EC6202- ELECTRONIC DEVICES AND CIRCUITS TWO MARK QUESTIONS AND ANSWERS UNIT- 1 PN JUNCTION DEVICES

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1 EC6202- ELECTRONIC DEVICES AND CIRCUITS TWO MARK QUESTIONS AND ANSWERS UNIT- 1 PN JUNCTION DEVICES 1. What is an ideal diode? An ideal diode is one which offers zero resistance when forward biased and infinite resistance when reverse biased. 2. Compare ideal diode as a switch. An ideal diode when forward biased is equivalent to a closed (ON) switch and when reverse biased, it is equivalent to an open (OFF) switch. 3. State the mathematical equation which relates voltage applied across the PN junction diode and current flowing through it. I = I 0 (e V/η V T -1) 4. Define knee/cut in threshold voltage of PN diode. It is the forward voltage applied across the PN diode below which practically no current flows. 5. What is the effect of junction temperature on cut-in voltage of PN diode? Cut-in voltage of a PN diode decreases as the junction temperature increases. 6. What is the effect of junction temperature on forward current and reverse current of a PN diode? For the same forward voltage, the forward current of PN diode increases and reverse saturation current increases with increase in junction temperature. 7. Differentiate between breakdown voltage and PIV of a PN diode. The breakdown voltage of a PN diode is the reverse voltage applied to it at which the PN junction breaks down with sudden rise in reverse current.whereas, the peak inverse voltage is the maximum reverse voltage that can be applied to PN-junction without damage to junction.

2 8. Differentiate Avalanche breakdown and Zener breakdowns. Zener breakdown: Breakdown occurs due to heavily doped junction and applied strong electric field. Doping level is high Breakdown occurs at lower voltage compared to avalanche breakdown. Avalanche breakdown: Breakdown occurs due to avalanche multiplication between thermally generated ions Doping level is low Breakdown occurs at higher voltage 9. Differentiate between drift current and diffusion current Drift current: It is developed due to potential gradient This phenomenon is found both in metals and semiconductors Diffusion current: It is developed to charge concentration gradient It is found only in semiconductors 10. List PN diode parameters Bulk resistance Static resistance/junction resistance or DC forward resistance Dynamic resistance or AC forward resistance Knee voltage Breakdown voltage Reverse or leakage current 11. State the PN diode ratings. Maximum forward current Peak reverse voltage Maximum power ratings

3 12. Define reverse recovery time. It is the maximum time taken by the device to switch from ON to OFF state. 13. Define transition capacitance of a diode. When a PN junction is reverse biased, the depletion region acts as an insulated or as a dielectric. The P and N junctions on either side have low resistance and act as the plates. It is hence, similar to a parallel plate capacitor. This Junction capacitance is known as transition or space charge capacitance (C T ). It is given by, C T = µa/d Where, A = Cross sectional area of depletion region d = Width of depletion region Since the thickness of depletion layer depends on the amount of reverse bias, C T can be controlled with the help of applied bias. 14. Define diffusion capacitance of a diode. Diffusion or storage capacitive effect is present when the junction is forward biased. Diffusion capacitance is formed due to the time delay in the minority charges across the junction by diffusion process. Due to this fact, this capacitance cannot be identified in terms of a dielectric and plates. It varies directly with forward current. When a forward biased PN junction is suddenly reverse biased, a reverse current flows which is large initially, but gradually decreases to the level of saturation current. 15. List some applications of zener diode. It is used as Voltage regulators as Peak clippers or voltage limiters for wave shaping For meter protection against damage from accidental application of excessive voltage. As fixed reference voltage network for biasing and comparisons and for calibrating voltmeters 16. State the ratings of Zener diode. Zener voltage: The voltage at which a zener diode breaks in the reverse bias condition is called Zener voltage. It is the voltage at which a zener diode operates. The value of zener voltage depends on amount of doping.

4 Tolerance: The range of voltages about the breakdown voltage in which a zener diode conducts in reverse direction is called tolerance Power rating: The maximum power which a zener diode can dissipate without damage is called power rating. PZM = IZM*VZ Maximum Current Rating (Izm): The maximum value of current which a zener diode can handle at its rated voltage without damage is called maximum current rating. Zener resistance: The opposition offered to the current flowing through the zener diode in the operating region is called zener resistance(rz) OR zener impedance 17. State the principle of operation of an LED. When a free electron from the higher energy level gets combined with the hole, it gives out the light output. Here, in case of LED s the supply of higher level electrons is provided by the battery connection. 18. State advantages of LED They are small in size Mechanically rugged Low operating temperature Small switch on time Longer lifetime Low cost Better linearity Compactable with IC 19. State disadvantages if LED Output power gets affected by temperature radiation Quantum efficiency is low Gets damaged due to over-voltage and over-current 20. List applications of LED Used in various types of displays Used as source in opto-couplers Used in infra-red remote controls Used as indicator lamps

5 Indicators in measuring devices 21. State the principles of operation of an LCD. This type of display consists of liquid crystal molecules. Their special property is that they change their orientation when an electrical signal is applied to them. The display consists of two glass plates and liquid crystal molecules are placed in between the glass plates. When no electrical signals are applied to the liquid crystal cells, all the liquid crystal molecules have random orientation with respect to their axis. The incoming light passes through the gap of molecules so light gets twisted. When an electrical signal is applied to this structure, all the liquid crystal molecules get oriented by 90 degrees to the glass plates. In this case, the light passes in a straight way along the molecular arrangement. 22. State advantages of LCD Less amount of power per digit Best contrast ratio No external interfacing circuitry is required Low threshold voltage Can be driven directly LCD and MOS are compatible Small size, low cost 23. Define rectifier. Mention the types Rectifier is a circuit that converts AC in to pulsating DC.It uses uni directional conducting devices like PN diodes. They are classified in to two types: Half wave rectifier Full wave rectifier Half wave rectifier: It uses the property of diodes that conducts current in one direction only. During the positive half cycle of the input AC voltage, the diode is forward biased and conducts. While conducting, diode acts as a short circuit so the circuit current flows and hence the positive half cycle of input ac voltage are dropped across the load. During the negative half cycle, Diode is reverse biased and there is no current flow. Hence the negative half cycles are suppressed. Full wave rectifier: Both the half cycles of input ac are utilized in a full wave rectifier. Alternate half cycles are inverted to give a unidirectional load current. There are two types of full wave rectifiers: Centre tapped Bridge rectifier

6 24. Define rectifier efficiency. It is the ratio of DC power output to the applied ac power input. 25. Define ripple factor of a rectifier. The pulsating output of a rectifier contains DC component and an ac component called ripple. The ratio of RMS value of ac components to the dc components in the rectifier output is called ripple factor. Ripple factor, r =RMS Value of ac component / RMS value of dc components It indicates the purity of dc power output. Smaller the ripple factor, lesser the effective ac component and hence more effective is the rectifier. 26. Define TUF of rectifier. Transformer utilization factor (TUF) is defined as the ratio of dc power delivered to the load to the ac power rating of the transformer secondary. TUF = P DC /P AC(RATED) For HWR, TUF = For Centre tapped FWR, TUF = For bridge Rectifier, TUF = Give the advantages and disadvantages of HWR and FWR HWR Advantages: Simple circuit Low cost Disadvantages: Rectification efficiency is low(40.6%) Very high amount of ripples(r = 1.21) Low TUF Saturation of transformer core occurs Centre tapped FWR Advantages: Rectification efficiency is high Low ripple Better TUF No problem of transformer core saturation

7 Disadvantages: Difficult to locate the centre tap on the secondary winding Dc output is small as each diode uses only one half of the transformer secondary voltage. Diode must have high PIV Bridge Rectifier: Advantages: High rectification efficiency Low ripple Better TUF Centre tap not required Output is twice that of centre tap circuit PIV is half of centre tap circuit. Disadvantages: Required four diodes Not suitable for low voltage rectifications 28. What is the need of filter in a rectifier? Filter circuits are used to remove ripples from the load output. It is kept between rectifier and load. 29. What is rectifier filter? List the different types of filters. A rectifier filter circuit passes the dc components and blocks the ac components. Ripples can be removed by using one of the following filtering methods: A capacitor in parallel to the load provides an easier bypass for the ripples and dc appears across the load. An inductor in series with the load prevents the passage of ripples due to high impedance. Various combinations of capacitors and inductors such as L section filters make use of the above properties. Types of filter circuits: Shunt capacitive filter Series inductive filter Choke input (LC) filter Capacitor input(pi) filter

8 30. List some advantages and disadvantages of CLC filters. Advantages: It can be used with both FWRs and HWRs More output voltage is obtained Output is almost pure DC Disadvantages: Cost, size and weight is more because of inductor applied in the circuit. 31. What is the need for voltage regulators? An ordinary power supply has following drawbacks: Poor regulation The DC output voltage varies with the ac supply voltage which fluctuates at different times of the day and is different in different locations. The DC output voltage varies with temperature, in case semiconductors are used For certain applications the output of the filter even with small amount of ripple is not acceptable. 32. What is a voltage regulator? List the types of regulators. A voltage regulator is a circuit which makes the rectifier filter output voltage constant regardless of the variations in the input voltage or load. Types of regulators: Shunt regulators Series regulators Switch mode regulators or switch mode power supply. 33. Define voltage regulation The variation of output voltage with respect to the amount of load current drawn from the power supply is called voltage regulation. % Voltage regulation = {(V NL -V FL )/V FL }*100 V NL = DC output voltage at no load V FL = DC output voltage at full load

9 1. What is bipolar junction transistor? UNIT 2 TRANSISTORS A bipolar junction Transistor is a three terminal semiconductor device in which the operations depends on the interaction of majority and minority carriers. 2. Give the biasing used in a BJT transistor. Forward bias is applied to the emitter base junction of a transistor and reverse bias is applied to the collector base junction for a common base bipolar junction transistor. 3. Define transistor current. The emitter current is the sum of the collector current and the base current.it is also known as the transistor current. The transistor current, I E = I B +I C 4. List different operating regions of transistors The different operating regions of transistors are: Active region Cutoff region Passive region 5. What is dc current gain? The DC current gain is defined as the ratio of the collector current, IC to base current,i B Denoted by h fe or β. h fe = β = I C / I B 6. What are the three transistor configurations? Common base configuration Common emitter configuration Common collector configuration 7. Explain npn and pnp transistors. Npn transistor In npn transistor, p-type semiconductor is sandwiched between two n-type semiconductors. The emitter region is made up of n-type semi conductor base region is made up of p-type semiconductor; collector region is made up of n type semiconductor.

10 Pnp transistor In pnp transistor, n type semiconductor is sandwiched between two p type semi conductors. Emitter region is made up of p type collector region is made of p type and base region is made up of n type semi conductors. 8. What are the types of breakdown in transistors? Different types of breakdowns in transistors are: Avalanche multiplication Reach through /punch through 9. What is FET? A field effect transistor is a three terminal semiconductor device in which current conduction is by one type of carrier, ie, either electrons or holes are controlled by the electric field. 10. Define pinch off voltage. It is the voltage, at which the channel is pinchedoff, ie, all the free charges from the channel get removed. 11. List the classification of FET. Based on the construction, FET is classified as: Junction field effect transistor Metal oxide semiconductor FET Based on majority charge carriers, FET is classified as: N-Channel FET P-Channel FET 12. What are the applications of FET? FET is used as a buffering instrument,receivers FETs are used in RF Amplifiers Used as voltage controlled devices Used as voltage variable resistor operational amplifier Used in mixer circuit in FM and TV receiver communication equipment. 13. List Various JFET Parameters DC drain resistance(r DS ) AC drain resistance (r d ) Transconductance(g m ) Amplification factor(µ) Input resistance(r i )

11 14. What is a MOSFET? MOSFET is metal oxide semiconductor field effect transistor. It is a three terminal semiconductor similar to FET, with the gate insulated from the channel. 15. What are the types of MOSFET? The two types of MOSFET are: Depletion MOSFET: The controlling electric field reduces the number of majority carriers available for condition. Enhancement MOSFET: Application of electric field causes an increase in the majority carrier density in the conducting region of transistor. 16. What are the applications of MOSFET? Applications of MOSFET are: It can be used as input amplifiers in oscilloscope, electronic voltmeters. In Logic circuits Phase shift Oscillators In FM and TV receivers 17. What are the internal capacitances present in MOSFET? Gate capacitances Source drain capacitances 18. What is the unique characteristic of UJT? A unique characteristic of UJT is : when it is triggered the emitter current increase regenerative until it is limited by emitter power supply. Due to this negative resistance property, a UJT can be employed in a variety of applications such as sawtooth wave generator, pulse generator. Switching, tuning and phase control circuits. 19. Define Holding current. It is that value of current at which the SCR switches from the conduction state to the forward blocking state. 20. Define latching current. Minimum current flowing from the anode to cathode when SCR goes from OFF to ON state and remains in ON state even after bias is removed. It is greater than the holding current. 21. Differentiate SCR and TRIAC SCR: It is unidirectional ie,it can conduct in forward direction only.

12 It is triggered by a narrow positive pulse applied at the gate terminal. SCRs are available with a large current rating It has a fast turnoff and therefore can be used to switch the ac supply frequencies up to few khz TRIAC: It is bidirectional,ie, it can conduct in forward as well as reverse direction It is triggered by a narrow pulse of either positive or negative polarity applied at the gate terminal TRIACs are available for lower current rating as compared to that of SCRs It has a comparatively longer turnoff time and therefore the use of triac is limited to supply frequencies up to 400 hz. 22. Mention few IGBT applications. AC and DC motor drives Power supplies Solid state relays

13 UNIT 3 AMPLIFIERS 1. What are the types of circuit connections or configurations for operating a transistor? Common base (CB) Common Emitter (CE) Common Collector (CC) 2. What are the benefits of h-parameters? Real numbers at audio frequencies Easy to measure Convenient to use in circuit analysis and design Can be obtained from the transfer static characteristics curve 3. What are the three configurations of h-parameters? Input resistance Common base -h ib Common Emitter-h ie Common Collector-h ic Reverse voltage gain Common base h rb Common Emitter-h re Common Collector-h rc Forward transfer current gain Common base h fb Common Emitter-h fe Common Collector-h fc Output admittance Common base h ob Common Emitter-h oe Common Collector-h oc

14 4. List out approximations of frequency response of CE C ce is very small and may be neglected r ce >> (R C R L ) so r ce may be neglected 5. Why collect is made larger than emitter and base? Collector is made physically larger than emitter and base because collector requires to dissipate much power. To remove the charge carriers away from the collector base junction. 6. Why CE configuration is most popular in amplifier circuits? CE configuration popular because its current,voltage and power gains are quite high and the ratio of output impedance and input impedance are quite moderate. 7. What are the parameters h r and h 0 called? h r Reverse transfer voltage h 0 output admittance 8. Why h parameters are called hybrid parameters? H parameters are called hybrid parameters as they have different units are mixed with other parameters 9. Which of the BJT configuration is suitable for impedance matching applications and why? CC configuration is suitable for impedance matching applications because of very high input impedance 10. What is meant by small signal equivalent circuit? Using small input signals, the transistor can be replaced with small signal linear model. Such a model is called small signal equivalent circuit. 11. In a common base circuit, I C =0.96 ma and I B =0.04 ma. Find α. Solution: I C = 0.96 ma I B = 0.04 ma α = I C /I E = I C /(I C +I B ) = 0.96/ ( ) = 0.96

15 12. Given α=0.95.find β and γ Solution: Given, α = 0.95 β = α/(1- α) =0.95(1-0.95) β = 19 γ =1+β =1+19 = In CE configuration, the voltage drop across load resistance R L of 1kΩ is 1.2V. If β = 100, find the base current. Base current I B = 12µA 14. A CE amplifier has an input resistance of 20Ω and an output resistance of 1 kω.if a signal voltage of 200mV is applied between the emitter and base, find the voltage gain. Assume α= 1. Voltage gain A v =50

16 UNIT 4 MULTISTAGE AMPLIFIERS AND DIFFERENTIAL AMPLIFIER 1. What is differential amplifier? A n amplifier which amplifies the difference between two signals is called differential amplifier. 2. Define differential gain.(a d ) The A d is the gain with which differential amplifier amplifies the difference between two input signals. Hence it is called differential gain of the differential amplifier. A d =V o /V d Where V d = difference voltage. V o = output voltage 3. Define common mode signal. The output voltage of the practical differential amplifier not only depends on the differential voltage, but also depends on the average common levels of the two inputs. Such an average of the two input signals is called common mode signal. It is denoted by V c V c = (V 1 + V 2 )/2 4. Define common mode gain (A c ) The gain which amplifies the common mode signal is called common mode gain V o = A c * V c 5. Define CMRR The ability of a differential amplifier to reject their common mode signal is expressed by the ratio called common mode rejection ratio denoted as CMRR.It is defined as the ratio of differential voltage gain A d to common mode voltage gain A c 6. What is Tuned Amplifier? The amplifier with a circuit that is capable of amplifying a signal over a narrow band of frequencies is called tuned amplifier. 7. What is dissipation factor? Dissipation factor is referred as the total loss within a component 1/Q. 8. What is the classification of Tuned Amplifier? Single tuned Double tuned Stagger tuned

17 9. What is a single tuned amplifier? An n amplifier circuit that uses a single parallel tuned circuit as a load is called single tuned amplifier. 10. What are the advantages of tuned amplifier? They amplify defined frequencies Signal to noise ratio at output is good They are suited for radio transmitters and receivers 11. What are the disadvantages of a tuned amplifier? The circuit is bulky and costly The design is complex They are not suited to amplify audio frequencies 12. How are amplifiers classified according to the input? Small signal amplifiers Large signal amplifiers 13. What are double tuned amplifiers? The amplifiers having two parallel resonance circuits in its load are called double tuned amplifiers. 14. What is stagger tuned amplifiers? It is a circuit in which two single tuned cascaded amplifiers having certain bandwidth are taken and their resonance frequencies are adjusted so that they are separated by an amount equal to bandwidth at each stage. Since resonant frequencies are displaced, it is called stagger tuned amplifier. 15. What is meant by neutralization? The technique used for the elimination of potential oscillation is called neutralization. 16. What are different types of neutralization? Hazeltine neutralization Rice neutralization Neutrodyne neutralization 17. What is Hazeltine neutralization? The effect of collector to base capacitance of the transistor is neutralized by introducing a signal that cancels the signal coupled through the collector to base capacitance. This process is called Hazeltine neutralization.

18 18. What is narrowband neutralization or rice neutralization? It uses centre tapped coils in the base circuit. The signal voltages at the end of tuned base coil are equal and out of phase. 19. What is the advantage of using output transformer for a class A amplifier? Efficiency of operation is higher than directly coupled amplifier. The DC bias current that flows through the load in case of directly coupled amplifier is stopped in case of transformer coupling. The impedence matching required for maximum power transfer is possible. 20. What are the drawbacks of class B amplifier? Two centre tapped transformers are necessary The transformers make the circuit bulky and costlier Frequency response is poor 21. What is the reason for instability in tuned amplifier? As reactance of C be at R f is low enough,it provides the feedback path from collector to base. With this circuit condition, if some feedback signals manage to reach the input from output, in a positive manner with proper phase shift. Then there is possibility of circuit being converted to an unstable state. 22. Which type of power amplifier among class A/B/C provides least distortion? The Power amplifiers are also called audio amplifiers or audio frequency power amplifier.they are present in class A and class B.Class C operation is not suitable for audio frequency power amplifiers. They are used in tuned circuits used in communication areas and in radio frequency RF circuits with tuned RLC loads. As they are used in tuned circuits, Class C amplifiers are called tuned amplifiers. So, class C is not a power amplifier. 23. How does the crossover distortion occur? If a is transistor to be in active region, the base emitter junction must be forward biased. The junction cannot be made forward biased till the voltage applied becomes greater than cut in voltage V r of the junction. Hence as long as the magnitude of the input signal is less than the cut-in voltage of the base emitter junction, the collector current remain zero and transistor remains in cutoff region.

19 24. Comparison of Push pull and complementary symmetry circuits. Pushpull class B Both the n-p-n and p-n-p transistors are similar The transformer is used to connect the load as well as input The impedance matching is possible due to the output transformer Frequency response is poor Dual power supply is not required Complementary symmetry class B Transistors are complementary type, ie, one is n-p-n and the other, p-n-p. The circuit does not have a transformer The impedance matching is possible due to common collector circuit. Frequency response is improved. Dual power supply is required

20 UNIT 5 FEEDBACK AMPLIFIERS AND OSCILLATORS 1. Define positive feed back If the feedback signal is applied in phase with the input signal, the net effect of the feedback increases according to the input signal given to the amplifier. This type of feedback is called positive feedback. 2. Define negative feed back If the feedback signal is applied out of phase with the input signal, the net effect of the feedback decreases according to the input signal given to the amplifier. This type of feedback is called negative feedback. 3. Define sensitivity Sensitivity is defined as the ratio of percentage change in voltage gain with feedback to the percentage change in voltage gain without feedback. 4. Define feedback. A portion of the output signal is taken from the output of the amplifier and is combined with the normal input signal. This is known as feedback. 5. What are the types of feedback? Different types of feedback are: Voltage series feedback Current series feedback Voltage shunt feedback Current shunt feedback 6. Give an example for voltage series feedback. The common collector or emitter follower amplifier is an example for voltage series feedback. 7. Give the properties of negative feedback. Negative feedback reduces gain Distortion is very much reduced 8. What is oscillator circuit A circuit with an effective device is used to produce an alternating current this type of circuit is called an oscillator circuit.

21 9. What are the conditions for oscillations The total phase shift of an oscillator should be 360 degree It should satisfy Barhausen criterion 10. Define desensitivity It is the ratio percentage change in voltage gain without feedback to the percentage gain with feedback. It is the reciprocal of sensitivity 11. What are the parameters on which feedback oscillators are classified Feedback oscillators are classified based on nature of output waveform parameters used, the range of frequency etc 12. Write the expression for input and output resistance of voltage series feedback Input resistance with feedback, R IF =(1+Aβ)R I Output resistance with feedback, R OF =R O /(1+Aβ) 13. Write the expression for input and output resistance of current shunt feedback Input resistance with feedback, R IF = R I /(1+Aβ) Output resistance with feedback, R OF =R O /(1+Aβ) 14. Define Barhausen criterion The condition for feedback oscillator given by βa V >1 is called Barhausen criterion We have, A VF =A V /(1- βa V ) A VF =α=1/0 ie; (1- βa V ) < 0 hence, βa V > 1..