EC6202-ELECTRONIC DEVICES AND CIRCUITS YEAR/SEM: II/III UNIT 1 TWO MARKS. 1. Define diffusion current.

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1 EC6202-ELECTRONIC DEVICES AND CIRCUITS YEAR/SEM: II/III UNIT 1 TWO MARKS 1. Define diffusion current. A movement of charge carriers due to the concentration gradient in a semiconductor is called process of diffusion. When charge carriers move, the current is constituted in a bar. This current due to diffusion is called diffusion current. 2. What is meant by zener break down? When a PN junction is heavily doped, the depletion region is very narrow. So under reverse bias condition, the electric field across the depletion region is intense. Such an intense field is enough to pull the electrons out of the valence band of the stable atoms. Such a creation of free electrons is called zener effect. These minority carriers constitute very large current and the mechanism is called zener break down. 3. List the diode parameters Bulk resistance Static resistance Dynamic resistance Reverse resistance Knee voltage Break down voltage Reverse current or leakage current 4. Compare the silicon and germanium diodes with respect to cut in voltage and reverse saturation current.

2 The barrier potential for germanium diode is 0.3 V. so the cut-in-voltage of germanium diode is greater then 0.3 V and the barrier potential of silicon diode is 0.7 V. So the cut-in-voltage of silicon diode is greater than 0.7 V. 5. A silicon diode has a saturation current of 7.5µA at room temperature 300K. calculate the saturation current at 460 K. Given: I01 = 7.5µA = 7.5 * 10-6 T1 = 300 K T2 = 400 K T = T2 T2 = T = 160 K I02 = 2(?T/10) * I01 = 2(160/10) * 7.5 * 10-6 I02 = A 6. Differentiate between drift and diffusion currents. Drift current Diffusion current It is developed due to potential gradient It is developed due to charge concentration gradient This phenomenon is found both in metals and semiconductors This is found only in semiconductors 7. Differentiate between avalanche and zener break down.

3 Zener break down Avalanche breakdown Breakdown occurs due to heavily doped junction and applied strong electric field Breakdown occurs due to avalanche multiplication between thermally generated ions Doping level is high Doping level is low Breakdown occurs at lower voltage compared to avalanche breakdown Break down occurs at higher voltage 8. List the PN diode switching times? Recovery time? Forward Recovery time? Reverse Recovery time? Storage and transition time 9. Define the cut-in-voltage and peak inverse voltage of pn junction diode. When diode is farward biased, some voltage is necessary to overcome the barrier potential,to make diode conduct. This is called cut-in-voltage. In reverse bias opposite polarity appears across diode. The maximum diode voltage which diode can withstand without break down is called peak inverse voltage. 10. Define barrier potential at the junction. Due to immobile positive charges on n-side and negative charges on p-side, there exists an electric field across the junction. This creates potential difference across the junction which is called barrier potential, junction potential, built-in potential or cut-in voltage o PN junction. 11. Give the diode current equation. I=Io[eV/?VT 1]

4 I = diode current Io = reverse saturation current in amperes V = applied voltage? = 1 for germanium diode, 2 for silicon diode VT = voltage equivalent of temperature 12. What is meant by depletion region? In PN junction, the diffusion of holes and electrons start initially. Near the junction, holes recombine in N-region to form immobile positive ions. Similarly electrons recombine in P-region to form immobile negative ions. With sufficient accumulation of such immobile ions on both sides, the diffusion stops. So near the junction, there exists a region in which immobile positive and negative charge reside while mobile charge carriers in this region get completely depleted. This region is called depletion region. 13. Define the transition capacitance of the diode. A capacitance existing at the PN junction when the diode is reverse biased where the two regions act as the plates while the depletion region acts as dielectric is called a transition capacitance of a diode. 14. Why a series resistor is necessary when a diode is reverse biased? A series resistor is necessary when a diode is reverse biased for limiting the forward current which increases exponentially with voltage. 15. List any four applications of light emitting diode. All kinds of visual displays In the optical devices such as optocouplers As on-off indicator in various types of electronic circuits. Remote control. burglar alarm

5 UNIT I(16 marks) 1. Explain the working of a PN junction forward bias and in reverse bias 2. Explain the operation of zener diode and define zener breakdown. Draw its VI characteristics. 3. With a neat sketch, explain the operation of PN junction diode 4. Define regulator. Explain the operation of any one type of regulator. 5. With a neat sketch, explain the operation of CLC and LC filter

6 UNIT II - BJT AND ITS APPLICATIONS ( TWO MARKS ) 1. Will a transistor result if two diodes are connected back to back? A transistor has two p-n junctions. One junction is between the emitter and the base and is called emitter base junction and the other junction is between the base and the collector and is called collector base junction. Thus transistor is like two pn junction diodes connected back to back. 2. State early effect. What are the consequences of it? In the common Base characteristics of BJT when reverse bias voltage VcB increases, the width of the depletion region also increases. This reduces the electrical base width. This effect is called "Early Effect" or "Base width modulation". The Early effect has two consequences. There is less chance of recombination within the base region. The charge gradient is increased within the base and consequently the current of the minority carriers injected across the junction increases. 3. What is thermal runaway? In a power transistor the increase in collector current increases the power dissipated at the collector junction. This, in turn further increases the temperature of the junction and hence increases the collector current. The process is cumulative and it is referred to as self heating. The excess heat produced at the collector base junction may even burn and destroy the transistor. The situation is called "Thermal runaway" of the transistor. 4. What are hybrid parameters? The parameters h11, h12, h21, h22 are the hybrid parameters. These four parameters are not same. They have different units. In other words, they are mixture of different units and hence referred to as hybrid parameters. They are commonly known as h-parameters.

7 5. Define the delay time and rise time in the switching characteristics of transistor. In the transistor switching characteristics the deley time is the time that elapses the application of the input pulse and current to rise to 10 percent of its maximum value.the time required for Ic to reach 90% of its maximum level from 10 % level is called the rise time. 6. Define the hie and hfe for a common emitter transistor configuration. hie =?VBE IB VCE constant hfe =?Ic IB VCE constant hie is input resistance in common emitter configuration hfe is short circuit current gain in common base configuration 7. When a transistor is used as a switch, in which region of output characteristics it is operated? 8. When a transistor is used as a switch it is operated alternately in the cut off region and saturation region of the output characteristics 9. Write the relation between Ic, ß, IB and IcBo in a BJT. i. In the active region: i. IC = ß IB + (1+ ß)ICO ii. In the cut-off region: i. IC = (1+ ß)ICO ICBO is the collector current when the emitter current is zero. ICBO is greater than ICO

8 10. Define a and ß of a transistor. a: It is defined as the ratio of the collector current resulting from carrier injection to the total emitter current a = IC IE ß: it is defined as the ratio of collector current to base current. ß = IC IB 11. In a bipolar transistor which region is wider and which region is thinner? Why? The middle region of bipolar junction transistor is called as the base of the transistor. Input signal of small amplitude is applied to the base. This region is thin and lightly doped.the magnified output signal is obtained at the collector. This region is thick and heavily doped. 12. State the advantages of optocoupler. a. It is compact and has less weight b. Much faster than isolation transformers and relays c. Noise transients, contact bounce etc, are completely eliminated d. Has wide band signal transmission capability e. Easy to interface with logic devices. f. Response time is small 13. What are the tools used for small signal analysis of BJT? Vi = hi Ii + hr Vo Io = hf Ii + ho Vo hi : input resistance Ii : input current hr : reverse voltage transfer ratio ho : output admittance Vi : input voltage Vo : output voltage

9 The above parameters are required for drawing the equivalent circuit which is necessary for the small signal analysis of BJT. 14. Give the current gain expression for a common emitter transistor configuration. Ai= IC = -hfe IB 1+hoeRL 15. What are power transistors? Power transistors are one which is used for power amplification. It means that the operating voltage and current is large. In the off-state they have to block large voltage and in the on-state they have to carry large current. 16. Which of the BJT configuration is suitable for impedance matching application? Common emitter configuration is used in impedance matching application because the ratio of output resistance to input resistance is small and may range from 10? to 100?. This makes the configuration an ideal for coupling between various transistor stages 17. For a npn transistor IE = 12 ma and ß = 140. Determine the value of IB and Ic. Ans: IE = (1+ ß) IB IB = IE / (1+ ß) = 12mA / (1+140)

10 = 85.1 ma Ic = ß IB = 140 * 85.1 ma = ma 18. Determine ß and IE for a transistor if IB = 50µA and Ic = 3.6 ma. Ans: I ß = Ic/IB = 3.6 ma/50 µa = 72 IE = IB + Ic = 50 µa ma = 3.65 ma UNIT II(16MARKS) 1. Draw and explain input and output characteristics of common emitter configuration of a transistor 2. Explain in detail on voltage gain and current gain expressions for CB configuration using transistor hybrid model 3. Draw the input and output characteristics of N-P-N transistor in common base configuration and explain it. How the different hybrid parameters are found out from the above characteristics 4. Explain how the hybrid parameters hfe, hoe, hie, hre can be determined from the CE characteristics of a transistor.

12 UNIT III - FET AND ITS APPLICATION ( TWO MARKS ) 1. Why is FET called a unipolar device? In FET current is carried by only one type of charge particles, either electrons or holes. Hence FET is called unipolar device. 2. How can a FET be used as a voltage controlled resistor? In FET, the voltage applied between gate and source controls the drain current ID. Therefore, FET is a voltage controlled device. 3. What are the advantages of FETs? In FET input resistance is high compared to BJT Construction is smaller than BJT Less sensitive to changes in applied voltage Thermal stability is more Thermal runaway does not exist in JFET Thermal noise is much lower 4. Draw the low frequency equivalent circuit of JFET 5. Define amplification factor in JFET. It is defined as the ratio of change in drain-source voltage VDS to the change in gate-source voltage VGS at constant drain current ID. It is also called mutual conductance

13 µ =?VDS?VGS ID constant 6. Give the small signal model of a FET. 7. Mention the three regions that are present in the drain source characteristics of JFET Saturation region Break down region Ohmic region 8. What is pinch-off voltage in FET? In the VI characteristics of JFET, at some value of VDS, drain current ID cannot be increased further, due to reduction in channel width. Any further increase in VDS does not increase the drain current ID. ID approaches the constant saturation value. The voltage VDS at which the current ID reaches to its constant saturation level is called " Pinch-off voltage", Vp 9. What are the parameters that control the Pinch off voltage? Electron charge, donor or acceptor concentration, permittivity of channel material and half width of channel bar. 10. List the characteristics of JFET. Drain characteristics Transfer characteristics 11. Differentiate between BJT and JFET

14 Sl.no BJT FET 1 Current controlled device Voltage controlled device 2 Bipolar device Unipolar device 3 Less input resistance Input resistance greater than BJT 4 Less thermal stability High thermal stability 5 More thermal noise Less thermal noise 12. Give the drain current equation of JFET. ID = IDSS (1 VGS )2 Vp ID = drain current IDSS = saturation drain current VGS = gate source voltage VP = pinch-off voltage 13. Why MOSFET is called IGFET? MOSFET is constructed with gete terminal insulated from the channel. So it is also called as insulated gate FET or IGFET. 14. Comparison between JFET and MOSFET

15 JFET MOSFET Gate is not insulated from channel Gate is insulated from channel by a thin layer of SiO2 There are two types N-channel and P-channel Four types - P-channel enhancement, P-channel depletion, N-channel enhancement, N-channel depletion Cannot be operated in depletion and enhancement modes Can be operated in depletion and enhancement modes There is a continuous channel There is a continuous channel only in depletion type, but not in enhancement type 15. List the JFET parameters. Transconductance (gm) Input resistance and capacitance Drain to source resistance (rd) Amplification factor (µ) UNIT III (16MARKS) 1. Explain the principle of operation of N-channel JFET 2. Give a detailed description of construction and operation of JFET 3. Explain the working principle and characteristics of Depletion N-MOSFET under depletion mode and enhancement mode in detail 4. Derive the expression for Av, Ri, and Ro for CS JFET amplifier

16 UNIT 04(2marks) 1. Define amplifier A device which accepts an input signal and produces an output signal proportional to the input is called an amplifier. 2. What is differential amplifier? An amplifier which amp[lifies the difference between two input signal is called differential amplifier 3. Define CMRR. The ability of differential amplifier to reject a common mode signal is expressed by a ratio called common mode rejection ratio denoted as CMRR. CMRR =? = Ad Ac 4. What is the significance of CMRR in differential amplifier? The significance of CMRR is that larger the value of CMRR, better the differential amplifier 5. What is voltage or node sampling? In amplifier if the output voltage is sampled by connecting the feedback network in shunt across the output, such a connection is referred to as voltage or node sampling 6. What is current or loop sampling? In amplifier if the output current is sampled by connecting the feedback network in series with the output, such a connection is referred to as current or loop sampling 7. What is positive feedback? If the input signal and the output signal are in phase, the feedback is called positive feedback 8. What is negative feedback? If the input signal and output signal are not in phase, the feedback is called negative feed back

17 9. List the advantages of negative feedback Less noise Less frequency distortion Less amplitude distortion Less phase distortion Less harmonic distortion Band width is increased 10. Define desensitivity. The fractional change in amplification with feedback divided by the fractional change without feedback is called the sensitivity of the transfer gain (1/ (1/ (1 + ßA) ). The reciprocal of the sensitivity is called the Desensitivity 11. What is the Barkhausen criterion for feedback oscillators? -Aß = 1 Aß = 1 The above to conditions are required to work a circuit as an oscillator. This is called Barkhausen criterion for oscillation. 12. State Barkhausen conditions for sinusoidal oscillators. The total phase shift around a loop, as the signal proceeds from a input through amplifier, feedback network back to input again, copeting a loop, is precisely 0 or 360, or of course an integral multiple of 2 radians. The magnitude of the product of the open loop gain of the amplifier (A) and the feedback factor ß is unity i.e. Aß = Mention the expression for frequency of oscillations for colpitts oscillator and Hartley oscillator. Hartley oscillator: 1 f = 2? C (L1 + L 2)

18 Colpitts oscillator: 1 f = 2? L Ce Ceq = C1 C 2 C1 + C2 14. What is the advantage of clap oscillator over a colpitts oscillator. The frequency is stable and accurate. Good frequency stability Stray capacitances have no effect on c3 which decides the frequency Keeping C3 variable, frequency can be varied in the desired range 15. Mention the features of crystal oscillator It is a tuned-circuit oscillator It uses piezoelectric crystal as its resonant tank circuit It is preferred when greater frequency stability is required Used in watches communication transmitters and receivers etc. 16. What are the advantages of crystal oscillator? Good frequency stability Good temperature stability Quartz crystal is used which is inexpensive and easily available in nature. 17. Draw the circuit diagram of one shot multivibrator. 18. Draw the typical wave form at base and collector coupled astable multivibrator 19. Draw the circuit diagram of a collector coupled astable multivibrator using complementry transistors. 20. Draw the circuit diagram of a emitter coupled astable multivibrator. 21. State the applications of Schmitt trigger circuit.

19 Used as an amplitude comparator Used as a squaring circuit Used as a flip-flop UNIT 04 (16marks) 1. Draw the circuit diagram of a phase shift oscillator using BJT. Derive the expression for its frequency of oscillation. Determine the minimum hfe for the transistor 2. With suitable diagram explain the operation of Colpitts oscillator 3. Explain the principle of operation of crystal oscillator and give its application 4. Briefly discuss the effect of negative feedback on band width and stability of an amplifier

20 UNIT V- AMPLIFIERS AND OSCILLATORS(2MARKS) 1. Define amplifier A device which accepts an input signal and produces an output signal proportional to the input is called an amplifier. 2. What is differential amplifier An amplifier which amp[lifies the difference between two input signal is called differential amplifier 3. Define CMRR. The ability of differential amplifier to reject a common mode signal is expressed by a ratio called common mode rejection ratio denoted as CMRR. CMRR =? = Ad Ac 4. What is the significance of CMRR in differential amplifier? The significance of CMRR is that larger the value of CMRR, better the differential amplifier 5. What is voltage or node sampling? In amplifier if the output voltage is sampled by connecting the feedback network in shunt across the output, such a connection is referred to as voltage or node sampling 6. What is current or loop sampling? In amplifier if the output current is sampled by connecting the feedback network in series with the output, such a connection is referred to as current or loop sampling 7. What is positive feedback? If the input signal and the output signal are in phase, the feedback is called positive feedback 8. What is negative feedback? If the input signal and output signal are not in phase, the feedback is called negative feed back 9. List the advantages of negative feedback Less noise

21 Less frequency distortion Less amplitude distortion Less phase distortion Less harmonic distortion Band width is increase 10. Define desensitivity. The fractional change in amplification with feedback divided by the fractional change without feedback is called the sensitivity of the transfer gain (1/ (1/ (1 + ßA) ). The reciprocal of the sensitivity is called the Desensitivity 11. What is the Barkhausen criterion for feedback oscillators? -Aß = 1 Aß = 1 The above to conditions are required to work a circuit as an oscillator. This is called Barkhausen criterion for oscillation. 12. State Barkhausen conditions for sinusoidal oscillators. The total phase shift around a loop, as the signal proceeds from a input through amplifier, feedback network back to input again, copeting a loop, is precisely 0 or 360, or of course an integral multiple of 2? radians. The magnitude of the product of the open loop gain of the amplifier (A) and the feedback factor ß is unity i.e. Aß = Mention the expression for frequency of oscillations for colpitts oscillator and Hartley oscillator. Hartley oscillator: 1 f = 2? C (L1 + L 2) Colpitts oscillator: 1

22 f = 2? L Ce Ceq = C1 C2 C1 + C2 14. What is the advantage of clap oscillator over a colpitts oscillator? The frequency is stable and accurate. Good frequency stability Stray capacitances have no effect on c3 which decides the frequency Keeping C3 variable, frequency can be varied in the desired range 15. Mention the features of crystal oscillator It is a tuned-circuit oscillator It uses piezoelectric crystal as its resonant tank circuit It is preferred when greater frequency stability is required Used in watches communication transmitters and receivers etc. 16. What are the advantages of crystal oscillator? Good frequency stability Good temperature stability Quartz crystal is used which is inexpensive and easily available in nature. 17. Draw the circuit diagram of one shot multivibrator. 18. Draw the typical wave form at base and collector coupled astable multivibrator 19. Draw the circuit diagram of a collector coupled astable multivibrator using complementry transistors. 20. Draw the circuit diagram of a emitter coupled astable multivibrator. 21. State the applications of Schmitt trigger circuit. Used as an amplitude comparator Used as a squaring circuit

23 Used as a flip-flop 22. Define intrinsic stand off ratio of UJT and draw its equivalent circuit The resistance ratio rb1/rbb is an important characteristics of UJT. It is known as the "intrinsic stand-off ratio" and is designated by "?" Hence? = rb1 = rb1 rb2 rb1 + rb What is a clamper? The circuit with which the waveform can be shifted, such that, a particular part of it (say positive or negative peak) is maintained at a specified level, is called a clamping circuit or simply a clamper 24. What is a clipper? The circuit with which waveform is shaped by removing a certain portion of the input signal voltage above or below a preset level is called clipper. They are also called limiters when they are used to limit the amplitude of the input signal. 25. List the types of clippers Based on limiting action positive clipper negative clipper biased clipper combination clipper Based on limiting action Diode clippertransistor clipper 26. What is the function of clamping circuits? The function of clamping circuit is to introduce a d.c level into an a.c level. The clamping circuits are also sometimes known as d.c restorers. 27. What is AMV? Why is it called as square wave generator? An AMV is essentially a two stage RC coupled amplifier with output of one stage supplied back to the input of another stage. An AMV generates square wave of known frequency. So it is called a "Square wave generator. UNIT V (16MARKS) 1. Explain the operation of Schmitt trigger using two transistors for a sinusoidal input 2. Explain the operation of UJT and draw the VI characteristics. 3. Explain series and shunt clipper with suitable diagram

24 4. Draw the circuit diagram of collector coupled astable multivibrator and explain its operation with relevant operation. Derive the expression for time period

FREQUENTLY ASKED QUESTIONS

FREQUENTLY ASKED QUESTIONS UNIT-1 SUBJECT : ELECTRONIC DEVICES AND CIRCUITS SUBJECT CODE : EC6202 BRANCH: EEE PART -A 1. What is meant by diffusion current in a semi conductor? (APR/MAY 2010, 2011, NOV/DEC