Digital Systems and Computer Architecture PDF

DIGITAL SYSTEMS AND COMPUTER ARCHITECTURE PN Junction Diodes Session 1.7: Focus ⚫ PN junction ◦ Diode construction and its symbols ⚫ Biasing of diodes ◦ Forward and Reverse biasing ⚫ Various types of diodes ⚫ Depletion region ⚫ Diode characte...

DIGITAL SYSTEMS AND COMPUTER ARCHITECTURE PN Junction Diodes Session 1.7: Focus ⚫ PN junction ◦ Diode construction and its symbols ⚫ Biasing of diodes ◦ Forward and Reverse biasing ⚫ Various types of diodes ⚫ Depletion region ⚫ Diode characteristics ◦ Forward biasing and Cut-in voltage ⚫ Reverse biasing ◦ Breakdown voltage ⚫ I-V characteristics of diode ◦ Q-point of a diode ⚫ Sample Circuits with Diodes PN Junction Diode Construction Cross-sectional Doping Concentration area Na Nd p n p n x=0 ⚫ A silicon crystal is taken, the left side is doped with acceptors (p-type) and the right side is doped with the donors (n-type). ⚫ This results in a junction between p-type and n-type material. ⚫ The diagram on the right shows the doping concentration of acceptors (Na) and donors (Nd) where Na > Nd ⚫ The doping of impurities is non-uniform on both p-type and n-type.  Diode: Symbols ID VD = 0 V Diode has two terminals, an anode and a cathode Zener Diode Light Emitting Diode Photo Diode Biasing of Diode  Diode: No Bias No bias, leads of PN junction are left open ⚫ No bias means that there is no external voltage applied to the diode. ⚫ It is equivalent to a diode kept on a table with both its terminals open.  Diode: Biasing Diode has different behaviours under each conditions. VD VD Forward bias Reverse bias Anode (p-type) is connected to +ve terminal of Anode (p-type) is connected to –ve terminal of bias voltage and the cathode (n-type) to -ve terminal bias voltage and the cathode (n-type) to +ve terminal  Anode Various Types Cathode of Diodes Cathode Anode Note: A silver line on the diode signifies the cathode terminal. LEDs: Light Emitting Diodes Power Diode Note: The longer terminal is the anode.  Diode: Depletion Region Depletion Region ⚫ Majority carriers on each side cross the junction of the p- type and n-type materials. ⚫ The electrons entering the p-region combine with the holes, depleting or reducing the holes in the p-region, near the junction. ⚫ Similarly electrons around the junction on n-type material are also depleted. ⚫ The region thus formed is called depletion region. ⚫ Further crossing of majority carriers prevented by the formation of depletion region. Diode Characteristics Forward Biasing  Forward Biasing:I Explained D Notice that the forward bias ID current is increasing exponentially with respect to the forward bias voltage. VD VD ⚫ Positive potential applied at the anode repels the holes (majority carriers) on p-type pushing them towards the junction. ⚫ Similarly the negative potential on the cathode repels the electrons (majority carriers) on n-type pushing them as well towards the junction. ⚫ When VD crosses a threshold, majority carriers on either side cross the  Cut-in Voltage ID VD Vcut-in ⚫ When the applied forward bias (VF) crosses Vcut-in or VKnee, diode starts conducting and allows current (IF) to flow through. ⚫ Cut-in voltage or knee voltage is the voltage at which the forward bias current of a diode starts increasing rapidly. ⚫ Cut-in voltage of a silicon diode is approximately 0.7V, and for germanium diode it is approximately 0.3V.  Diode Characteristics Reverse Biasing  Reverse Biasing: Explained ID -VBV VD Zener breakdown Depletion Region region ⚫ Because of reverse bias, large number of free electrons are drawn to the positive potential of the applied voltage (on n-region), which in turn increases the width of the depletion region at the junction. ⚫ The diode is said to be in OFF state because current passing through it is very minimal. ⚫ There are a very few charges crossing the junction, in micro amperes. ⚫ When the reverse voltage is increased continuously  Reverse Bias Current -V ID BV VD Zener breakdown region ⚫ When the reverse voltage goes beyond -VBV the minority carriers move quickly across the depletion region. ◦ Minority carriers in p material are electrons. ◦ Minority carriers in n material are holes. ⚫ Note that electrons flow from left to right and the holes from right to left.  I-V Characteristics ID mA Forward Operating Region V V -VBV VD VD Vcut-in Reverse Operating Vknee = 0.3 V (Germanium) Region Vknee = 0.7 V (Silicon) µA Vknee = 1.2 V (Gallium Arsenide) ⚫ It’s characteristics have non-linear shapes at different operating voltages/currents, so diode is a non-linear element.  Q-Point of a Diode I D Q-point VD ⚫ Q-point is the operating or quiescent point at which a diode operates. ⚫ It is specific to a circuit with which a diode is part of. ⚫ The Q-point of a diode is decided by the circuit elements (resistors and power supplies) that are used to bias a diode. ⚫ The circuit designer chooses the Q-point such that diode operates within its safe limits. Sample Circuits using Diodes Sample Circuits using Diodes: Full- wave Rectifier Session 1.7: Summary ⚫ PN junction ◦ Diode construction and its symbols ⚫ Biasing of diodes ◦ Forward and Reverse biasing ⚫ Various types of diodes ⚫ Depletion region ⚫ Diode characteristics ◦ Forward biasing and Cut-in voltage ⚫ Reverse biasing ◦ Breakdown voltage ⚫ I-V characteristics of diode ◦ Q-point of a diode ⚫ Sample Circuits with Diodes  Transistors and Circuits using Transistors Session 1.8: Focus ⚫ Invention of Transistor ◦ Construction ◦ Types and symbols NPN and PNP ◦ Biasing of Transistors ⚫ Double and Single Subscript Conventions ⚫ Transistors ◦ Doping and Construction ◦ Operation ⚫ Biasing of NPN Transistor ⚫ Biasing of PNP Transistor ⚫ Currents in the Transistor ◦ IE, IB and IC ◦ Relationship among them ⚫ Circuits using Transistors Transistor s  Invention of Transistor ⚫ On 23rd December 1947 electronics industry experienced a new direction of interest and development. ⚫ Three scientists, Dr. S William Shockley, Walter H Brattain and John Bardeen at the Bell Telephone Laboratories (Bell Labs), New Jersey, USA, demonstrated the amplifying action of the first transistor. ⚫ They received the Nobel Prize for Physics in 1956 for this wonderful contribution!!! ⚫ Transistors replaced vacuum tubes in electronics circuits because of being smaller in size, lightweight, rugged and being more power efficient.  Transistors: An Introduction ⚫ Transistor is a three terminal device that amplifies the input AC signal when it is biased appropriately with a DC supply. ⚫ It is named as transistor because it transfers the resistance value experienced by the input signal at the input to a different value at the output. ⚫ The names of the three terminals are: ◦ Emitter, base and collector. ⚫ It is also called Bipolar Junction Transistor (BJT). ◦ Because current through this device is contributed by both electrons and holes (bipolar). ◦ But mostly in electronics industry it is just called as transistor.  Transistor Construction ⚫ Transistor can be thought of as two diodes connected back-to-back. ⚫ However, during the transistor construction, many important NPN Transistor PNP Transistor modifications are done. ◦ The base region of the transistor is very thin compared to the collector and the emitter regions. ◦ Doping of emitter is higher compared to the collector and base is lightly doped. ⚫ These are all very crucial for the functioning of transistor. ⚫ Connecting of two diodes back to back will not behave like a transistor!!! Transistor Types and Symbols NPN Transistor PNP Transistor NPN Transistor PNP Transistor Symbol Symbol  Biasing of Transistors Emitter Base Junction Collector Base Junction (EBJ) (CBJ) EBJ CBJ Forward Biased Reverse Biased Forward Biased Reverse Biased - + + - Base VBE + -VCE VBE - + VCE NPN Transistor PNP Transistor ⚫ For normal operations, EBJ is forward biased and CBJ is reverse biased. ⚫ Transistors can be made either with Silicon (Si) or Germanium (Ge). ◦ But, both the materials are not used in making a single transistor. ⚫ The forward biased EBJ junction will have the cut-in voltage of either 0.7V (Si) or 0.3V (Ge) based on the material used to construct the transistor.  Doping of Transistors NPN Transistor PNP Transistor ⚫ The Emitter is heavily doped and has a wider width. ⚫ The Base region is very thin in terms width as well as it is very lightly doped. ⚫ The Collector is heavily doped compared to the Base but little lightly doped with respect to the Emitter.  NPN Transistor: Operation EBJ CBJ Forward Biased Reverse Biased - + Base VEE + -VCC ⚫ The emitter is heavily doped to inject more electrons into the base region. ⚫ Since the base region is thin and lightly doped it passes all of the electrons injected by the emitter on to the collector. ⚫ Because base is thin and lightly doped, the electrons will have less chance to recombine with the holes in the base region. ⚫ Once electrons cross the junction of collector-base, due to the positive potential at the collector, these electrons quickly pass through the collector region and cause collector current to flow. ◦ Collector is named so, because it collects or gathers all the Biasing of NPN Transistor  Base Biasing of NPN Transistor ⚫ For making the transistor operational some DC voltage needs to be supplied, which is called biasing of transistor. ⚫ The EBJ is forward biased with a positive voltage (VBB) connected between the base and the emitter of NPN transistor. ⚫ The current flowing through the base-emitter diode is called the base current (IB). ⚫ Though the collector terminal is left open here, current still flows through the EBJ.  Problem 1: Find Base current (IB) 3 kΩ N P 4V I N B ⚫ Assume that the transistor is made of Si. ⚫ Since EBJ is forward biased, and the transistor is made of Si, the cut-in voltage across EBJ is 0.7 V. ⚫ The polarity of 0.7 V across the EBJ is as shown above. ⚫ The KVL for the base-emitter loop: -4 + /0.7 = 0 ⚫ Then, the base current (IB) = (4 – 0.7) 3k = 1.B1 mA + 3k * I  NPN Transistor: Collector Biasing R R C C IC + R + V V -C C B -C C + VB I B - B IE Collector Biasing Base and Collector Biasing ⚫ For the transistor to function as an amplifier of input signal, the collector-base junction (CBJ) needs to be reverse biased – This mode is called an active mode. ⚫ The figure above shows that positive terminal of VCC is connected to the Collector, which reverse biases the CBJ. ⚫ The Base bias voltage VBB forward biases the EBJ of the transistor. ⚫ Transistor is biased with DC supply voltages to maintain proper junction  Biasing of PNP Transistor R C IC R - B VC - +C VB I IE B + B ⚫ The above circuit shows the base and collector biasing of a PNP transistor. ⚫ Since the EBJ needs to be forward biased the negative terminal of VBB is connected to the Base. ⚫ For the transistor to function as an amplifier of input signal, the collector-base junction (CBJ) needs to be reverse biased, thus negative terminal of VCC is connected to the Collector. Currents in the Transist or  IE: Emitter Current ⚫ The IE is flowing out of the Emitter terminal because electrons are moving in from the Emitter to the Base region, contributing to IE. ⚫ Due to the lighter doping in the Base, the electrons moving into the Base region from the Emitter is much larger than the holes moving into the Emitter from the Base. ⚫ So, IE is dominated by the flow of electrons from the Emitter to the Base.  IB: Base Current ⚫ The IB flowing into the Base from the supplied voltage is due to small amount of holes moving into the Emitter. ⚫ Small number of holes lost due to the recombination of electrons in the Base region is also supplied by the IB. ⚫ VBE applied across EBJ forward biases the junction, IB flows through base terminal. ⚫ Thus, IB is in terms of μA.  IC: Collector Current ⚫ Majority of electrons injected by the Emitter flows into the Collector crossing the lightly doped narrow Base region. ⚫ Wider Collector cross section enables electrons to be collected at the Collector. ⚫ These electrons get attracted towards the positive VCB applied at the Collector. ◦ Thus, IC is shown as going into the Collector terminal because the electrons flow out of it. ⚫ Thus, IC will be slightly lower than IE. Relationship between Transistor Currents R C IC R + V B -CC + VB I B - B IE ⚫ As per KCL, currents entering the Transistor must leave it too. ⚫ Assuming the transistor as a node, the currents entering the transistor should be equal the total current leaving the node. ◦ IE = IB + IC ⚫ Transistor starts conducting when the VBE supplied is above the cut-in voltage. Circuits using the Transistor s Transistor as a Switch  Audio Amplifier ⚫ It increases the amplitude of the audio signal fed to it without distorting the original waveform.  Oscillator: Square Wave Generator ⚫ Generates a Square waveforms. ⚫ The values of R2 and C2, R3 and C3, decide the frequency of the signal generated.  Oscillator: Sine Wave Generator ⚫ Generates a sine waveform. ⚫ The values of L and C in the tank cricuit, decide the frequency of the signal generated. Session 1.8: Summary ⚫ Invention of Transistor ◦ Construction ◦ Types and symbols NPN and PNP ◦ Biasing of Transistors ⚫ Double and Single Subscript Conventions ⚫ Transistors ◦ Doping and Construction ◦ Operation ⚫ Biasing of NPN Transistor ⚫ Biasing of PNP Transistor ⚫ Currents in the Transistor ◦ IE, IB and IC ◦ Relationship among them ⚫ Circuits using Transistors