ELECTRONICS QUESTION

Q.1 Define the following and give their units of measurement:-(i) Resistance. (ii) Electric Potential.(iii) Electric current.

Ans: (i) Resistance: - The opposition offered by a substance to the flow of electric current. The unit of resistance is ohm and given by the symbol Ω.

(ii) Electric Potential: -The capacity of a charged body to do work. The unit of electric potential is volt (V).

(iii) Electric Current: - The flow of free electrons constitutes electric current. The unit of electric current is called ampere (A).

Q.2 For a p n junction diode, draw a typical V-I characteristic. What is meant by a. forward resistance

b. static resistancec. dynamic resistance of a diode. (7

Ans:

VI Characteristics of Diode

Forward Resistance: The resistance offered by a diode in the circuit, when forward biased,is known as the forward-resistance. Thus resistance is not the same for dc as well as changing-current.DC or Static Resistance: R is the resistance offered by a diode to the direct-current. It is the simply the ration of the dc-voltage across the diode to the direct-current flowing through it. At any point P on the V-I characteristic of the diode, the voltage across the diode is OA and corresponding current is OB. So dc or static resistance,

R = V I = OA

OB

Thus at any point on the V-I characteristic of the diode, the dc or static resistance R is equal to the reciprocal of the slope of the line joining the operating point to the origin.

AC or Dynamic Resistance: r is a resistance offered by a diode to the changing forward- current. It may also defined as the reciprocal of the slope of the forward characteristic of the diode.

Q.3 Explain the Zener phenomenon. How does it differ from Avalanche breakdown?

Ans Under a very high-reverse voltage, the depletion region expands and the potential barrie increases leading to a very high electric field across the junction. The electric-field will break some of the covalent-bonds of the semiconductor atoms leading to a large number of free minority carriers, which suddenly increase the reverse current. This is also called the Zener- Effect.

Zener-breakdown or Avalanche breakdown may occur independently or both of these mayoccur simultaneously. Diode junctions that breakdown below 5v are caused by ZenerEffect. Junctions that experience breakdown above 5v are caused by avalanche-effect.The Zener-breakdown occurs in heavily doped junctions which produce narrow depletion layers. The avalanche breakdown occurs in lightly doped junctions, which produce wide depletion layers.

Q.4 What is a PN junction? Draw its circuit symbol. What is the convention followed in writing its symbol? Illustrate its characteristic and make it self explanatory.

Ans: The PN junction is produced by placing a layer of P type semiconductor next to the layer of N type semiconductor. The contact surface is called PN junction.

(a) Circuit Symbol (b) Graphical Symbol

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The graph plotted between potential difference across the PN junction and the circuit current is known as volt–ampere characteristics.Forward Characteristics: When the external voltage is zero, i.e., when the circuit is open, the potential barrier at the junction does not allow the flow of current and, therefore, the circuit current is zero.With forward bias to PN junction, very little current, called the forward current flows until the forward voltage exceeds the junction barrier potential. As the forward voltage increased to the knee of characteristics, the potential barrier is completely eliminated, forward current increases linearly with the increase in forward voltage.Reverse characteristics: When the reverse bias is applied, the potential barriers at junction is increased. Therefore, the junction resistance becomes very high and there is no possibility of a majority carriers flowing across a reverse-biased junction. But still minority carriers generated on each side can cross the junction. This results in a very small current which is known as reverse current.

Q.5 What are the three modes in which a transistor can operate? Explain the meaning of each mode of operation.

Ans:

The three modes of operations of a transistor are(1) Common – Base configuration – In common base configuration, input is connected between emitter base and output is taken across collector and base.

(2) Common – Emitter configuration – In common emitter configuration, input is connected between emitter base and output is taken across collector and emitter. This emitter is common to both input and output circuits.

(3) Common – Collector configuration – In common collector configuration, input is applied between base and collector while the output is taken across collector

and emitter. Thus the collector forms the terminal is common to both input and output circuits.

Q.6 Draw the circuits of an NPN and a PNP transistor in CE configuration. Define the following:

(1) CE dc current gain.(2) CE ac current gain.

Ans:

CE-P-N-P-Transistor

CE-N-P-N-Transistor

The output characteristics used to determine the dc- current gain β and ac current gain βo is as follows.DC current gain β = IC / IBand AC current gain, βo = ∆ IC / ∆ IBVCE = constant.

Q.7 What is Schottky diode? Why is it also called Hot-Carrier diode? How does it differ in construction from a normal P-N junction diode?

Ans :

Schemaic Symbols of Schottky Diode

(a) Equivalent Circuit (b) Approximate Equivalent Circuit

The reverse recovery time is so short in small signal diodes that its effect cannot be noticed at frequencies below 10MHZ or so. It becomes very important well above 10 MHZ. The solution is a special purpose device called Schottky diode. Such a diode has no depletion layer eliminating the stored charges at the junction. Due to the lack of charge storage the Schottky diode can switch off faster than an ordinary diode.

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Its construction is very different from the normal PN junction in which metal semiconductor junction is developed. On one side of the junction a metal is used and the other side of the junction N-type doped silicon is used.

In both materials, the electrons are the majority carriers. In the metal, the level of minority carriers is insignificant. When diode is unbiased, electrons on N-side have low energy levels than the electrons in the metal and so the electrons cannot cross the ju Schottky barrier. But when the diode is forward biased the electrons on the N-side gainenough energy to cross the junction and enter the metal. Since these electrons plung into themetal with very large energy they are usually called the hot carriers and the diode is calledthe hot carrier diode.

Q.8 What is an SCR? Explain the construction, working and V-I characteristics of an SCR for different gate currents and indicate there-upon holding current, latching current and break over voltage.

Ans :The SCR (Silicon controlled rectifier) is a controlled rectifier constructed of a siliconsemiconductor material with a third terminal for control purposes. Silicon was chosen because of its high temperature and power capabilities. The third

terminal gate, determines when the rectifier switches from the open circuit to short circuit state.

Schematic Diagram & Symbolic representation of SCR

Construction – SCR is essentially an ordinary rectifier (PN) and a junction transistor (NPN) combined in one unit to form PNPN device. It consists of a four layer pellet of P and N type silicon semiconductor materials. The junctions are diffused or alloyed. The material which may be used for P- diffusion is aluminium and for N diffusion is phosphorous. The contact with anode can be made with an aluminium foil through cathode and gate by metal sheet.Working - SCR is a switch .Ideally it remains off or appears to have an infinite impedance until both the anode and gate terminals have suitable positive voltages with respect to the cathode terminal.

VI Characteristics of SCR

When anode is made positive w.r.t. the cathode, junction J1 and J3 are forward biased and junction J2 is reverse biased and only the leakage current will flow through the device. The SCR is then said to be in the forward blocking state or in the forward mode or OFF state. But when cathode is made positive w.r.t. the anode, junctions J1 and J3 are reverse biased a small reverse leakage current will flow through the SCR and the SCR is said to be in the reverse blocking or reverse mode.

When the SCR is in forward mode the SCR conducts when the forward voltage exceeds certain value called the forward breakover voltage VFBO.If a positive gate current is supplied, the SCR can become conducting at a voltage less thanforward break over voltage. The larger the gate current, lower the break over voltage. With sufficiently large gate current the SCR behaves identical to PN rectifier.Once the SCR has been switched ON, it has no control on the amount of current flowing through it. The current through the SCR is entirely controlled by the external impedance connected in the circuit and the supplied voltage. There is a very small about 1v, potential drop across the SCR. The forward current through the SCR can be reduced by reducing the applied voltage or by increasing circuit impedance. There is, however, a minimum forward current that must be maintained to keep the SCR in conducting state. This is called the holding current rating of SCR. If the current through the SCR is reduced below the level of holding current, the device returns to OFF state or blocking state.

OBJECTIVE TYPE QUESTIONS

Choose correct or the best alternative in the following:

Q.1 The breakdown mechanism in a lightly doped p-n junction under reverse biased condition is called(A) avalanche breakdown.(B) zener breakdown.(C) breakdown by tunnelling.(D) high voltage breakdown.Ans: A

Q.2 In a CE – connected transistor amplifier with voltage – gain Av, the capacitance Cbc is amplified bya factor(A)Av (B) 1Av

(C) 1Av (D) 2AvAns: B

Q.3 For a large values of |VDS|, a FET – behaves as(A) Voltage controlled resistor.(B) Current controlled current source.(C) Voltage controlled current source.(D) Current controlled resistor.Ans: C

Q.4 Removing bypass capacitor across the emitter-leg resistor in a CE amplifier causes(A) increase in current gain. (B) decrease in current gain.(C) increase in voltage gain. (D)decrease in voltage gain.Ans: D

Q.6 When a step-input is given to an op-amp integrator, the output will be(A) a ramp.(B) a sinusoidal wave.(C) a rectangular wave.(D) a triangular wave with dc bias.Ans: A

Q.7 Hysteresis is desirable in Schmitt-trigger, because(A) energy is to be stored/discharged in parasitic capacitances.(B) effects of temperature would be compensated.(C) devices in the circuit should be allowed time for saturation and desaturation.(D) it would prevent noise from causing false triggering.Ans: C

Q.8 In a full-wave rectifier without filter, the ripple factor is(A) 0.482 (B) 1.21(C) 1.79 (D) 2.05Ans: A

Q.9 A minterm of the Boolean-function, f(x, y, x) is(A) xy z (B) x y z(C) x z (D) (y +z) xAns: B

Q.10 The minimum number of flip-flops required to construct a mod-75 counter is(A) 5 (B) 6(C) 7 (D) 8Ans: C

Q.11 Space charge region around a p-n junction(A) does not contain mobile carriers(B) contains both free electrons and holes(C) contains one type of mobile carriers depending on the level of doping of the p or nregions(D) contains electrons only as free carriersAns: A

Q.12 The important characteristic of emitter-follower is(A) high input impedance and high output impedance(B) high input impedance and low output impedance(C) low input impedance and low output impedance(D) low input impedance and high output impedance

Ans: B

Q.13 In a JFET, at pinch-off voltage applied on the gate(A) the drain current becomes almost zero(B) the drain current begins to decrease(C) the drain current is almost at saturation value.(D) the drain-to-source voltage is close to zero volts.Ans: C

Q.14 When an amplifier is provided with current series feedback, its(A) input impedance increases and output impedance decreases(B) input and output impedances both decrease(C) input impedance decreases and output impedance increases(D) input and output impedances both increaseAns: D

Q.15 The frequency of oscillation of a tunnel-collector oscillator having L= 30μH and C = 300pf isnearby(A) 267 kHz (B) 1677 kHz(C) 1.68 kHz (D) 2.67 MHzAns: B KHz

Lcfo 1677 .422 30 300 1012112

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Q.16 The open-loop gain of an op-amp available in the market may be around.(A) 10–1 (B) 10(C) 105 (D) 102

Ans: CQ.17 The control terminal (pin5) of 555 timer IC is normally connected to ground through a capacitor(~ 0.01μF). This is to(A) protect the IC from inadvertent application of high voltage(B) prevent false triggering by noise coupled onto the pin(C) convert the trigger input to sharp pulse by differentiation(D) suppress any negative triggering pulseAns: BQ.18 The value of ripple factor of a half-wave rectifier without filter is approximately(A) 1.2 (B) 0.2(C) 2.2 (D) 2.0Ans: A

Q.19 The three variable Boolean expression xy + xyz + x y + x y zAE05 BASIC ELECTRONICS4(A) y xz (B) x yz(C) y xz (D) y xzAns: Cy(x x) xz(y y) y xzQ.20 The fan-out of a MOS-logic gate is higher than that of TTL gates because of its(A) low input impedance (B) high output impedance(C) low output impedance (D) high input impedanceAns: DQ.21 In an intrinsic semiconductor, the Fermi-level is(A) closer to the valence band(B) midway between conduction and valence band(C) closer to the conduction band(D) within the valence bandAns: CQ.22 The reverse – saturation current of a silicon diode(A) doubles for every 10C increase in temperature(B) does not change with temperature(C) halves for every 1C decrease in temperature(D) increases by 1.5 times for every 2C increment in temperatureAns: AQ.23 The common collector amplifier is also known as(A) collector follower (B) Base follower(C) Emitter follower (D) Source followerAns: CQ.24 In class–A amplifier, the output current flows for(A) a part of the cycle or the input signal.(B) the full cycle of the input signal.(C) half the cycle of the input signal.(D) 3/4th of the cycle of the input signal.Ans: BQ.25 In an amplifier with negative feedback(A) only the gain of the amplifier is affected(B) only the gain and bandwidth of the amplifier are affected(C) only the input and output impedances are affectedAE05 BASIC ELECTRONICS5(D) All of the four parameters mentioned above would be affectedAns: DQ.26 Wien bridge oscillator can typically generate frequencies in the range of(A) 1KHz – 1MHz(B) 1 MHz – 10MHz(C) 10MHz – 100MHz(D) 100MHz – 150MHzAns: AQ.27 A differential amplifier, amplifies(A) and mathematically differentiates the average of the voltages on the two input lines(B) and differentiates the input waveform on one line when the other line is grounded(C) the difference of voltages between the two input lines(D) and differentiates the sum of the two input waveformsAns: C

Q.28 The transformer utilization factor of a half-wave rectifier is approximately(A) 0.6 (B) 0.3(C) 0.9 (D) 1.1Ans: B0.286 0.3Q.29 The dual of the Boolean expression: x + y + z is(A) x .y z (B) x yz(C) x . y . z (D) x.y.zAns: Cx y z x.y.zQ.30 It is required to construct a counter to count upto 100(decimal). The minimum number of flipflopsrequired to construct the counter is(A) 8 (B) 7(C) 6 (D) 5Ans: AQ.31 The power conversion efficiency of an output stage is defined as_______.(A) (Load power + supply power) / supply power(B) (Load power + supply power) / (load power-supply power)(C) Load power / supply power(D) Supply power / load powerAE05 BASIC ELECTRONICS6Ans. (C)Power gain is defined as the ratio of output signal power to that of input signal power.Q.32 A highly stable resonance characteristic is the property of a ____