(a) BJT-OPERATING MODES & CONFIGURATIONS

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2 (a) BJT-OPERATING MODES & CONFIGURATIONS 1. The leakage current I CBO flows in (a) The emitter, base and collector leads (b) The emitter and base leads. (c) The emitter and collector leads. (d) The base and collector leads. 2. Early effect in BJT refers to (a) avalanche breakdown (b) thermal breakdown (c) base narrowing (d) Zener breakdown 3. The emitter of the transistor is generally doped the heaviest because it (a) has to dissipate maximum power (c) is the first region of transistor (b) has to supply the charge carriers (d) must possess low resistance 4. In a properly Biased NPN transistor most of the electrons from the emitter (a) recombine with holes in the base (b) recombine in the emitter its self (c) pass through the base to the collector (d) are stopped by the junction barrier 5. The following relationship between α and β are correct except (a) (c) (b) (d) ( ) 6. The value of total collector current in a CB circuit is (a) (c) (b) (d)

3 7. In a transistor amplifier, the reverse saturation current I CO (a) double for every 10 0 rise in temperature (b) doubles for every 1 0 rise in temperature (c) increase linearly with the temperature (d) doubles for every 5 0 rise in temperature 8. The collector characteristics of a common- emitter connected transistor may be used to find its (a) input resistance (b) base current (c) output resistance (d) voltage gain 9. Which of the following approximation is often used in electronic circuit (a) (b) (c) (d) 10. Which of the following transistor configuration circuit is much less temperature dependent (a) common base (b) common emitter (c) common collector (d) none of the above 11. The CE amplifier circuit are preferred over CB amplifier circuit because they have (a) lower amplification factor (b) larger amplification factor (c) high input resistance and low output resistance (d) none of these 12. A transistor when connected in CE mode has (a) a low input resistance and a low output resistance (b) a high input resistance and high output resistance (c) a high input resistance and low output resistance (d) a medium input resistance and high output resistance 13. A transistor connected in common base configuration has

4 (a) a high input resistance and low output resistance (b) a low input resistance and high output resistance (c) a low input resistance and low output resistance (d) a high input resistance and a high output resistance Answers 1. (d) 2. (c) 3. (b) 4. (c) 5. (b) 6. (b) 7. (a) 8. (c) 9. (a) 10. (c) 11. (b) 12. (d) 13. (b)

5 (b) BJT CHARACTERISTICS 1. In CE configuration the output V- I characteristics are drawn by taking (a) V CE vs. I C for constant value of I E (b) V CE vs. I C for constant value of I B (c) V CE vs. I C for constant value of V CB (d) None of these 2. In CE configuration the input V-I characteristics are drawn by taking (a) V CE vs. I C for constant value of I E (b) V BE vs. I E for constant value of V CE (c) V BE vs. I B for constant value of I C (d) V BE vs. I B for constant value of V CB 3. The transistor is said to be in quiescent state when (a) it is unbiased (b) no current flows through it (c) no signal is applied to the input (d) emitter junction is just biased equal to collector junction 4. In CB configuration, the output V- I characteristics of the transistor are drawn by taking (a) V CB vs. I C for constant I E (b) V CB vs. I B for constant I E (c) V CB vs. I C for constant I E (d) V CB vs. I B for constant I E 5. When the collector junction in a transistor is biased in reverse direction and the emitter junction in the forward direction, the transistor is said to be is the (a) active region (c) saturation (b) cutoff region (d) none of them 6. To avoid thermal runaway in the design of analog circuits, the operating point of the BJT should be such that it satisfies the condition (a) (b) (c) (d)

6 7. Thermal runaway will take place if the quiescent point is such that (a) (c) (b) (d) (UPSC Engg. Service 1999) 8. The power dissipated by a transistor approximately equals the collector current times (a) base emitter voltage (c) base supply voltage (b) collector emitter voltage (d) 0.7V 9. Leakage current in CE configuration is (a) very high (b) very small (c) normal (d) not present 10. The dc current gain in common collector configuration is given by (a) α (c) β + 1 (b) β (d) α + 1 Answers 1. (b) 2. (d) 3. (c) 4. (a) 5. (a) 6. (c) 7. (d) 8. (b) 9. (a) 10. (c)

7 (c) BJT BIASING AND STABILIZATION 1. The dc load line of transistor circuit (a) is a graph between I C and V CE (b) is a graph between I C and I B (c) does not contain the Q Point (d) is a curved line 2. The ac load line of a transistor circuit is steeper than its dc line because (a) ac signal sees less load resistance (c) input signal varies in magnitude (b) I C is higher (d) none of the above 3. The maximum peak to peak output voltage swing is obtained when Q point of a circuit is located (a) near saturation point (c) at the center of the load line (b) near cutoff point (d) atleast on the load line 4. The positive part of the output signal in a transistor circuit starts clipping, if Q point of the circuit moves (a) toward the saturation point (c) toward the center of the load line (b) toward the cutoff point (d) none of the above 5. The negative part of the output signal in a transistor circuit starts clipping if the Q point of the circuit moves (a) toward the saturation point (c) toward the center of the load line (b) toward the cutoff point (d) none of the above 6. The Q-point in a voltage amplifier is selected in the middle of the active region because (a) it gives better stability (b) the circuit needs a small (c) the biasing circuit then need less number of resistors (d) it gives distortions less output 7. The biasing circuit that gives best stability to Q point is (a) base resistance biasing (b) feedback resistor biasing

8 (c) potential divider biasing (d) emitter resistor biasing 8. The potential divider biasing is used in amplifiers to (a) limit the input ac signal going to the base (b) reduce dc base current (c) reduce the cost of the circuit by limiting the number of resistors (d) make the operating point almost independent of β 9. The ideal value of stability factor of a biasing circuit is (a) 1 (c) 10 (b) 5 (d) The universal bias stabilization circuit is the most popular because (a) I C does not depend on transistor characteristic (b) its β sensitivity is low (c) voltage divider is heavily loaded by transistor base (d) I C equals to I E 11. Improper biasing of the transistor circuit leads to (a) excessive heat production at collector terminal (b) distortion in output signal (c) faulty location of load line (d) heavy loading of emitter terminal 12. The voltage divider biasing circuit is used in amplifiers quite often because it (a) limits the ac signal going to base (b) makes the operating point almost independent of β (c) reduces the dc base current (d) reduces the cost of the circuit 13. For a transistor amplifier with self- biasing network, the following components are used: R 1 = 4KΩ, R 2 = 4KΩ and R E = 1KΩ, the approximate value of stability factor will be (a) 4 (c) 2 (b) 3 (d) 1.5

9 14. Which of the following components are used for bias compensation in transistor circuit (a) resistors (c) thermistors (b) rectifier diodes (d) both (b) and (c) above Answers 1. (a) 2. (a) 3. (c) 4. (b) 5. (a) 6. (d) 7. (c) 8. (d) 9. (a) 10. (b) 11. (b) 12. (b) 13. (b) 14. (d)

10 BJT Circuits (MCQs of Moderate Complexity) 1. The current ib through base of a silicon npn transistor is cos (1000πt) ma. At 300K, the rπ in the small signal model of the transistor is i b B C r π r 0 E (a) 250Ω (b) 27.5Ω (c) 25Ω (d) 22.5Ω Answer (c) The current ib through the base of a silicon npn transistor is cos (10000 πt) ma. At 300 K, the rπ in the small signal model of the transistor is given by rπ= β. re = β V T I E β V T βi b = V T i b V T = 25mv, i b = 1ma r π = 25 Ω 2. The amplifier circuit shown below uses a silicon transistor. The capacitors CC and CE can be assumed to be short at signal frequency and the effect of output resistance r0can be ignored. If CE is disconnected from the circuit, which one of the following statements is TRUE?

11 V CC = 9V R B = 800 K R C = 2.7 K C C V S C C V i AC β =100 R E = 0.3 K C R E R 0 i (a) The input resistance Ri increases and the magnitude of voltage gain AV decreases. (b) The input resistance Ri decreases and the magnitude of voltage gain AV increases. (c) Both input resistance Riand the magnitude of voltage gain AV decrease. (d) Both input resistance Riand the magnitude of voltage gain AV increase. Answer (a) If CE is disconnected from the circuit, this is negative feedback. The input impedance increases and voltage gain decreases. 3. In an ideal differential amplifier shown in the figure, a large value of (RE). V CC R C R C V 1 R E V 2 V EE (a) Increases both the differential and common-mode gains. (b) Increases the common-mode gain only (c) Decreases the differential-mode gain only (d) Decreases the common-mode gain only

12 [GATE 2005: 2 Marks] Answer (d) Only common mode gain depends on RE and differential mode gain is independent of RE 4. The cascode amplifier is a multistage configuration of (a) CC-CB (c) CB-CC (b) CE-CB (d) CE-CC [GATE 2005: 1 Mark] Answer (b) Cascode amplifier provides a high input impedance with low voltage gain to ensure minimum input miller capacitance, thus suitable for high frequency operation. 5. Assuming VCEsat = 0.2 V and β = 50, the minimum base current (IB) required to drive the transistor in the figure to saturation is 3V I C 1K I B (a) 56 μa (b) 140 μa Answer (a) V CE sat = 0. 2V & β = 50 V CE = V CC I C 1K 0. 2 = 3 I C 1K (c) 60 μa (d) 3 μa [GATE 2004: 1 Marks]

13 I C = 2. 8 ma, I B = 2.8 ma 50 = 56 μa 6. Generally, the gain of a transistor amplifier falls at high frequency due to the (a) Internal capacitance of the device (b) Coupling capacitor at the input (c) Skin effect (d) Coupling capacitor at the output [GATE 2003: 1 Mark] Answer (a) The gain of the transistor amplifier falls at high frequency due to internal capacitance of the device. 7. The current gain of a BJT is (a) gmr0 (b) gm / ro (c) gmrπ (d) gm / rπ [GATE 2002: 1 Mark] The current gain of a BJT is β or hfe I B B C β.i B r π r 0 E g m = I C V i = β I B I B r π or g m = β r π so β = g m r π

14 8. The current gain of a bipolar transistor drops at high frequencies because of (a) Transistor capacitances (c) Parasitic inductive elements (b) High current effects in the base (d) The Early effect [GATE 2000: 1 Mark] Answer (a) The current gain of a bipolar transistor drops at high frequencies because of transistor internal capacitances. 9. In the differential amplifier of the figure, if the source resistance of the current source IEE is infinite, then common-mode gain is V CC R R V in1 V in2 I EE V EE (a) Zero (b) Infinite (c) Indeterminate (d) (Vin1 + Vin2) + 2VT [GATE 2000: 1 Mark] Answer (a) The Common mode gain, V C = A C V i (V i1 = V i2 = V i ). If the source resistance of current source (Rs) is infinite then due to symmetry common mode gain VC is zero. 10. In the cascode amplifier shown in the figure, if the common-emitter stage (Q1) has a trans conductance gm1 and the common base stage (Q2) has a trans conductance gm2 then the overall trans conductance g(=i0 / Vi) of the cascode amplifier is

15 Q 2 i 0 V 0 i c1 V i Q 1 R L (a) gm1 (b) gm2 Answer (a) (c) gm1 / 2 (d) gm2 / 2 [GATE 1999: 1 Mark] Q1 has transconductanceg m1 Q2 has transconductanceg m2 Overall transconductanceg = i 0 V i i 0 = i E2 = i C1 so g = g m1 11. the unit of q / KT are (a) V (b) V -1 (c) J (d) J / K [GATE 1998: 1 Mark] Answer (b) Thermal voltage = V T = KT q 12. A multistage Amplifier has a low-pass Response with three real poles at s = -ω1, ω2 and ω 3 The approximate overall bandwidth B of the Amplifier will be given by (a) B = ω 1 + ω 2 + ω 3

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BJT Circuits (MCQs of Moderate Complexity)

BJT Circuits (MCQs of Moderate Complexity) 1. The current ib through base of a silicon npn transistor is 1+0.1 cos (1000πt) ma. At 300K, the rπ in the small signal model of the transistor is i b B C r