Digital Electronics and Computer Architecture (DECA) - Introduction to Basic Electronics - PDF

Digital Electronics and Computer Architecture (DECA) Introduction to Basic Electronics (Unit-1): (Complete Unit) Introduction to Basic Electronics, Digital Electronics and Computer Architecture, Active and passive components, Ohm’s la...

Digital Electronics and Computer Architecture (DECA) Introduction to Basic Electronics (Unit-1): (Complete Unit) Introduction to Basic Electronics, Digital Electronics and Computer Architecture, Active and passive components, Ohm’s law, Concept and various types of Resistors, Capacitors, Inductors and their series and parallel combinations, n-type and p-type semiconductor, P-N Junction Diode, V-I Characteristics, Ideal Diode, Diode application as a switch, Rectifiers: Half Wave, Full Wave and Bridge Rectifiers, Efficiency, Zener Diode, Light Emitting Diode, Bipolar Transistors : NPN and PNP transistor. B.E.-CSE 1st Sem. Department of Interdisciplinary Courses in Engineering (DICE) & Department of Computer Science and Engineering 1  Introduction to Basic Electronics Electronics is a branch of science and technology that deals with the behavior and control of electrons. It is a fundamental field that has revolutionized the way we live and has become an integral part of our everyday lives. At its core, electronics involves the study and manipulation of electrical circuits. An electrical circuit consists of various components such as resistors, capacitors, inductors, diodes, transistors, and integrated circuits, which work together to control the flow of electric current. 2  Cont…. 3  Cont….. 4  Digital Electronics and Computer Architecture Digital electronics and computer architecture are two closely related fields that play a fundamental role in the design and operation of modern computing systems. Digital electronics is a branch of electronics that deals with the manipulation and control of digital signals. It involves the use of logic gates and other digital circuits to perform various operations on binary data. Digital electronics is the foundation of all modern electronic devices, including computers, smartphones, and digital cameras. Fig. 1. Block diagram of Digital computer 5  Digital Electronics and Computer Architecture The building blocks of digital electronics are logic gates, which are electronic components that perform basic logical operations, such as AND, OR, and NOT. These gates are used to design more complex circuits, such as adders, multiplexers, and flip- flops, which form the backbone of digital systems. The behavior of these circuits can be described using Fig. 2. Analogy of Digital computers Boolean algebra, a mathematical framework for working with binary variables and logical operations. 6  Digital Electronics and Computer Architecture Computer architecture, on the other hand, focuses on the structure and organization of computer systems. It involves the design of hardware components, such as processors, memory modules, and input/output devices, as well as the development of instruction sets and system-level protocols. Computer architects strive to optimize the performance, power efficiency, and cost-effectiveness of computer systems. Fig. 3. Computer architecture 7 Digital Electronics and Computer Architecture One of the key concepts in computer architecture is the von Neumann architecture, which is the basis for most modern computers. This architecture consists of a central processing unit (CPU) that executes instructions stored in memory. It also includes input and output units, as well as a system bus that facilitates communication between different components. The von Neumann architecture provides Fig. 4. Von Neuman Computer architecture a framework for the sequential execution of instructions, which is the foundation of general-purpose computing. 8  Cont… In addition to the von Neumann architecture, computer architects also explore alternative architectures, such as parallel and distributed systems. These architectures leverage the power of multiple processors or computers to solve complex problems more efficiently. Parallel processing, for example, involves dividing a task into smaller subtasks that can be executed simultaneously, leading to faster computation. 9 Active and passive components Active components are capable of amplifying, switching, or controlling the flow of electrical signals. These components require a power source to function properly. Transistors, integrated circuits (ICs), operational amplifiers (op-amps), and diodes are examples of active components. Transistors, such as bipolar junction transistors (BJTs) and field-effect transistors (FETs), are widely used in amplification and switching applications. ICs, which can contain multiple transistors and other circuit elements, are the building blocks of many electronic devices. Op-amps are used for signal amplification and filtering in various applications, including audio systems and instrumentation. Diodes, with their ability to allow current flow in one direction, are commonly used in rectification and switching circuits. Passive components, on the other hand, do not require an external power source to operate. They do not actively control or amplify electrical signals but instead respond to changes in voltage or current. Resistors, capacitors, inductors, and transformers are examples of passive components. Resistors are used to limit current flow, control voltage levels, and divide voltages in circuits. Capacitors store and release electrical energy, making them essential in filtering, timing, and energy storage applications. Inductors store energy in a magnetic field and are used in applications such as filtering, energy storage, and signal coupling. Transformers, with their ability to transfer electrical energy from one circuit to another, are commonly used in power supply systems. 10. Active and passive components Fig. 5. Active and passive components 11  Ohm's Law Ohm's Law is a fundamental concept in electronics that relates the current flowing through a conductor to the voltage applied across it and the resistance of the conductor. It provides a mathematical relationship between these three quantities and is widely used in the analysis and design of electrical circuits. According to Ohm's Law, the current through a conductor is directly proportional to the voltage applied across it and inversely proportional to the resistance of the conductor. This relationship can be expressed using the equation: I=V/R where I is the current in amperes (A), V is the voltage in volts (V), and R is the resistance in ohms (Ω). Ohm's Law can also be rearranged to solve for voltage or resistance. For example: V=I*R This equation allows us to calculate the voltage drop across a resistor given the current flowing through it and its resistance, or to determine the resistance needed to achieve a desired voltage drop at a given current. 12 Ohm's Law Fig. 6. verification of Ohm’s Law 13  Various Types of Resistors, Capacitors, Inductors Resistors, capacitors, and inductors are fundamental electronic components used in various electrical circuits. Understanding different types of resistors, capacitors, and inductors, as well as their series and parallel combinations, is crucial for circuit design and analysis. They play a crucial role in regulating the flow of current, storing electrical energy, and creating magnetic fields. 14  Resistors Resistors are passive two-terminal electrical components that resist the flow of electric current. They are used to control the amount of current flowing through a circuit or to create specific voltage drops. There are several types of resistors, including: 1. Carbon Composition Resistors: These resistors are made of a mixture of carbon powder and a binder. They are known for their high stability and reliability. 2. Metal Film Resistors: Metal film resistors are constructed by depositing a thin layer of metal alloy on a ceramic or plastic substrate. They offer low noise levels, high precision, and good stability. 3. Wire wound Resistors: These resistors are made by winding a resistance wire around a ceramic or fiberglass core. They have high power handling capabilities and are suitable for high-current applications. 15  Cont…. 16  Color Coding of Resistors. 17 Resistor in parallel and series When resistors are connected in parallel, they are placed side by side, with both ends of each resistor connected to the same points in the circuit. In this configuration, the voltage across each resistor is the same, while the total current flowing through the resistors is divided among them. The total resistance of resistors in parallel can be calculated using the formula: Fig. 7. Resistors in parallel 18  Connecting resistors in series involves placing them in a sequential manner, with one end of each resistor connected to the end of the previous resistor. In this arrangement, the total resistance of the resistors is simply the sum of their individual resistances. The total resistance of resistors in series can be calculated by adding the resistances: Fig. 8. Resistors in series 19  Capacitors Capacitors are passive electronic components that store and release electrical energy. They consist of two conductive plates separated by an insulating material called a dielectric. Different types of capacitors include: 1. Ceramic Capacitors: Ceramic capacitors are small, inexpensive, and widely used in electronic circuits. They provide high capacitance values and are suitable for high-frequency applications. 2. Electrolytic Capacitors: Electrolytic capacitors are polarized capacitors that use an electrolyte as the dielectric. They offer high capacitance values and are commonly used in power supply circuits. 3. Film Capacitors: Film capacitors are made by depositing a thin film of metal on a plastic or ceramic substrate. They have excellent self-healing properties and are suitable for high-temperature applications. 20  Capacitors. 21  Cont…. 22  Capacitors In parallel when capacitors are connected in parallel, the total capacitance is simply the sum of the individual capacitances: C_total = C1 + C2 + C3 +... In a parallel configuration, the total capacitance increases, while the voltage across each capacitor remains the same. This means that when capacitors are connected in parallel, the overall capacitance is greater than any of the individual capacitances. Fig. 9. capacitors in Parallel 23  Capacitors In series When capacitors are connected in series, the total capacitance (C_total) is calculated using the following formula: 1/C_total = 1/C1 + 1/C2 + 1/C3 +... In other words, the reciprocal of the total capacitance is equal to the sum of the reciprocals of the individual capacitances. This means that when capacitors are connected in series, the total capacitance decreases. It is important to note that the voltage across each capacitor in a series configuration is the same. Fig. 10. capacitors in Serial 24  Inductors Inductors, also known as coils or chokes, are passive components that store energy in a magnetic field when current flows through them. They are commonly used in filters, oscillators, and power supply circuits. Different types of inductors include: 1. Air Core Inductors: Air core inductors have a coil wound around a non- magnetic core. They offer high inductance values and low resistance but are susceptible to interference. 2. Iron Core Inductors: Iron core inductors have a coil wound around a ferromagnetic core, such as iron or ferrite. They provide high inductance values and are widely used in power applications. 3. Toroidal Inductors: Toroidal inductors have a coil wound around a donut-shaped core. They offer high inductance values, low electromagnetic interference, and compact size. 25  Series Connection When inductors are connected in series, their total inductance is the sum of individual inductances. This can be represented by the equation: Ltotal = L1 + L2 + L3 +... + Ln In a series connection, the current passing through each inductor is the same. However, the total voltage across the series combination is divided among the inductors based on their individual inductance values. It is important to note that inductors in series do not share the same magnetic field. Instead, the magnetic fields generated by each inductor add up, resulting in increased total magnetic energy. Fig. 11.inductors in series 26  Parallel Connection When inductors are connected in parallel, their total inductance can be calculated using the following equation: 1/Ltotal = 1/L1 + 1/L2 + 1/L3 +... + 1/Ln In a parallel connection, the voltage across each inductor is the same. However, the total current passing through the combination is divided among the individual inductors based on their individual inductance values. Fig. 12.inductors in parallel 27  Series-Parallel Connections Resistors 28  Cont… Q. 1 Calculate the equivalent resistance for the circuit which is connected to 24 V battery and also find the potential difference across 4 Ω and 6 Ω resistors in the circuit. 29  Cont… Q. 2 Calculate the equivalent resistance in the following circuit and also find the current I, I1 and I2 in the given circuit. 30  Cont… Q. 3 Calculate the equivalent resistance between A and B in the given circuit. 31  Capacitors 32  Cont… Inductor 33 Semiconductors 34 Types of Semiconductor 35 Intrinsic Semiconductor 36 Intrinsic Semiconductor 37 Extrinsic Semiconductor 38 Semiconductor 39 P and n type semiconductors 40  P and n type semiconductors Trivalent Impurities for p-type Pentavalent Impurities for n-type semiconductor: Aluminium, Boron, semiconductor: Pentavalent impurities Gallium, and Indium are added in the p- like Arsenic, Antimony, Phosphorus and type semiconductor Bismuth 41 Resistance Effects of Doping If you use lots of arsenic atoms for doping, there will be lots of extra electrons so the resistance of the material will be low and current will flow freely. If you use only a few boron atoms, there will be fewer free electrons so the resistance will be high and less current will flow. By controlling the doping amount, virtually any resistance can be achieved.  Current Flow in N-type Semiconductors The DC voltage source has a positive terminal that attracts the free electrons in the semiconductor and pulls them away from their atoms leaving the atoms charged positively. Electrons from the negative terminal of the supply enter the semiconductor material and are attracted by the positive charge of the atoms missing one of their electrons. Current (electrons) flows from the positive terminal to the negative terminal.  Current Flow in P-type Semiconductors Electrons from the negative supply terminal are attracted to the positive holes and fill them. The positive terminal of the supply pulls the electrons from the holes leaving the holes to attract more electrons. Inside the semiconductor current flow is actually by the movement of the holes from positive to negative.  In Summary In its pure state, semiconductor material is an excellent insulator. The commonly used semiconductor material is silicon. Semiconductor materials can be doped with other atoms to add or subtract electrons. An N-type semiconductor material has extra electrons. A P-type semiconductor material has a shortage of electrons with vacancies called holes. The heavier the doping, the greater the conductivity or the lower the resistance. By controlling the doping of silicon the semiconductor material can be made as conductive as desired.  PN Junction Diode A p–n junction is a boundary or interface between two types of semiconductor material, p-type and n-type, inside a single crystal of semiconductor. It is created by doping. 46  Formation of depletion layer At the interface of two semiconductors the excess electrons in the combine with the excess holes in the P region. N region loses its electrons and have immobile positive ions. P region accepts the electrons and have immobile negative ions. At one point , the migratory action is stopped. And depletion region is created. 47  PN Junction Diode  An additional electrons from the N region are repelled by the net negative charge of the p region.  An additional holes from the P region are repelled by the net positive charge of the n region.  Depletion layer : The formation of a narrow region on either side of the junction which becomes free from mobile charge carriers is called depletion layer.  The depletion layer contains no free and mobile charge carriers but only fixed and immobile ions.  Its width depends upon the doping level. 48  PN Junction Diode The electrons in the N region have to climb the potential hill in order to reach the P region Electrons trying to cross from the N region to P region experience a retarding field of the battery and therefore repelled. Similarly for holes from P region. Thus, this potential is called built-in potential. tial barrier 49 PN Junction Diode 50 Forward Biased PN Junction 51  PN Junction Diode  If a suitable positive voltage (forward bias) is applied between the two ends of the PN junction, it can supply free electrons and holes with the extra energy they require to cross the junction as the width of the depletion layer around the PN junction is decreased.  Once the junction is crossed, a number of electrons and the holes will recombine.  For each hole in the P section that combines with an electron from the N section, a covalent bond breaks and an electron is liberated which enters the positive terminal  Thus creating an electron hole pair.  Current in the N region is carried by electrons  Current in the P region is carried by holes.  By applying a negative voltage (reverse bias) results in the free charges being pulled away from the junction resulting in the depletion layer width being increased. This has the effect of increasing or decreasing the effective resistance of the junction itself allowing or blocking the flow of current through the diodes pn-junction. 52 Reverse Biased PN Junction 53 V-I Characteristics 54  PN Junction Diode VI characteristics of P-N junction diodes is a curve between the voltage and current through the circuit. Voltage is taken along the x-axis while the current is taken along the y-axis. With the help of the curve, we can understand that there are three regions in which the diode works. Zero bias Forward bias Reverse bias When the P-N junction diode is in zero bias condition, there is no external voltage applied and this means that the potential barrier at the junction does not allow the flow of current. 55  PN Junction Diode When the P-N junction diode is in forward bias condition, the p-type is connected to the positive terminal while the n-type is connected to the negative terminal of the external voltage. When the diode is arranged in this manner, there is a reduction in the potential barrier. For silicone diodes, when the voltage is 0.7 V and for germanium diodes, when the voltage is 0.3 V, the potential barriers decrease, and there is a flow of current. When the diode is in forward bias, the current increases slowly, and the curve obtained is non-linear as the voltage applied to the diode overcomes the potential barrier. Once the diode overcomes the potential barrier, the diode behaves normally, and the curve rises sharply as the external voltage increases, and the curve obtained is linear. 56  PN Junction Diode When the P-N junction diode is in negative bias condition, the p- type is connected to the negative terminal while the n-type is connected to the positive terminal of the external voltage. This results in an increase in the potential barrier. Reverse saturation current flows in the beginning as minority carriers are present in the junction. When the applied voltage is increased, the minority charges will have increased kinetic energy which affects the majority charges. This is the stage when the diode breaks down. This may also destroy the diode. 57 IdealDiode 58  Diode as Switch An ideal diode just functions as a switch. When an ordinary switch is turned on or closed, both its contacts combine together, due to which its resistance becomes zero. However, when the switch is turned off (open), an infinite resistance takes place between contacts due to the opening of the contacts. Exactly the same way, when a diode is forward biased, it functions as a closed switch i.e. it practically becomes a diode short circuit. 59  Diode as Switch However, when a diode is reversed bias, it offers unlimited resistance, due to which the flow of currents stops completely from within it. Thus, an ideal diode (a diode with no forward drop, nor a reverse flow of current, further with no breakdown on such a diode) is a device or instrument that, just like a switch, acts as a short circuit in the forward direction and as an open circuit in the reverse direction. Thus, the ideal diode is considered as a bi-stable or two-directional switch, which closes in the forward direction and opens in the reverse direction. Thus, it has two conditions i.e. on or off. In case of high frequency, as a diode turns on or off (due to a rapid change in direction of the cycle)(i.e. it has to change consistently from a conducting condition to a non-conducting condition). Thus, it functions as a switch on high frequency as well. Remember, the on /off of a diode depends on the frequency being provided to the power supply. Suppose, if the value of supply frequency is 50 cycles per second, it turns on or off continuously 50 times. Due to this characteristic of a diode, it assumes the capacity of a switch. 60 Use of Diode in Rectifiers Use of Diode in Rectifiers 61  RECTIFIERS: INTRODUCTION A rectifier is an electrical device that converts alternating current (AC), into pulsating current, which is in only one direction, a process known as rectification. TYPES OF RECTIFIERS Half wave Rectifier Full wave Rectifier  HALF WAVE RECTIFIER In half wave rectification, either the positive or negative half of the AC wave is passed, while the other half is blocked. Because only one half of the input waveform reaches the output, it is very inefficient if used for power transfer.  HW Half-Wave rectifier 64  Cont…. 65  Peak Inverse Voltage (PIV) PIV 66  Output DC Voltage. 67  Cont…. 68  Cont…. 69  Important Formulas for Numerical. Maximum Peak Voltage= Maximum Secondary Coil Voltage= DC Load Voltage= Half Wave rectifier, 70  Cont…. 71  Full-Wave Rectifier Types of Full-Wave Rectifier Centre-tap Rectifier Bridge Rectifier 72  Centre-Tap Rectifier. 73  Cont…. 74  PIV Peak Inverse Voltage (PIV) for Centre-Tap Rectifier 75  Bridge Rectifier In a circuit with a non - center tapped transformer, four diodes are required instead of the one needed for half-wave rectification.  Bridge rectifier. 77  Cont…. 78  PIV for Bridge rectifier PIV for Bridge rectifier 79  Output DC Voltage Output DC Voltage 80  Numerical. 81 FULL WAVE BRIDGE RECTIFIER BRIDGE RECTIFIER  Advantages of Half Wave rectifiers Simplicity Low cost Efficient Compact design High reliability DISADVANTAGES OF HALF WAVE RECTIFIERS Low efficiency Limited output Increased ripple Limited voltage range Power loss  ADVANTAGES OF FULL WAVE RECTIFIERS The ripple frequency is two times the input frequency. Efficiency is higher. The large DC power output. Ripple factor is less. The ripple voltage is low and the higher frequency in case full-wave rectifier so simple filtering circuit is required. Higher output voltage. Higher transformer utilization factor. Utilizes both halves of the AC waveform. Easier to provide smoothing as a result of using the ripple frequency.  DISADVANTAGES OF FULL WAVE RECTIFIERS More complicated than half-wave rectifier. It requires more diodes, two for center tap rectifier and four for bridge rectifier. PIV rating of the diode is higher. Higher PIV diodes are larger in size and too much costlier. The cost of the center tap transformer is high. The twice frequency hum on an audio circuit maybe it is more audible. This rectifier is difficult to locate the center tap on the secondary winding. The DC output is small as using each of diode utilized only one-half of the transformer secondary voltages. When a small voltage is required to be rectified, the full-wave rectifier circuit is not suitable.  APPLICATIONS OF HALF WAVE RECTIFIERS Modulation Voltage Multiplier Soldering Firing Circuits  APPLICATIONS OF FULL WAVE RECTIFIERS Power supply Battery charging Motor control Welding equipment Audio amplifiers Signal demodulation  APPLICATIONS OF BRIDGE RECTIFIERS Electric welding Circuits Audio systems Lighting systems Power electronics(UPS, Invertors, Motor drivers) Electric welding Circuits Audio systems Lighting systems Power electronics(UPS, Invertors, Motor drivers) Ripple factor 90 Half wave 91 Half wave 92 Full Wave 93 Full Wave 94 Difference 95 Filters 96  Shunt capacitors filters This is the simplest and the cheapest filter. The capacitance offers a low- reactance path to the ac components of current. All the dc current passes through the load. Only a small part of the ac component passes through the load, producing a small ripple voltage. The capacitor changes the conditions under which the diodes (of the rectifier) conduct. When the rectifier output voltage is increasing, the capacitor charges to the peak voltage Vm. Just past the positive peak, the rectifier output voltage tries to fall. But at point B, the capacitor has +Vm volts across it. Since the source voltage becomes slightly less than Vm, the capacitor willi try to send current back through the diode (of the rectifier). This reverse-biases the diode, i.e., it becomes open-circuited. 97  Contiue… The diode (open circuit) disconnects or separates the source from the load. The capacitor starts to discharge through the load. This prevents the load voltage from falling to zero. The capacitor continues to discharge until the source voltage (the dotted curve) becomes more than the capacitor voltage (at point C). The diode again starts conducting and the capacitor is again charged to peak value Vm. During the time the capacitor is charging (from point C to point D) the rectifier supplies the charging current ic through the capacitor branch as well as the load current iL. When the capacitor discharges (from point B to point C), the rectifier does not supply any current; the capacitor sends current iL through the load. The current is maintained through the load all the time. The rate at which the capacitor discharges between points Band C depends upon the time constant CRL· The longer this time constant is, the steadier is the output voltage. If the load current is fairly small (i.e., RL is sufficiently large) the capacitor does not discharge very much, and the average load voltage Vdc is slightly less than the peak value Vm. 98  Continue.. An increase in the load current (i.e., decrease in the value of Rd makes the time constant of the discharge path smaller. The capacitor then discharges more rapidly, and the load voltage is not constant. The ripple increase with increase in load current. Also, the de output voltage, Vdc decreases. 99  Series Inductor Filter An inductor has the fundamental property of opposing any change in current flowing through it. This property is used in the series inductor filter of Figure. Whenever the current through an inductor tends to change, a 'back emf' is induced in the inductor. This induced back emf prevents the current from changing its value. For dc (zero frequency), the choke resistance R in series with the load resistance RL forms a voltage divider as shown in Figure. If Vdc is the dc voltage from a full wave rectifier, the dc voltage Vdc across the load is given as Usually, R is much smaller than RL; therefore, almost all of the de voltage reaches the load. 100  The operation of a series inductor filter depends upon the current through it. Therefore, this filter (and also the choke-input LC filter discussed in the next section) can only be used together with a full-wave rectifier (since it requires current to flow at all times). Furthermore, the higher the current flowing through it, the better is its filtering action. Therefore, an increase in load current results in reduced ripple. 101  Chok input LC Filter Figure on the next slide shows a choke-input filter using an inductor L in series and capacitor C in shunt with load. An LC filter combines the features of both the series inductor filter and shunt capacitor filter. Therefore, the ripples remain fairly the same even when the load current changes. The choke (iron-core inductor) allows the de component to pass through easily because its de resistance R is very small. For dc, the capacitor appears as open circuit and all the dc current passes through the load resistance RL. Therefore, the circuit acts like a dc voltage divider of Figure on next slide, and the output de voltage is given by Equation on this slide. The fundamental frequency of the ac component in the output of the rectifier is 100 Hz (twice the line frequency). For this ac, the reactance XL(= 2pi*fL) is high. The ac current has difficulty in passing through the inductor. Even if some ac current manages to pass through the choke, it flows through the low reactance XC(~ l/2pi*c/C) rather than through load resistance RL · The ripples are reduced very effectively because XL is much greater than XC, and XC is much smaller than RL. The circuit works like the ac voltage divider of Fig. If v; is the rms value of the ripple voltage from the full-wave rectifier, then the rms value of the output ripple is given as 102 Continue.. 103  Pi Filter Very often, in addition to the LC filter, we use an additional capacitor C1 for providing smoother output voltage. This filter is called Pi filter (its shape is like the Greek pi letter Pi). The rectifier now feeds directly into the capacitor C1 Therefore, the filter is also called capacitor-input filter. Since the rectifier feeds into the capacitor C1, this type of filter can be used together with a half-wave rectifier. (The choke-input filter cannot be used with a half-wave rectifier.) Typical values for C1 and C2 for a half-wave rectifier are 32 μF each; and for L, 30 H. The half-wave rectifier ripple frequency being 50 Hz, these components have reactance's of XL = I 00 Q and Xe = 9492 Q approximately. The reactance's of L and C2 act as an ac potential divider. This reduces the ripple voltage to approximately 100/9426 times its original value. In the full-wave rectifier, the ripple frequency is 100 Hz. It means that a filter using the same component values would be more efficient in reducing the ripple. In other words, for a given amount of ripple smaller components can be used.. 104  Zener Diode A Zener Diode, also referred to as a breakdown diode, is a specially doped semiconductor device engineered to function in reverse direction. As the reverse voltage increases and reaches the predetermined breakdown voltage () current begins to flow through the diode. This current reaches a maximum level determined by the series resistor, after which it stabilizes and remains constant for wide ranges of voltages. Device symbol is different from ordinary diode symbol. 105  Zener Diode (contd..) V-I characteristics of the diode is given by : 106  Zener Diode As Voltage Regulator Zener diode voltage regulator After the ripples have been smoothed or filtered from the rectifier output, we get a sufficiently steady dc output. But for many applications, even this sort of power supply may not serve the purpose. First, this supply does not have a good enough voltage regulation. That is, the output voltage reduces as the load (current) connected to it is increased. Secondly, the dc output voltage varies with the change in the ac input voltage. To improve the constancy of the dc output voltage as the load and/or the ac input voltage vary, a voltage- regulator circuit is used. The stabilizer circuit is connected between the output of the filter and the load. The simplest regulator circuit consists merely of a resistor Rs connected in series with the input voltage, and a zener diode connected in parallel with the load. The voltage from an unregulated power supply is used as the input voltage Vi to the regulator circuit. As long as the voltage across RL is less than the zener breakdown voltage Vz, the zener diode does not conduct. If the zener diode does not conduct, the resistors Rs and RL make a potential divider across Vi. At an increased Vi, the voltage across RL becomes greater than the zener breakdown voltage. It then operates in its breakdown region. The resistor Rs limits the zener current from exceeding its rate maximum /zmax· 107  Zener Diode As Voltage Regulator The maximum permissible current is Izmax= Power/Vz = P/Vz 108  Zener Diode As Voltage Regulator Reasons why we need Regulator Circuit 1. Output Voltage decreases as the load connected to it is increased. 2. The DC output voltage varies with the change in AC input voltage. To improve the constancy of the DC output voltage as the load/ ac input voltage vary a Voltage Regulator circuit is used. 109 LED (Light Emitting Diode) The LED is a special type of diode, and it has similar electrical characteristics to a PN junction diode. It allows the flow of current in the forward direction and blocks the current in the reverse direction. It is a specially doped diode and made up of a special type of semiconductors such as GaAs (Gallium Arsenide). 110  LED (Light Emitting Diode) LEDs work on the principle of Electroluminescence. On passing a current through the diode, minority charge carriers and majority charge carriers recombine at the junction. On recombination, energy is released in the form of photons. 111  LED (Light Emitting Diode) (Contd..) Construction: It is designed through the deposition of three semiconductor material layers over a substrate. These three layers are arranged one by one where the top region is a P-type region, the middle region is active, and the bottom region is N- type. Working: When the diode is forward biased, then the electrons & holes are moving fast across the junction and they are combined constantly, removing one another out. Soon after the electrons are moving from the n-type to the p-type silicon, it combines with the holes, then it disappears. Hence it recombines and gives the little burst of energy in the form of a tiny packet or photon of light. 112 LED (Light Emitting Diode) (Contd..) 113 Bipolar Junction Transistors The basic of electronic system nowadays is semiconductor device. The famous and commonly use of this device is BJTs (Bipolar Junction Transistors). The transistor is a 3 layer semiconductor consisting of either 2-n and 1-p or 2-p and 1-n type of layer of material. The former is called npn transistor while other is called the pnp transistor 114  BJT (BIPOLAR JINCTION TRANSISTOR) Some important points: There are 3 terminals. The middle section is thin than other. There are two junction, so transistor can be considered as two diode connected back to back Transistors are current-driven devices. 115  Position of the terminals and symbol of BJT. The emitter and collector terminals are made of the same type of semiconductor material, their functions cannot be interchanged. Collector region is made larger than emitter since it required to dissipate more heat. Emitter – Heavily doped Base – Lightly Doped Collector- Moderately doped 116 Construction of NPN The NPN transistor is made of semiconductor materials like silicon or germanium. When a p- type semiconductor material is fused between two n-type semiconductor materials, an NPN transistor is formed. 117  Transistor Operation Working of npn transistor Forward bias is applied to emitter base junction and reverse bias is applied to collector-base junction The forward bias in the emitter base junction causes electrons to move towards base. This constitutes the emitter current. 118  Transistor Operation Working of NPN transistor: As this electrons flow toward p- type base, they try to recombine with holes. As base is lightly doped only few electrons recombine with holes within the base. These recombined electrons constitute small base current. The remainder electrons crosses base and constitute collector current. Ie = Ib +Ic 119  Transistor operation Working of PNP transistor Forward bias is applied to emitter- base junction and reverse bias is applied 120 to collector- base junction. The forward bias in the emitter-base junction causes holes to move toward base. This constitute emitter current, Ie  Transistor Operation Working of pnp transistor: As this holes flow toward n-type base, they try to recombine with electrons. As base is lightly doped only few holes recombine with electrons within the base. These recombined holes constitute small base current. The remainder holes crosses base and constitute collector current. 121 Difference Between NPN and PNP Transistor NPN PNP The current flows from collector terminal The current flows from emitter to to emitter terminal. collector terminal. One P-type semiconductor is sandwiched It is made of up two P-type material between the two N-type semiconductors. layers with N-type sandwiched between them. The current flow from the collector is The current flow from the emitter to generated by keeping a +ve voltage there. collector is generated at emitter terminal by keeping a +ve voltage there. The transistor switches ON with the The transistors switch ON when there is increase in current in the base terminal no current flow at the base terminal When the current is reduced in the base, When a current is present at the base of a the transistor doesn’t function across the PNP transistor, then the transistor collector terminal and switches OFF switches OFF. 122 Thank you