for various materials’ are given as follows. Insulator Semiconductor Conductor -6 10 Ω-cm (Cu) 50Ω-cm (Ge) 1012 Ω-cm (mica) 50x103 Ω-cm (Si) Typical resistivity values Semiconductor Types A pure form of semiconductors is called as intrinsic semiconductor. Conduction in intrinsic sc is either due to thermal excitation or crystal defects. Si and Ge are the two most important semiconductors used. Other examples include Gallium arsenide GaAs, Indium Antimonide (InSb) etc.  Let us consider the structure of Si. A Si atomic no. is 14 and it has 4 valance electrons. These 4 electrons are shared by four neighboring atoms in the crystal structure by means of covalent bond. Fig. 1.2a shows the crystal structure of Si at absolute zero temperature (0K). Hence a pure SC acts has poor conductivity (due to lack of free electrons) at low or absolute zero temperature. Covalent bond Valence electron Fig. 1.2a crystal structure of Si at 0K At room temperature some of the covalent bonds break up to thermal energy as shown in fig 1.2b. The valance electrons that jump into conduction band are called as free electrons that are available for conduction. Free electron Valance electron hole Fig. 1.2b crystal structure of Si at room temperature0K  The absence of electrons in covalent bond is represented by a small circle usually referred to as hole which is of positive charge. Even a hole serves as carrier of electricity in a manner similar to that of free electron. The mechanism by which a hole contributes to conductivity is explained as follows: When a bond is in complete so that a hole exists, it is relatively easy for a valance electron in the neighboring atom to leave its covalent bond to fill this hole. An electron moving from a bond to fill a hole moves in a direction opposite to that of the electron. This hole, in its new position may now be filled by an electron from another covalent bond and the hole will correspondingly move one more step in the direction opposite to the motion of electron. Here we have a mechanism for conduction of electricity which does not involve free electrons. This phenomenon is illustrated in fig1.3 Electron movement Hole movement Fig. 1.3a Fig. 1.3b Fig. 1.3c  Fig 1.3a show that there is a hole at ion 6.Imagine that an electron from ion 5 moves into the hole at ion 6 so that the configuration of 1.3b results. If we compare both fig1.3a &fig 1.3b, it appears as if the hole has moved towards the left from ion6 to ion 5. Further if we compare fig 1.3b and fig 1.3c, the hole moves from ion5 to ion 4. This discussion indicates the motion of hole is in a direction opposite to that of motion of electron. Hence we consider holes as physical entities whose movement constitutes flow of current. In a pure semiconductor, the number of holes is equal to the number of free electrons. EXTRINSIC SEMICONDUCTOR Intrinsic semiconductor has very limited applications as they conduct very small amounts of current at room temperature. The current conduction capability of intrinsic semiconductor can be increased significantly by adding a small amounts impurity to the intrinsic semiconductor. By adding impurities it becomes impure or extrinsic semiconductor. This process of adding impurities is called as doping. The amount of impurity added is 1 part in 106 atoms. N type semiconductor: If the added impurity is a pentavalent atom then the resultant semiconductor is called N-type semiconductor. Examples of pentavalent impurities are Phosphorus, Arsenic, Bismuth, Antimony etc. A pentavalent impurity has five valance electrons. Fig 1.4a shows the crystal structure of N-type semiconductor material where four out of five valance electrons of the impurity atom(antimony) forms covalent bond with the four intrinsic semiconductor atoms. The fifth electron is loosely bound to the impurity atom. This loosely bound electron can be easily Fifth valance electron of SB CB Ec Ed Donor energy level Ev VB Fig. 1.4a crystal structure of N type SC Fig. 1.4bEnergy band diagram of N type Excited from the valance band to the conduction band by the application of electric field or increasing the thermal energy. The energy required to detach the fifth electron form the impurity atom is very small of the order of 0.01ev for Ge and 0.05 eV for Si. The effect of doping creates a discrete energy level called donor energy level in the forbidden band gap with energy level Ed slightly less than the conduction band (fig 1.4b). The difference between the energy levels of the conducting band and the donor energy level is the energy required to free the fifth valance electron (0.01 eV for Ge and 0.05 eV for Si). At room temperature almost all the fifth electrons from the donor impurity atom are raised to conduction band and hence the number of electrons in the conduction band increases significantly. Thus every antimony atom contributes to one conduction electron without creating a hole. In the N-type sc the no. of electrons increases and the no. of holes decreases compared to those available in an intrinsic sc. The reason for decrease in the no. of holes is that the larger no. of electrons present increases the recombination of electrons with holes. Thus current in N type sc is dominated by electrons which are referred to as majority carriers. Holes are the minority carriers in N type sc P type semiconductor: If the added impurity is a trivalent atom then the resultant semiconductor is called P-type semiconductor. Examples of trivalent impurities are Boron, Gallium , indium etc. The crystal structure of p type sc is shown in the fig1.5a. The three valance electrons of the impurity (boon) forms three covalent bonds with the neighboring atoms and a vacancy exists in the fourth bond giving rise to the holes. The hole is ready to accept an electron from the neighboring atoms. Each trivalent atom contributes to one hole generation and thus introduces a large no. of holes in the valance band. At the same time the no. electrons are decreased compared to those available in intrinsic sc because of increased recombination due to creation of additional holes. hole Fig. 1.5a crystal structure of P type sc  Thus in P type sc , holes are majority carriers and electrons are minority carriers. Since each trivalent impurity atoms are capable accepting an electron, these are called as acceptor atoms. The following fig 1.5b shows the pictorial representation of P type sc hole (majority carrier) Electron (minority carrier) Acceptor atoms Fig. 1.5b crystal structure of P type sc  The conductivity of N type sc is greater than that of P type sc as the mobility of electron is greater than that of hole.  For the same level of doping in N type sc and P type sc, the conductivity of an Ntype sc is around twice that of a P type sc QUANTITATIVE THEORY OF PN JUNCTION DIODE PN JUNCTION WITH NO APPLIED VOLTAGE OR OPEN CIRCUIT CONDITION: In a piece of sc, if one half is doped by p type impurity and the other half is doped by n type impurity, a PN junction is formed. The plane dividing the two halves or zones is called PN junction. As shown in the fig the n type material has high concentration of free electrons, while p type material has high concentration of holes. Therefore at the junction there is a tendency of free electrons to diffuse over to the P side and the holes to the N side. This process is called diffusion. As the free electrons move across the junction from N type to P type, the donor atoms become positively charged. Hence a positive charge is built on the N-side of the junction. The free electrons that cross the junction uncover the negative acceptor ions by filing the holes. Therefore a negative charge is developed on the p –side of the junction..This net negative charge on the p side prevents further diffusion of electrons into the p side. Similarly the net positive charge on the N side repels the hole crossing from p side to N side. Thus a barrier sis set up near the junction which prevents the further movement of charge carriers i.e. electrons and holes. As a consequence of induced electric field across the depletion layer, an electrostatic potential difference is established between P and N regions, which are called the potential barrier, junction barrier, diffusion potential or contact potential, Vo. The magnitude of the contact potential Vo varies with doping levels and temperature. Vo is 0.3V for Ge and 0.72 V for Si.  Fig 1.6: Symbol of PN Junction Diode The electrostatic field across the junction caused by the positively charged N-Type region tends to drive the holes away from the junction and negatively charged p type regions tend to drive the electrons away from the junction. The majority holes diffusing out of the P region leave behind negatively charged acceptor atoms bound to the lattice, thus exposing a negatives pace charge in a previously neutral region. Similarly electrons diffusing from the N region expose positively ionized donor atoms and a double space charge builds up at the junction as shown in the fig. 1.7a  Fig 1.7a It is noticed that the space charge layers are of opposite sign to the majority carriers diffusing into them, which tends to reduce the diffusion rate. Thus the double space of the layer causes an electric field to be set up across the junction directed from N to P regions, which is in such a direction to inhibit the diffusion of majority electrons and holes as illustrated in fig 1.7b. The shape of the charge density, ρ, depends upon how diode id doped. Thus the junction region is depleted of mobile charge carriers. Hence it is called depletion layer, space region, and transition region. The depletion region is of the order of 0.5µm thick. There are no mobile carriers in this narrow depletion region. Hence no current flows across the junction and the system is in equilibrium. To the left of this depletion layer, the carrier concentration is p= NA and to its right it is n= ND.  Fig 1.7b FORWARD BIASED JUNCTION DIODE When a diode is connected in a Forward Bias condition, a negative voltage is applied to the N- type material and a positive voltage is applied to the P-type material. If this external voltage becomes greater than the value of the potential barrier, approx. 0.7 volts for silicon and 0.3 volts for germanium, the potential barriers opposition will be overcome and current will start to flow. This is because the negative voltage pushes or repels electrons towards the junction giving them the energy to cross over and combine with the holes being pushed in the opposite direction towards the junction by the positive voltage. This results in a characteristics curve of zero current flowing up to this voltage point, called the "knee" on the static curves and then a high current flow through the diode with little increase in the external voltage as shown below. Forward Characteristics Curve for a Junction Diode Fig 1.8a: Diode Forward Characteristics The application of a forward biasing voltage on the junction diode results in the depletion layer becoming very thin and narrow which represents a low impedance path through the junction thereby allowing high currents to flow. The point at which this sudden increase in current takes place is represented on the static I-V characteristics curve above as the "knee" point. Forward Biased Junction Diode showing a Reduction in the Depletion Layer Fig 1.8b: Diode Forward Bias This condition represents the low resistance path through the PN junction allowing very large currents to flow through the diode with only a small increase in bias voltage. The actual potential difference across the junction or diode is kept constant by the action of the depletion layer at approximately 0.3v for germanium and approximately 0.7v for silicon junction diodes. Since the diode can conduct "infinite" current above this knee point as it effectively becomes a short circuit, therefore resistors are used in series with the diode to limit its current flow. Exceeding its maximum forward current specification causes the device to dissipate more power in the form of heat than it was designed for resulting in a very quick failure of the device. 1.1.2 PN JUNCTION UNDER REVERSE BIAS CONDITION: Reverse Biased Junction Diode When a diode is connected in a Reverse Bias condition, a positive voltage is applied to the N-type material and a negative voltage is applied to the P-type material. The positive voltage applied to the N- type material attracts electrons towards the positive electrode and away from the junction, while the holes in the P-type end are also attracted away from the junction towards the negative electrode. The net result is that the depletion layer grows wider due to a lack of electrons and holes and presents a high impedance path, almost an insulator. The result is that a high potential barrier is created thus preventing current from flowing through the semiconductor material. Reverse Biased Junction Diode showing an Increase in the Depletion Fig 1.9a: Diode Reverse Bias This condition represents a high resistance value to the PN junction and practically zero current flows through the junction diode with an increase in bias voltage. However, a very small leakage current does flow through the junction which can be measured in microamperes, (μA). One final point, if the reverse bias voltage Vr applied to the diode is increased to a sufficiently high enough value, it will cause the PN junction to overheat and fail due to the avalanche effect around the junction. This may cause the diode to become shorted and will result in the flow of maximum circuit current, and this shown as a step downward slope in the reverse static characteristics curve below. Reverse Characteristics Curve for a Junction Diode Fig 1.9b: Diode Reverse Characteristics Sometimes this avalanche effect has practical applications in voltage stabilizing circuits where a series limiting resistor is used with the diode to limit this reverse breakdown current to a preset maximum value thereby producing a fixed voltage output across the diode. These types of diodes are commonly known as Zener Diodes VI CHARACTERISTICS AND THEIR TEMPERATURE DEPENDENCE Diode terminal characteristics equation for diode junction current:  v ID  I 0 (e vT  1) Where VT = KT/q; VD_ diode terminal voltage, Volts Io _ temperature-dependent saturation current, µA T _ absolute temperature of p-n junction, K K _ Boltzmann’s constant 1.38x 10 -23J/K) q _ electron charge 1.6x10-19 C  = empirical constant, 1 for Ge and 2 for Si Fig 1.10: Diode Characteristics Temperature Effects on Diode Temperature can have a marked effect on the characteristics of a silicon semiconductor diode as shown in Fig. 11 It has been found experimentally that the reverse saturation current Io will just about double in magnitude for every 10°C increase in temperature. Fig 1.11 Variation in Diode Characteristics with temperature change It is not uncommon for a germanium diode with an I o in the order of 1 or 2 A at 25°C to have a leakage current of 100 A - 0.1 mA at a temperature of 100°C. Typical values of Io for silicon are much lower than that of germanium for similar power and current levels. The result is that even at high temperatures the levels of Io for silicon diodes do not reach the same high levels obtained. For germanium—a very important reason that silicon devices enjoy a significantly higher level of development and utilization in design. Fundamentally, the open-circuit equivalent in the reverse bias region is better realized at any temperature with silicon than with germanium. The increasing levels of Io with temperature account for the lower levels of threshold voltage, as shown in Fig. 1.11. Simply increase the level of I o in and not rise in diode current. Of course, the level of TK also will be increase, but the increasing level of I o will overpower the smaller percent change in TK. As the temperature increases the forward characteristics are actually becoming more “ideal,” IDEAL VERSUS PRACTICAL RESISTANCE LEVELS DC or Static Resistance The application of a dc voltage to a circuit containing a semiconductor diode will result in an operating point on the characteristic curve that will not change with time. The resistance of the diode at the operating point can be found simply by finding the corresponding levels of VD and ID as shown in Fig. 1.12 and applying the following Equation: The dc resistance levels at the knee and below will be greater than the resistance levels obtained for the vertical rise section of the characteristics. The resistance levels in the reverse-bias region will naturally be quite high. Since ohmmeters typically employ a relatively constant-current source, the resistance determined will be at a preset current level (typically, a few mill amperes).  Fig 1.12 Determining the dc resistance of a diode at a particular operating point. AC or Dynamic Resistance It is obvious from Eq. 1.3 that the dc resistance of a diode is independent of the shape of the characteristic in the region surrounding the point of interest. If a sinusoidal rather than dc input is applied, the situation will change completely. The varying input will move the instantaneous operating point up and down a region of the characteristics and thus defines a specific change in current and voltage as shown in Fig. 1.13. With no applied varying signal, the point of operation would be the Q- point appearing on Fig. 1.13 determined by the applied dc levels. The designation Q-point is derived from the word quiescent, which means “still or unvarying.” A straight-line drawn tangent to the curve through the Q-point as shown in Fig. 1.13 will define a particular change in voltage and current that can be used to determine the ac or dynamic resistance for this region of the diode characteristics. In equation form, Where Δ Signifies a finite change in the quantity Fig 1.13: Determining the ac resistance of a diode at a particular operating point.  DIODE EQUIVALENT CIRCUITS An equivalent circuit is a combination of elements properly chosen to best represent the actual terminal characteristics of a device, system, or such in a particular operating region. In other words, once the equivalent circuit is defined, the device symbol can be removed from a schematic and the equivalent circuit inserted in its place without severely affecting the actual behavior of the system. The result is often a network that can be solved using traditional circuit analysis techniques. Piecewise-Linear Equivalent Circuit One technique for obtaining an equivalent circuit for a diode is to approximate the characteristics of the device by straight-line segments, as shown in Fig. 1.31. The resulting equivalent circuit is naturally called the piecewise-linear equivalent circuit. It should be obvious from Fig. 1.31 that the straight-line segments do not result in an exact duplication of the actual characteristics, especially in the knee region. However, the resulting segments are sufficiently close to the actual curve to establish an equivalent circuit that will provide an excellent first approximation to the actual behaviour of the device. The ideal diode is included to establish that there is only one direction of conduction through the device, and a reverse-bias condition will result in the open- circuit state for the device. Since a silicon semiconductor, diode does not reach the conduction state until VD reaches 0.7 V with a forward bias (as shown in Fig. 1.14a), a battery VT opposing the conduction direction must appear in the equivalent circuit as shown in Fig. 1.14b. The battery simply specifies that the voltage across the device must be greater than the threshold battery voltage before conduction through the device in the direction dictated by the ideal diode can be established. When conduction is established, the resistance of the diode will be the specified value of rav. Fig: 1.14aDiode piecewise-linear model characteristics  Fig: 1.14b Diode piecewise-linear model equivalent circuit The approximate level of rav can usually be determined from a specified operating point on the specification sheet. For instance, for a silicon semiconductor diode, if IF _ 10 mA (a forward conduction current for the diode) at VD _ 0.8 V, we know for silicon that a shift of 0.7 V is required before the characteristics rise. Fig 1.15 Ideal Diode and its characteristics Fig 1.16: Diode equivalent circuits(models)  BREAK DOWN MECHANISMS When an ordinary P-N junction diode is reverse biased, normally only very small reverse saturation current flows. This current is due to movement of minority carriers. It is almost independent of the voltage applied. However, if the reverse bias is increased, a point is reached when the junction breaks down and the reverse current increases abruptly. This current could be large enough to destroy the junction. If the reverse current is limited by means of a suitable series resistor, the power dissipation at the junction will not be excessive, and the device may be operated continuously in its breakdown region to its normal (reverse saturation) level. It is found that for a suitably designed diode, the breakdown voltage is very stable over a wide range of reverse currents. This quality gives the breakdown diode many useful applications as a voltage reference source. The critical value of the voltage, at which the breakdown of a P-N junction diode occurs, is called the breakdown voltage. The breakdown voltage depends on the width of the depletion region, which, in turn, depends on the doping level. The junction offers almost zero resistance at the breakdown point. There are two mechanisms by which breakdown can occur at a reverse biased P-N junction: 1. avalanche breakdown and 2. Zener breakdown. Avalanche breakdown The minority carriers, under reverse biased conditions, flowing through the junction acquire a kinetic energy which increases with the increase in reverse voltage. At a sufficiently high reverse voltage (say 5 V or more), the kinetic energy of minority carriers becomes so large that they knock out electrons from the covalent bonds of the semiconductor material. As a result of collision, the liberated electrons in turn liberate more electrons and the current becomes very large leading to the breakdown of the crystal structure itself. This phenomenon is called the avalanche breakdown. The breakdown region is the knee of the characteristic curve. Now the current is not controlled by the junction voltage but rather by the external circuit. Zener breakdown Under a very high reverse voltage, the depletion region expands and the potential barrier increases leading to a very high electric field across the junction. The electric field will break some of the covalent bonds of the semiconductor atoms leading to a large number of free minority carriers, which suddenly increase the reverse current. This is called the Zener effect. The breakdown occurs at a particular and constant value of reverse voltage called the breakdown voltage, it is found that Zener breakdown occurs at electric field intensity of about 3 x 107 V/m.  Fig 1.18: Diode characteristics with breakdown Either of the two (Zener breakdown or avalanche breakdown) may occur independently, or both of these may occur simultaneously. Diode junctions that breakdown below 5 V are caused by Zener effect. Junctions that experience breakdown above 5 V are caused by avalanche effect. Junctions that breakdown around 5 V are usually caused by combination of two effects. The Zener breakdown occurs in heavily doped junctions (P-type semiconductor moderately doped and N-type heavily doped), which produce narrow depletion layers. The avalanche breakdown occurs in lightly doped junctions, which produce wide depletion layers. With the increase in junction temperature Zener breakdown voltage is reduced while the avalanche breakdown voltage increases. The Zener diodes have a negative temperature coefficient while avalanche diodes have a positive temperature coefficient. Diodes that have breakdown voltages around 5 V have zero temperature coefficient. The breakdown phenomenon is reversible and harmless so long as the safe operating temperature is maintained. ZENER DIODES The Zener diode is like a general-purpose signal diode consisting of a silicon PN junction. When biased in the forward direction it behaves just like a normal signal diode passing the rated current, but as soon as a reverse voltage applied across the zener diode exceeds the rated voltage of the device, the diodes breakdown voltage VB is reached at which point a process called Avalanche Breakdown occurs in the semiconductor depletion layer and a current starts to flow through the diode to limit this increase in voltage. The current now flowing through the zener diode increases dramatically to the maximum circuit value (which is usually limited by a series resistor) and once achieved this reverse saturation current remains fairly constant over a wide range of applied voltages. This breakdown voltage point, V B is called the "zener voltage" for zener diodes and can range from less than one volt to hundreds of volts. The point at which the zener voltage triggers the current to flow through the diode can be very accurately controlled (to less than 1% tolerance) in the doping stage of the diodes semiconductor construction giving the diode a specific zener breakdown voltage, (Vz) for example, 4.3V or 7.5V. This zener breakdown voltage on the I-V curve is almost a vertical straight line. Zener Diode I-V Characteristics Fig 1.19 : Zener diode characteristics The Zener Diode is used in its "reverse bias" or reverse breakdown mode, i.e. the diodes anode connects to the negative supply. From the I-V characteristics curve above, we can see that the zener diode has a region in its reverse bias characteristics of almost a constant negative voltage regardless of the value of the current flowing through the diode and remains nearly constant even with large changes in current as long as the zener diodes current remains between the breakdown current I Z(min) and the maximum current rating IZ(max). This ability to control itself can be used to great effect to regulate or stabilize a voltage source against supply or load variations. The fact that the voltage across the diode in the breakdown region is almost constant turns out to be an important application of the zener diode as a voltage regulator. The function of a regulator is to provide a constant output voltage to a load connected in parallel with it in spite of the ripples in the supply voltage or the variation in the load current and the zener diode will  continue to regulate the voltage until the diodes current falls below the minimum I Z(min) value in the reverse breakdown region. SPECIAL PURPOSE DIODES VARACTOR DIODE Varactor diode is a special type of diode which uses transition capacitance property i.e voltage variable capacitance.These are also called as varicap,VVC(voltage variable capacitance) or tuning diodes. The varactor diode symbol is shown below with a diagram representation. Fig 1.21a:symbol of varactor diode When a reverse voltage is applied to a PN junction, the holes in the p-region are attracted to the anode terminal and electrons in the n-region are attracted to the cathode terminal creating a region where there is little current. This region ,the depletion region, is essentially devoid of carriers and behaves as the dielectric of a capacitor. The depletion region increases as reverse voltage across it increases; and since capacitance varies inversely as dielectric thickness, the junction capacitance will decrease as the voltage across the PN junction increases. So by varying the reverse voltage across a PN junction the junction capacitance can be varied.This is shown in the typical varactor voltage-capacitance curve below.  Fig 1.21b:voltage- capacitance curve Notice the nonlinear increase in capacitance as the reverse voltage is decreased. This nonlinearity allows the varactor to be used also as a harmonic generator. Major varactor considerations are: (a) Capacitance value (b) Voltage (c) Variation in capacitance with voltage. (d) Maximum working voltage (e) Leakage current Applications:  Tuned circuits.  FM modulators  Automatic frequency control devices  Adjustable bandpass filters  Parametric amplifiers  Television receivers. RECTIFIERS: INTRODUCTION For the operation of most of the electronics devices and circuits, a d.c. source is required. So it is advantageous to convert domestic a.c. supply into d.c.voltages. The process of converting a.c. voltage into d.c. voltage is called as rectification. This is achieved with i) Step-down Transformer, ii) Rectifier, iii) Filter and iv) Voltage regulator circuits. These elements constitute d.c. regulated power supply shown in the fig 1 below.  Fig 2.1: Block Diagram of regulated D.C Power Supply  Transformer – steps down 230V AC mains to low voltage AC.  Rectifier – converts AC to DC, but the DC output is varying.  Smoothing – smooth the DC from varying greatly to a small ripple.  Regulator – eliminates ripple by setting DC output to a fixed voltage. The block diagram of a regulated D.C. power supply consists of step-down transformer, rectifier, filter, voltage regulator and load. An ideal regulated power supply is an electronics circuit designed to provide a predetermined d.c. voltage Vo which is independent of the load current and variations in the input voltage ad temperature. If the output of a regulator circuit is a AC voltage then it is termed as voltage stabilizer, whereas if the output is a DC voltage then it is termed as voltage regulator. RECTIFIER Any electrical device which offers a low resistance to the current in one direction but a high resistance to the current in the opposite direction is called rectifier. Such a device is capable of converting a sinusoidal input waveform, whose average value is zero, into a unidirectional Waveform, with a non- zero average component. A rectifier is a device, which converts a.c. voltage (bi-directional) to pulsating d.c. voltage (Unidirectional). Characteristics of a Rectifier Circuit: Any electrical device which offers a low resistance to the current in one direction but a high resistance to the current in the opposite direction is called rectifier. Such a device is capable of converting a sinusoidal input waveform, whose average value is zero, into a unidirectional waveform, with a non- zero average component. A rectifier is a device, which converts a.c. voltage (bi-directional) to pulsating d.c..Load currents: They are two types of output current. They are average or d.c. current and RMS currents. Average or DC current: The average current of a periodic function is defined as the area of one cycle of the curve divided by the base. It is expressed mathematically as Area over one period i) Average value/dc value/mean value= Total time period 1T T 0 Vdc  Vd (wt) ii) Effective (or) R.M.S current: The effective (or) R.M.S. current squared ofa periodic function of time is given by the area of one cycle of the curve, which represents the square of the function divided by the base. 1T 2 T 0 Vrms  V d (wt) iii) Peak factor: It is the ratio of peak value to Rms value peakvalue Peak factor = rmsvalue iv) Form factor: It is the ratio of Rms value to average value Rmsvalue Form factor= averagevalue v) Ripple Factor (  ) : It is defined as ration of R.M.S. value of a.c. component to the d.c. component in the output is known as “Ripple Factor”. Vac  Vdc Vac  Vrms 2 Vdc2 vi) Efficiency ( ): It is the ratio of d.c output power to the a.c. input power. It signifies, how efficiently the rectifier circuit converts a.c. power into d.c. power. o / p power  i / p power vii) Peak Inverse Voltage (PIV): It is defined as the maximum reverse voltage that a diode can withstand without destroying the junction. viii) Transformer Utilization Factor (UTF): The d.c. power to be delivered to the load in a rectifier circuit decides the rating of the Transformer used in the circuit. So, transformer utilization factor is defined as Pdc TUF  pac(rated) ix) % Regulation: The variation of the d.c. output voltage as a function of d.c. load current is called regulation. The percentage regulation is defined as VNL  VFL % Re gulation  *100 VFL For an ideal power supply, % Regulation is zero. CLASSIFICATION OF RECTIFIERS Using one or more diodes in the circuit, following rectifier circuits can be designed. 1) Half - Wave Rectifier 2) Full – Wave Rectifier 3) Bridge Rectifier HALF-WAVE RECTIFIER: A Half – wave rectifier as shown in fig 1.2 is one, which converts a.c. voltage into a pulsating voltage using only one half cycle of the applied a.c. voltage. Fig 1.2: Basic structure of Half-Wave Rectifier The a.c. voltage is applied to the rectifier circuit using step-down transformer-rectifying element i.e., p- n junction diode and the source of a.c. voltage, all connected is series. The a.c. voltage is applied to the rectifier circuit using step-down transformer  V=Vm sin (wt) The input to the rectifier circuit, Where Vm is the peak value of secondary a.c. voltage. Operation: For the positive half-cycle of input a.c. voltage, the diode D is forward biased and hence it conducts. Now a current flows in the circuit and there is a voltage drop across RL. The waveform of the diode current (or) load current is shown in fig 3. For the negative half-cycle of input, the diode D is reverse biased and hence it does not Conduct. Now no current flows in the circuit i.e., i=0 and Vo=0. Thus for the negative half- cycle no power is delivered to the load. Analysis: In the analysis of a HWR, the following parameters are to be analyzed. 1. DC output current 2. DC Output voltage 3. R.M.S. Current 4. R.M.S. voltage 5. Rectifier Efficiency (η ) 6. Ripple factor (γ )  7. Peak Factor 8. % Regulation 9. Transformer Utilization Factor (TUF) 10. form factor 11. o/p frequency Let a sinusoidal voltage Vi be applied to the input of the rectifier. Then V=Vm sin (wt) Where Vm is the maximum value of the secondary voltage. Let the diode be idealized to piece-wise linear approximation with resistance Rf in the forward direction i.e., in the ON state and Rr (=∞) in the reverse direction i.e., in the OFF state. Now the current ‘i’ in the diode (or) in the load resistance RL is given by V=Vm sin (wt) i) AVERAGE VOLTAGE 1T Vdc  T Vd (wt) 0 2 1 Vdc  T 0 V ( )d 1 2 Vdc   V ( )d 2   1 Vdc  2 V m sin(wt) 0 Vm Vdc    ii).AVERAGE CURRENT: Im I dc    iii) RMS VOLTAGE: T 1 Vrms   T V 2 d (wt) 0   2 1 2  Vrms  (Vm sim(wt)) 2 d (wt)  0     Vm Vrms  2   IV) RMS CURRENT Im I rms   V) PEAK FACTOR peakvalue Peak factor = rmsvalue Vm Peak Factor = (Vm / 2) Peak Factor =2 vi) FORM FACTOR Rmsvalue Form factor= averagevalue (Vm / 2) Form factor= Vm /   Form Factor =1.57  Vac vii) Ripple Factor:  Vdc  Vac  Vrms 2 Vdc2 V 2 V 2  rms dc Vac 2 Vrms  1 Vdc2  1.21 viii) Efficiency ( ): o / ppower  *100 i / ppower pac = *100 Pdc  =40.8 ix) Transformer Utilization Factor (TUF): The d.c. power to be delivered to the load in a rectifier circuit decides the rating of the transformer used in the circuit. Therefore, transformer utilization factor is defined as pdc TUF  Pac(rated)  TUF =0.286. The value of TUF is low which shows that in half-wave circuit, the transformer is not fully utilized. If the transformer rating is 1 KVA (1000VA) then the half-wave rectifier can deliver 1000 X 0.287 = 287 watts to resistance load. x) Peak Inverse Voltage (PIV): It is defined as the maximum reverse voltage that a diode can withstand without destroying the junction. The peak inverse voltage across a diode is the peak of the negative half- cycle. For half-wave rectifier, PIV is Vm. DISADVANTAGES OF HALF-WAVE RECTIFIER: 1. The ripple factor is high. 2. The efficiency is low. 3. The Transformer Utilization factor is low. Because of all these disadvantages, the half-wave rectifier circuit is normally not used as a power rectifier circuit. FULL WAVE RECTIFIER: A full-wave rectifier converts an ac voltage into a pulsating dc voltage using both half cycles of the applied ac voltage. In order to rectify both the half cycles of ac input, two diodes are used in this circuit. The diodes feed a common load RL with the help of a center-tap transformer. A center-tap transformer is the one, which produces two sinusoidal waveforms of same magnitude and frequency but out of phase with respect to the ground in the secondary winding of the transformer. The full wave rectifier is shown in the fig 4 below Fig. 5 shows the input and output wave forms of the ckt. During positive half of the input signal, anode of diode D1 becomes positive and at the same time the anode of diode D2 becomes negative. Hence D1 conducts and D2 does not conduct. The load current flows through D1 and the voltage drop across RL will be equal to the input voltage. During the negative half cycle of the input, the anode of D1 becomes negative and the anode of D2 becomes positive. Hence, D1 does not conduct and D2 conducts. The load current flows through D2 and the voltage drop across RL will be equal to the input voltage. It is noted that the load current flows in the both the half cycles of ac voltage and in the same direction through the load resistance. i) AVERAGEVOLTAGE ii) AVERAGE CURRENT  iii) RMS VOLTAGE: T 1 Vrms   T V 2 d (wt) 0   2 1 2  Vrms  (Vm sim(wt)) 2 d (wt)  0    IV) RMS CURRENT 2I m I rms   V) PEAK FACTOR peakvalue Peak factor = rmsvalue  Vm Peak Factor = (Vm / 2) Peak Factor =2 vi) FORM FACTOR Rms value Form factor= averagevalue Form factor= (Vm / 2) 2Vm /   Form Factor =1.11 vii) Ripple Factor: viii) Efficiency ( ): o / ppower  *100 i / ppower ix) Transformer Utilization Factor (TUF): The d.c. power to be delivered to the load in a rectifier circuit decides the rating of the transformer used in the circuit. So, transformer utilization factor is defined as pdc TUF  Pac(rated) x) Peak Inverse Voltage (PIV): It is defined as the maximum reverse voltage that a diode can withstand without destroying the junction. The peak inverse voltage across a diode is the peak of the negative half- cycle. For half- wave rectifier, PIV is 2Vm xi) % Regulation. Advantages 1) Ripple factor = 0.482 (against 1.21 for HWR) 2) Rectification efficiency is 0.812 (against 0.405 for HWR) 3) Better TUF (secondary) is 0.574 (0.287 for HWR) 4) No core saturation problem Disadvantages: 1) Requires center tapped transformer. BRIDGE RECTIFIER. Another type of circuit that produces the same output waveform as the full wave rectifier circuit above, is that of the Full Wave Bridge Rectifier. This type of single phase rectifier uses four individual rectifying diodes connected in a closed loop "bridge" configuration to produce the desired output. The main advantage of this bridge circuit is that it does not require a special centre tapped transformer, thereby reducing its size and cost. The single secondary winding is connected to one side of the diode bridge network and the load to the other side as shown below. The Diode Bridge Rectifier The four diodes labelled D1 to D4 are arranged in "series pairs" with only two diodes conducting current during each half cycle. During the positive half cycle of the supply, diodes D1 and D2 conduct in series while diodes D3 and D4 are reverse biased and the current flows through the load as shown below (fig 7). The Positive Half-cycle The Negative Half-cycle During the negative half cycle of the supply, diodes D3 and D4 conduct in series (fig 8), but diodes D1 and D2 switch "OFF" as they are now reverse biased. The current flowing through the load is the same direction as before. As the current flowing through the load is unidirectional, so the voltage developed across the load is also unidirectional the same as for the previous two diode full-wave rectifier, therefore the average DC voltage across the load is 0.637Vmax. However in reality, during each half cycle the current flows through two diodes instead of just one so the amplitude of the output voltage is two voltage drops ( 2 x 0.7 = 1.4V ) less than the input VMAX amplitude. The ripple frequency is now twice the supply frequency (e.g. 100Hz for a 50Hz supply) FILTERS The output of a rectifier contains dc component as well as ac component. Filters are used to minimize the undesirable ac i.e., ripple leaving only the dc component to appear at the output. Some important filters are: 1. Inductor filter 2. Capacitor filter 3. LC or L section filter 4. CLC or Π-type filter CAPACITOR FILTER This is the most simple form of the filter circuit and in this arrangement a high value capacitor C is placed directly across the output terminals, as shown in figure. During the conduction period it gets charged and stores up energy to it during non-conduction period. Through this process, the time duration during which Ft is to be noted here that the capacitor C gets charged to the peak because there is no resistance (except the negligible forward resistance of diode) in the charging path. But the discharging time is quite large (roughly 100 times more than the charging time depending upon the value of RL) because it discharges through load resistance RL. The function of the capacitor filter may be viewed in terms of impedances. The large value capacitor C offers a low impedance shunt path to the ac components or ripples but offers high impedance to the dc component. Thus ripples get bypassed through capacitor C and only dc component flows through the load resistance RL Capacitor filter is very popular because of its low cost, small size, light weight and good characteristics. CAPACITOR FILTER WITH HWR CAPACITOR FILTER WITH FWR  Electronic Circuits - Positive Clipper Circuits The Clipper circuit that is intended to attenuate positive portions of the input signal can be termed as a Positive Clipper. Among the positive diode clipper circuits, we have the following types − Positive Series Clipper Positive Series Clipper with positive Vr referencevoltage Positive Series Clipper with negative Vr Positive Shunt Clipper Positive Shunt Clipper with positive Vr Positive Shunt Clipper with negative Vr Let us discuss each of these types in detail. Positive Series Clipper A Clipper circuit in which the diode is connected in series to the input signal and that attenuates the positive portions of the waveform, is termed as Positive Series Clipper. The following figure represents the circuit diagram for positive series clipper. Positive Cycle of the Input − When the input voltage is applied, the positive cycle of the input makes the point A in the circuit positive with respect to the point B. This makes the diode reverse biased and hence it behaves like an open switch. Thus the voltage across the load resistor becomes zero as no current flows through it and hence V0 will be zero. Negative Cycle of the Input − The negative cycle of the input makes the point A in the circuit negative with respect to the point B. This makes the diode forward biased and hence it conducts like a closed switch. Thus the voltage across the load resistor will be equal to the applied input voltage as it completely appears at the output V0. Waveforms In the above figures, if the waveforms are observed, we can understand that only a portion of the positive peak was clipped. This is because of the voltage across V0. But the ideal output was not meant to be so. Let us have a look at the following figures. Unlike the ideal output, a bit portion of the positive cycle is present in the practical output due to the diode conduction voltage which is 0.7v. Hence there will be a difference in the practical and ideal output waveforms. Positive Series Clipper with positive Vr A Clipper circuit in which the diode is connected in series to the input signal and biased with positive reference voltage Vr and that attenuates the positive portions of the waveform, is termed as Positive Series Clipper with positive Vr. The following figure represents the circuit diagram for positive series clipper when the reference voltage applied is positive. During the positive cycle of the input the diode gets reverse biased and the reference voltage appears at the output. During its negative cycle, the diode gets forward biased and conducts like a closed switch. Hence the output waveform appears as shown in the above figure. Positive Series Clipper with negative Vr A Clipper circuit in which the diode is connected in series to the input signal and biased with negative reference voltage Vr and that attenuates the positive portions of the waveform, is termed as Positive Series Clipper with negative Vr. The following figure represents the circuit diagram for positive series clipper, when the reference voltage applied is negative. During the positive cycle of the input the diode gets reverse biased and the reference voltage appears at the output. As the reference voltage is negative, the same voltage with constant amplitude is shown. During its negative cycle, the diode gets forward biased and conducts like a closed switch. Hence the input signal that is greater than the reference voltage, appears at the output. Positive Shunt Clipper A Clipper circuit in which the diode is connected in shunt to the input signal and that attenuates the positive portions of the waveform, is termed as Positive Shunt Clipper. The following figure represents the circuit diagram for positive shunt clipper. Positive Cycle of the Input − When the input voltage is applied, the positive cycle of the input makes the point A in the circuit positive with respect to the point B. This makes the diode forward biased and hence it conducts like a closed switch. Thus the voltage across the load resistor becomes zero as no current flows through it and hence V0 will be zero. Negative Cycle of the Input − The negative cycle of the input makes the point A in the circuit negative with respect to the point B. This makes the diode reverse biased and hence it behaves like an open switch. Thus the voltage across the load resistor will be equal to the applied input voltage as it completely appears at the output V0. Waveforms In the above figures, if the waveforms are observed, we can understand that only a portion of the positive peak was clipped. This is because of the voltage across V0. But the ideal output was not meant to be so. Let us have a look at the following figures. Unlike the ideal output, a bit portion of the positive cycle is present in the practical output due to the diode conduction voltage which is 0.7v. Hence there will be a difference in the practical and ideal output waveforms. Positive Shunt Clipper with positive Vr A Clipper circuit in which the diode is connected in shunt to the input signal and biased with positive reference voltage Vr and that attenuates the positive portions of the waveform, is termed as Positive Shunt Clipper with positive Vr. The following figure represents the circuit diagram for positive shunt clipper when the reference voltage applied is positive. During the positive cycle of the input the diode gets forward biased and nothing but the reference voltage appears at the output. During its negative cycle, the diode gets reverse biased and behaves as an open switch. The whole of the input appears at the output. Hence the output waveform appears as shown in the above figure. Positive Shunt Clipper with negative Vr A Clipper circuit in which the diode is connected in shunt to the input signal and biased with negative reference voltage Vr and that attenuates the positive portions of the waveform, is termed as Positive Shunt Clipper with negative Vr. The following figure represents the circuit diagram for positive shunt clipper, when the reference voltage applied is negative. During the positive cycle of the input, the diode gets forward biased and the reference voltage appears at the output. As the reference voltage is negative, the same voltage with constant amplitude is shown. During its negative cycle, the diode gets reverse biased and behaves as an open switch. Hence the input signal that is greater than the reference voltage, appears at the output.  Electronic Circuits - Negative Clipper Circuits The Clipper circuit that is intended to attenuate negative portions of the input signal can be termed as a Negative Clipper. Among the negative diode clipper circuits, we have the following types. Negative Series Clipper Negative Series Clipper with positive Vr referencevoltage Negative Series Clipper with negative Vr Negative Shunt Clipper Negative Shunt Clipper with positive Vr Negative Shunt Clipper with negative Vr Let us discuss each of these types in detail. Negative Series Clipper A Clipper circuit in which the diode is connected in series to the input signal and that attenuates the negative portions of the waveform, is termed as Negative Series Clipper. The following figure represents the circuit diagram for negative series clipper. Positive Cycle of the Input − When the input voltage is applied, the positive cycle of the input makes the point A in the circuit positive with respect to the point B. This makes the diode forward biased and hence it acts like a closed switch. Thus the input voltage completely appears across the load resistor to produce the output V0. Negative Cycle of the Input − The negative cycle of the input makes the point A in the circuit negative with respect to the point B. This makes the diode reverse biased and hence it acts like an open switch. Thus the voltage across the load resistor will be zero making V0 zero. Waveforms In the above figures, if the waveforms are observed, we can understand that only a portion of the negative peak was clipped. This is because of the voltage across V0. But the ideal output was not meant to be so. Let us have a look at the following figures. Unlike the ideal output, a bit portion of the negative cycle is present in the practical output due to the diode conduction voltage which is 0.7v. Hence there will be a difference in the practical and ideal output waveforms. Negative Series Clipper with positive Vr A Clipper circuit in which the diode is connected in series to the input signal and biased with positive reference voltage Vr and that attenuates the negative portions of the waveform, is termed as Negative Series Clipper with positive Vr. The following figure represents the circuit diagram for negative series clipper when the reference voltage applied is positive. During the positive cycle of the input, the diode starts conducting only when the anode voltage value exceeds the cathode voltage value of the diode. As the cathode voltage equals the reference voltage applied, the output will be as shown. Negative Series Clipper with negative Vr A Clipper circuit in which the diode is connected in series to the input signal and biased with negative reference voltage Vr and that attenuates the negative portions of the waveform, is termed as Negative Series Clipper with negative Vr. The following figure represents the circuit diagram for negative series clipper, when the reference voltage applied is negative. During the positive cycle of the input the diode gets forward biased and the input signal appears at the output. During its negative cycle, the diode gets reverse biased and hence will not conduct. But the negative reference voltage being applied, appears at the output. Hence the negative cycle of the output waveform gets clipped after this reference level. Negative Shunt Clipper A Clipper circuit in which the diode is connected in shunt to the input signal and that attenuates the negative portions of the waveform, is termed as Negative Shunt Clipper. The following figure represents the circuit diagram for negative shunt clipper. Positive Cycle of the Input − When the input voltage is applied, the positive cycle of the input makes the point A in the circuit positive with respect to the point B. This makes the diode reverse biased and hence it behaves like an open switch. Thus the voltage across the load resistor equals the applied input voltage as it completely appears at the output V0 Negative Cycle of the Input − The negative cycle of the input makes the point A in the circuit negative with respect to the point B. This makes the diode forward biased and hence it conducts like a closed switch. Thus the voltage across the load resistor becomes zero as no current flows through it. Waveforms In the above figures, if the waveforms are observed, we can understand that just a portion of the negative peak was clipped. This is because of the voltage across V0. But the ideal output was not meant to be so. Let us have a look at the following figures. Unlike the ideal output, a bit portion of the negative cycle is present in the practical output due to the diode conduction voltage which is 0.7v. Hence there will be a difference in the practical and ideal output waveforms. Negative Shunt Clipper with positive Vr A Clipper circuit in which the diode is connected in shunt to the input signal and biased with positive reference voltage Vr and that attenuates the negative portions of the waveform, is termed as Negative Shunt Clipper with positive Vr. The following figure represents the circuit diagram for negative shunt clipper when the reference voltage applied is positive. During the positive cycle of the input the diode gets reverse biased and behaves as an open switch. So whole of the input voltage, which is greater than the reference voltage applied, appears at the output. The signal below reference voltage level gets clipped off. During the negative half cycle, as the diode gets forward biased and the loop gets completed, no output is present. Negative Shunt Clipper with negative Vr A Clipper circuit in which the diode is connected in shunt to the input signal and biased with negative reference voltage Vr and that attenuates the negative portions of the waveform, is termed as Negative Shunt Clipper with negative Vr. The following figure represents the circuit diagram for negative shunt clipper, when the reference voltage applied is negative. During the positive cycle of the input the diode gets reverse biased and behaves as an open switch. So whole of the input voltage, appears at the output Vo. During the negative half cycle, the diode gets forward biased. The negative voltage up to the reference voltage, gets at the output and the remaining signal gets clipped off. Explore our latest online courses and learn new skills at your own pace. Enroll and become a certified expert to boost your career. Two-way Clipper This is a positive and negative clipper with a reference voltage Vr. The input voltage is clipped two-way both positive and negative portions of the input waveform with two reference voltages. For this, two diodes D1 and D2 along with two reference voltages Vr1 and Vr2 are connected in the circuit. This circuit is also called as a Combinational Clipper circuit. The figure below shows the circuit arrangement for a two-way or a combinational clipper circuit along with its output waveform. During the positive half of the input signal, the diode D1 conducts making the reference voltage Vr1 appear at the output. During the negative half of the input signal, the diode D2 conducts making the reference voltage Vr1 appear at the output. Hence both the diodes conduct alternatively to clip the output during both the cycles. The output is taken across the load resistor. With this, we are done with the major clipper circuits. Let us go for the clamper circuits in the next chapter.  Electronic Circuits - SMPS The topics discussed till now represent different sections of power supply unit. All these sections together make the Linear Power Supply. This is the conventional method of obtaining DC out of the input AC supply. Linear Power Supply The Linear Power Supply LP S is the regulated power supply which dissipates much heat in the series resistor to regulate the output voltage which has low ripple and low noise. This LPS has many applications. A linear power supply requires larger semiconductor devices to regulate the output voltage and generates more heat resulting in lower energy efficiency. Linear power supplies have transient response times up to 100 times faster than the others, which is very important in certain specialized areas. Advantages of LPS The power supply is continuous. The circuitry is simple. These are reliable systems. This system dynamically responds to load changes. The circuit resistances are changed to regulate the output voltage. As the components operate in linear region, the noise is low. The ripple is very low in the output voltage. Disadvantages of LPS The transformers used are heavier and large. The heat dissipation is more. The efficiency of linear power supply is 40 to 50%  Power is wasted in the form of heat in LPS circuits. Single output voltage is obtained. We have already gone through different parts of a Linear Power supply. The block diagram of a Linear Power Supply is as shown in the following figure. In spite of the above disadvantages, Linear Power Supplies are widely used in low-noise amplifiers, test equipment, control circuits. In addition, they are also used in data acquisition and signal processing. All the power supply systems that needs simple regulation and where efficiency is not a concern, the LPS circuits are used. As the electrical noise is lower, the LPS is used in powering sensitive analog circuitry. But to overcome the disadvantages of Linear Power Supply system, the Switched Mode Power Supply SM P S is used. Switched Mode Power Supply SMPS The disadvantages of LPS such as lower efficiency, the need for large value of capacitors to reduce ripples and heavy and costly transformers etc. are overcome by the implementation of Switched Mode Power Supplies. The working of SMPS is simply understood by knowing that the transistor used in LPS is used to control the voltage drop while the transistor in SMPS is used as a controlled switch. Working The working of SMPS can be understood by the following figure. Let us try to understand what happens at each stage of SMPS circuit. Input Stage The AC input supply signal 50 Hz is given directly to the rectifier and filter circuit combination without using any transformer. This output will have many variations and the capacitance value of the capacitor should be higher to handle the input fluctuations. This unregulated dc is given to the central switching section of SMPS. Switching Section A fast switching device such as a Power transistor or a MOSFET is employed in this section, which switches ON and OFF according to the variations and this output is given to the primary of the transformer present in this section. The transformer used here are much smaller and lighter ones unlike the ones used for 60 Hz supply. These are much efficient and hence the power conversion ratio is higher. Output Stage The output signal from the switching section is again rectified and filtered, to get the required DC voltage. This is a regulated output voltage which is then given to the control circuit, which is a feedback circuit. The final output is obtained after considering the feedback signal. Control Unit This unit is the feedback circuit which has many sections. Let us have a clear understanding about this from The following figure. The above figure explains the inner parts of a control unit. The output sensor senses the signal and joins it to the control unit. The signal is isolated from the other section so that any sudden spikes should not affect the circuitry. A reference voltage is given as one input along with the signal to the error amplifier which is a comparator that compares the signal with the required signal level. By controlling the chopping frequency the final voltage level is maintained. This is controlled by comparing the inputs given to the error amplifier, whose output helps to decide whether to increase or decrease the chopping frequency. The PWM oscillator produces a standard PWM wave fixed frequency. We can get a better idea on the complete functioning of SMPS by having a look at the following figure. The SMPS is mostly used where switching of voltages is not at all a problem and where efficiency of the system really matters. There are few points which are to be noted regarding SMPS. They are SMPS circuit is operated by switching and hence the voltages vary continuously. The switching device is operated in saturation or cut off mode. The output voltage is controlled by the switching time of the feedback circuitry. Switching time is adjusted by adjusting the duty cycle. The efficiency of SMPS is high because, instead of dissipating excess power as heat, it continuously switches its input to control the output. Disadvantages There are few disadvantages in SMPS, such as The noise is present due to high frequency switching. The circuit is complex. It produces electromagnetic interference. Advantages The advantages of SMPS include, The efficiency is as high as 80 to 90%  Less heat generation; less power wastage. Reduced harmonic feedback into the supply mains. The device is compact and small in size. The manufacturing cost is reduced. Provision for providing the required number of voltages. Applications There are many applications of SMPS. They are used in the motherboard of computers, mobile phone chargers, HVDC measurements, battery chargers, central power distribution, motor vehicles, consumer electronics, laptops, security systems, space stations, etc. Types of SMPS SMPS is the Switched Mode Power Supply circuit which is designed for obtaining the regulated DC output voltage from an unregulated DC or AC voltage. There are four main types of SMPS such as DC to DC Converter AC to DC Converter Fly back Converter Forward Converter The AC to DC conversion part in the input section makes the difference between AC to DC converter and DC to DC converter. The Fly back converter is used for Low power applications. Also there are Buck Converter and Boost converter in the SMPS types which decrease or increase the output voltage depending upon the requirements. The other type of SMPS include Self-oscillating fly-back converter, Buck-boost converter, Cuk, Sepic, etc.  7-Segment LED Display LED (Light Emitting Diode) is a semiconductor device that emits either visible light or infrared light when it is forward biased. Thus, it is widely used in different electronic devices like TV screens, mobile screens, watches and clocks, remote controls, etc. A light emitting diode (LED) basically converts electrical energy into light energy when an electric current flows through it. In this article, we will discuss an application of LED in Seven Segment Display. The seven segment displays are extensively used in in different electronic gadgets like calculators, counters, watches, electronic measuring instruments, etc. What is a Seven Segment LED Display? Seven Segment Display is an LED based display screen that can display information in the form of decimal numbers. The seven segment display is a used in place of the more complex dot matrix displays. It is called so because it consists of seven segments of light emitting diodes (LEDs) that are assembled like decimal 8 as shown in the following figure. Seven segments LED display can be used in applications where an electronic display device is required for showing decimal numbers from 0 to 9, sometimes, basic characters as well. Since, it uses LEDs, therefore it is an energy efficient display device. Therefore, it is most widely used in those devices that are powered by a small battery or a cell. Working of Seven Segment LED Display A seven segment display consists of seven LED segments arranged like a decimal 8. These LED segments are illuminated to form a pattern that represents a decimal number from 0 to 9. Now, let us understand, how the seven segment LED display work to display different numbers. When electrical energy is supplied to all the segments, then the seven segment LED display shows the decimal number 8. When power is given to all the segments and if we disconnect power from the segment ‘g’, then it displays the decimal number 0. When the power is given to segments "b" and "c" only, then it displays the number 1. When the power is given to the segments "a", "b", "g", "e", "d", then it displays the number 2. When the power is given to segments "a", "b", "g", "c", "d", then it displays the number 3. When the power is given to segments "b", "c", "f", "g", then it displays the number 4. When the power is given to segments "a", "c", "d", "f", "g", then it displays the number 5. When the power is given to segments "a", "c", "d", "e", "f", "g", then it displays the number 6. When the power is given to segments "a", "b", "c", then it displays the number 7. When the power is given to segments "a", "b", "c", "d", "f", "g", then it displays the number 9. In this way, we can display any decimal number from 0 to 9 by illuminating a set of LED segments of the seven segment LED display. Explore our latest online courses and learn new skills at your own pace. Enroll and become a certified expert to boost your career. Truth Table of Seven Segment LED Display The following table shows the truth table of a seven segment LED display to display the decimal numbers from 0 to 9. LED Segments Inputs Seven Segment Display Output a b c d e f g 0 1 1 1 1 1 1 0 1 0 1 1 0 0 0 0 2 1 1 0 1 1 0 1 3 1 1 1 1 0 0 1  4 0 1 1 0 0 1 1 5 1 0 1 1 0 1 1 6 1 0 1 1 1 1 1 7 1 1 1 0 0 0 0 8 1 1 1 1 1 1 1 9 1 1 1 1 0 1 1 Types of Seven Segment LED Displays There are two types of seven segment displays available − Common Cathode Seven Segment Display In this type of seven segment display, the cathode terminal of all LED segments are connected together to logic 0 (lower voltage level). The logic 1 (higher voltage level) is applied through a current limiting resistor to forward bias the individual LED segments at their anode terminals. Common Anode Seven Segment Display In this type of seven segment LED display, the anode terminals of all the LED segments are connected together to the logic 1 (higher voltage level), and the logic 0 (lower voltage level) is used through a current limiting resistor to the individual cathode terminals of LED segments. Note − Common Anode Seven Segment LED Displays are more popular than Common Cathode Seven Segment Display because logic circuit can sink higher current as compared to they can source. Applications of Seven Segment LED Displays Seven Segment LED Displays are widely used in the following devices − Digital watches and clocks Calculators Microwaves Remote controls Speedometers  Vehicle odometers Clock radios, etc. Conclusion Seven segment displays are very commonly used in low power electronic devices like remote controls, watches, clocks, digit measuring instruments, etc. From the above discussion, we may conclude that a seven segment display consists of seven LED (Light Emitting Diode) segments that are illuminated in a pattern to display the numbers from 0 to 9. Seven segment displays are also used to display some basic characters.  SEVEN SEGMENT DISPLAY A display consisting of seven LEDs arranged in seven segments is called seven segment display. It is shown in the Fig. The seven LEDs are arranged in a rectangular fashion and are labeled A through G. Each LED is called a segment because it forms a part of the digit being displayed. An additional LED is used for the indication of a decimal point (DP). By forward biasing different LEDs we can display the digits 0 through 9. For example, to display a zero, the LEDs A, B, C, D, E and F are forward biased. To light up a 5, we need to forward bias segments A, F, C, C, D. Thus in a seven segment display depending upon the digit to be displayed, the particular set of LEDs is forward biased. The various digits from 0 to 9 which can be displayed using seven segment display are shown in the Fig. Types of Seven Segment Display The two types of seven segment display are available called, 1) Common anode type 2) Common cathode type Common Anode Type In this type, all anodes of LEDs are connected together and common point is connected to + V which is positive supply voltage. A current limiting resistor is required i be connected between each LED and ground. Common Cathode Type In this type, all cathodes of LEDs are connected together and common point is connected to the ground. A current limiting resistor is connected between each LED and the supply + Vcc. The anodes of the respective segments are to be connected to + for the required operation of LEDs.  LED Driver Circuit The output of a digital circuit is logical i.e. either 0 or 1. The ‘0’ means low while 1’ means high. In the high state the output voltage is nearly 5V while in low state, it is almost OV. If LED is to be driven by such digital circuit, it can be connected as shown in the Fig. 4.21. When output of digital circuit is high, both ends of LED are at 5V and it can not be forward biased hence will not give light. While when output of digital circuit is low, then high current will flow through LED as it becomes forward biased, and it will give light. Source : http://mediatoget.blogspot.in/2011/09/seven-segment-display.html  Figure 4.1 A simplified LCD cell showing each of the major components. LCD Basics The basic display using liquid crystals is composed of six main components: a polarizing filter, a glass plate that has a transparent electrode pattern, the liquid crystal material, a clear common electrode on glass, a polarizer whose axis is crossed compared to the first polarizer, and either a reflective surface or a light source. Without the liquid crystal between the polarizers, the crossed polarizers would block out the light, making the screen appear dark. Adjusting the voltages on the electrodes changes the amount of twist in the liquid crystal and varies the amount of light passing through. While most of the components of the LCD might be familiar, we will provide a brief overview of each component. A simplified design of a liquid crystal cell is shown in Fig. 4.1. The structure of the LCD includes the alignment layers in contact with the liquid crystal, the electrical contacts composed of indium-tin-oxide (ITO) (which are transparent), glass layers, and polarizing films. The order of the layers on the glass is shown in Fig. 4.1 as ITO–glass–alignment layer; however, this ordering can be glass–ITO–alignment layer, which can reduce overall capacitance. Display cells have a reflector on the rear side that allows the displays to be used with ambient light. The next sections expand on the function related to each of these components as they apply to the LCD cell. Polarization revisited Polarization is a fundamental property of light and, in fact, all oscillating waves. Light waves are actually electromagnetic waves that have both an electric and a magnetic component. The electromagnetic component is a transverse wave and can be thought of as a string vibrating orthogonally to a line joining both ends. The electromagnetic wave is considered to be unpolarized if it is vibrating in more than one plane and is polarized if the vibrations occur in a single plane. In light, polarization is tied to the electric field rather than the magnetic field. 3 The main polarization forms are linear and circular polarization. Light vibrating in a single plane is referred to as linearly polarized, or, if the plane rotates at the optical frequency, it is referred to as circularly polarized or, sometimes, elliptically polarized. 1 Unpolarized light can be converted to polarized light, often, with a loss of intensity. Consider the string introduced above, vibrating in many planes and viewed on end through a thin slit. Every so often, the full oscillation of the string fills the view of the slit; however, the remainder of the time, the slit blocks the view of the string. If the string were a light wave and could pass through the slit, the slit would be acting as a polarizer. When using two polarizers with their polarizing axes aligned, the light passing through the first polarizer will also be able to pass through the second polarizer. If the angle between the two polarizers is changed, there will be a reduction in the amount of light that can pass through the second polarizer. Complete blocking of the light will occur when the two polarizers are 90 deg to each other. 4 For the light to pass through two crossed polarizers, something would have to be placed between the polarizers to transform the polarization from one plane to another. This effect is illustrated in Fig. 4.2, where a pair of Figure 4.2 Illustration of the “picket fence” model demonstrating how either a single or a pair of aligned polarizers work to pass a single polarization of light. aligned polarizers and an incoming unpolarized light source allows only a single polarization to pass through. Twisted nematic effect in liquid crystals The development of the twisted nematic effect changed liquid crystals from an interesting material to the commercially viable display technology that has virtually replaced all other approaches. The strength of this technology is the combination of optical performance and low-operating-power requirements. The latter is very important in mobile devices and is the result of the LCD not requiring any significant current flow and using low voltage levels. In a twisted nematic liquid crystal device, the director rotates through an angle from one side of the cell to the other, typically with the exit polarization 90 deg from the input. Light entering a liquid crystal cell with its electric field vector parallel to the director of the liquid crystal will remain parallel to the director as it passes through the cell in a waveguiding effect known as adiabatic following. 5 When light enters parallel to the director, the setup is known as O-mode operation and provides an exit beam with polarization 90 deg to the input beam. This waveguiding effect also occurs for light entering the cell perpendicular to the director, known as E-mode operation. The liquid crystal material operates by transitioning between two states controlled by the applied electric field. When there is no electric field, the molecules in the liquid crystal align to provide a light path that rotates the polarization vector of the light. When the electric field is on, the molecules align with the field and do not affect the propagation of light, providing a direct path for the light. This interesting effect is controlled by alignment films on the electrodes that create a preferred alignment of the liquid crystal molecules. As such, a twist can be introduced in the light path that will guide the light between the two polarizers. 6 The off state is then defined as the state in which light can pass through the liquid crystal. In the on state, the light path in the liquid crystal is not modified and passes straight through, but then is blocked by the second polarizer. This is the principle behind the black segments of a traditional seven-segment LCD. Figure 4.3 illustrates the changes to the liquid crystal that allow the polarized light to pass through. Backlighting Liquid crystals do not generate light of their own, so another means of providing light is required to allow the screen to be viewed. The light source can either be ambient light or an artificial light source located behind or to the side of the screen. 7 In the case of a display using ambient light, a reflector is required to take the incoming light and reflect it back to the viewer; however, such reflectors are also useful in side-lit or back-lit displays. Ambient light will normally be located in front of the screen, on the side of the viewer. This light can be maximized by placing a reflector behind the LCD Figure 4.3 Illustration of the twisted nematic cell in which (a) the cell is off and (b) the cell is on. A and P indicated analyzer and polarizer, respectively. Figure 4.4 A typical front-illuminated display, in which the light source is located on the left and the display is viewed from the left. to reflect the unpolarized ambient light toward the viewer, as shown in Fig. 4.4. Artificial light sources can take many forms, such as conventional light - emitting diodes, electroluminescent panels, incandescent light bulbs, or cold, hot or external electrode fluorescent lamps. Electroluminescent panels provide uniform illumination of the display; however, the other sources require the use of a diffuser to provide well-distributed, uniform light for the display. White light is preferred for larger displays, and, typically, colored lighting is only used in small, single-purpose displays. Liquid Crystal Operating Modes The three main operating modes for LCDs are (1) the in-plane switching mode, (2) the vertical-alignment mode, and (3) the twisted nematic mode, which is currently the most often used. While other operating modes do exist, their commercial viability has not yet been shown. The quest for higher-speed operation and improved viewing angles has led to the development of new and improved operating modes. Twisted nematic mode In the basic operation of the twisted nematic mode, the amount of twist of the liquid crystal controls the amount of light that is allowed to pass. When no voltage is applied across the liquid crystal, the twist allows light to pass through the crossed polarizers. Applying a voltage removes the twist from the liquid crystal, directly affecting how much light can pass, and resulting in a change in the intensity of light seen. The twisted nematic mode is one of the most practical operating approaches and is responsible for making LCD technology viable, particularly for mobile, battery-operated displays. The low operating voltages and extremely low current flow have led to the popularity of LCDs and have almost completely removed the need for cathode ray tubes. The twisted nematic mode provides good response time and brightness in a structure that is simple to construct. When manufacturing a color display with a large number of pixels, the liquid crystal material is injected between thin films of transistor layers. Such transistor layers provide the data and gate bus structure for the display and pixel electronics. Finally, a black matrix and colored filter layers are added, as well as a common electrode layer. The crossed polarizers complete the LCD. This is illustrated in Fig. 4.3. The key to the operation of the twisted nematic liquid crystal device is the placement of the alignment layers on the conducting contacts on either side of the liquid crystal material. This forces the ‘twist’ in the material that allows light to pass through without an applied electrical potential. An applied voltage changes the orientation of the liquid crystal, blocking light from passing. 7 The twisted nematic liquid crystal technology was developed in the 1960s8 but has only recently become a major part of display technology, due in large part to the needs of the mobile computing market. However, the twisted nematic display is not without its issues. It has a long response time to signal changes and works best in a narrow viewing angle. Additionally, it is often noted that color reproduction is not considered very high quality, at least when compared to older cathode ray tube displays. In-plane switching mode In-plane switching mode is a technology used to align liquid crystals in a plane parallel to the glass substrates so that, by changing the applied voltage, the orientation of the liquid crystal molecules can be changed or switched in the same plane. In-plane switching mode was developed to overcome some of Figure 4.5 Schematic of a traditional LC cell using (a) thin film transistor (TFT) technology and (b) in-plane switching technology. The arrows below the displays show the direction of the incoming unpolarized light. the problems encountered by twisted nematic LCDs, in particular, the narrow viewing angle. The in-plane switching mode operates by arranging and switching the molecules of the liquid crystal layer parallel to the glass plates, as shown in Fig. 4.5. The basic idea of the in-plane switching mode display is that the polarizers are oriented in the same plane, and the switching effect is through the liquid crystal molecule’s rotation around the axes perpendicular to its length. This is accomplished by the use of interdigitated electrodes on the conducting surface, as shown in Fig. 4.5. In-plane switching displays show good color from all viewing angles and work well in touch-sensitive screens because they do not lighten when pressed. The clarity of the image is improved over the twisted nematic displays, which have a better response time. Vertical alignment mode Vertical alignment mode liquid crystals naturally align in a direction perpendicular or vertical to the glass substrate. In this state, with no external voltage applied, there is no effect on the polarization of incoming light such that a darkened display is created when the cell is placed between crossed polarizers. The application of an electric field causes the crystals to tilt away