Bipolar Junction Transistors Questions and Answers

Q: Which of the following transistor amplifier configuration is often used in impedance-matching application ?

The configuration is the emitter follower.The emitter follower (common-collector amplifier) has a high input impedance and a low output impedance, with voltage gain close to 1. It can therefore connect a high-impedance source to a low-impedance load without loading the source, which is the definition of an impedance-matching buffer.The common-base stage has a low input impedance; the common-emitter stage is used for voltage gain, with moderate impedances; the common-source FET amplifier is the analogue of common-emitter, not of the follower.

Q: What information does the measurement of dc beta provide about a bipolar junction transistor ?

The dc beta tells about the current gain characteristics of the transistor.The dc beta, \(\beta_{dc} = h_{FE} = I_{C}/I_{B}\), is the ratio of dc collector current to dc base current in the common-emitter configuration. It shows how much the transistor amplifies current and how that gain varies with collector current and temperature, which is needed in biasing and switching design.The voltage drop across the collector resistor depends on the circuit. The gain-bandwidth product (fT) is a high-frequency property found from ac measurements. The base-emitter threshold voltage, about 0.6-0.7 V for silicon, is a junction property, not a current gain.

Q: The phase difference between the input and output voltages in a common base arrangement is .........

The phase difference in a common base amplifier is 0°.In the common base configuration the input is applied to the emitter and the output taken from the collector, with the base common. When the emitter voltage rises (for an npn transistor), the emitter-base forward bias falls, collector current falls, the drop across the collector resistor falls, and the collector voltage rises. Input and output therefore move together, in phase.A phase shift of 180° belongs to the common emitter amplifier, where a rise in base voltage raises collector current and lowers the collector voltage. 90° and 270° are not the phase relations of any of the three basic configurations at mid-band frequencies.

121.

Which of the following transistor amplifier configuration is often used in impedance-matching application ?

A.The common-base amplifier
B.The emitter follower
C.The common-emitter amplifier
D.The common-source amplifier

Exam Year(s)

MH SET - 2024, SET Exam - 2024,

Answer: B

The configuration is the emitter follower.

The emitter follower (common-collector amplifier) has a high input impedance and a low output impedance, with voltage gain close to 1. It can therefore connect a high-impedance source to a low-impedance load without loading the source, which is the definition of an impedance-matching buffer.

The common-base stage has a low input impedance; the common-emitter stage is used for voltage gain, with moderate impedances; the common-source FET amplifier is the analogue of common-emitter, not of the follower.

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122.

What information does the measurement of dc beta provide about a bipolar junction transistor ?

A.Voltage drop across the collector resistor
B.Gain bandwidth product
C.Current gain characteristics
D.Base-emitter voltage threshold

Exam Year(s)

MH SET - 2025, SET Exam - 2025,

Answer: C

The dc beta tells about the current gain characteristics of the transistor.

The dc beta, \(\beta_{dc} = h_{FE} = I_{C}/I_{B}\), is the ratio of dc collector current to dc base current in the common-emitter configuration. It shows how much the transistor amplifies current and how that gain varies with collector current and temperature, which is needed in biasing and switching design.

The voltage drop across the collector resistor depends on the circuit. The gain-bandwidth product (f_T) is a high-frequency property found from ac measurements. The base-emitter threshold voltage, about 0.6-0.7 V for silicon, is a junction property, not a current gain.

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123.

The phase difference between the input and output voltages in a common base arrangement is .........

A.180°
B.90°
C.270°
D.0°

Exam Year(s)

MH SET - 2025, SET Exam - 2025,

Answer: D

The phase difference in a common base amplifier is 0°.

In the common base configuration the input is applied to the emitter and the output taken from the collector, with the base common. When the emitter voltage rises (for an npn transistor), the emitter-base forward bias falls, collector current falls, the drop across the collector resistor falls, and the collector voltage rises. Input and output therefore move together, in phase.

A phase shift of 180° belongs to the common emitter amplifier, where a rise in base voltage raises collector current and lowers the collector voltage. 90° and 270° are not the phase relations of any of the three basic configurations at mid-band frequencies.

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124.

Arrange the following configurations of BJT in the increasing order of their input impedance.

I. CE II. CB
III. CC IV. Darlington pair

A.III, II, IV, I
B.I, II, III, IV
C.IV, III, II, I
D.II, I, III, IV

Exam Year(s)

UGC NET - 2012,

Answer: D

The input impedance of the common base configuration is the lowest, the common emitter is higher, the common collector is higher still, and the Darlington pair, which is two transistors in cascade, gives the highest input impedance of all, so the increasing order is CB, CE, CC and Darlington, which is II, I, III, IV. In figures, the common base stage has an input impedance of a few tens of ohms, the common emitter about a kilo-ohm or so, and the common collector and the Darlington pair hundreds of kilo-ohms or more. The other sequences place the common collector or the Darlington pair before the common base. The answer is II, I, III, IV.

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125.

In an npn transistor, the expression for avalanche multiplication factor is given by

Where, \(V_{CB}\) is collector to base voltage and \(BV_{CBO}\) is maximum reverse biasing voltage which may be applied before breakdown between the collector and base terminals.

A.\(M = \dfrac{1}{(1 + V_{CB}/BV_{CBO})^{n}}\)
B.\(M = (1 + V_{CB}/BV_{CBO})^{n}\)
C.\(M = (1 - V_{CB}/BV_{CBO})^{n}\)
D.\(M = \dfrac{1}{[1 - (V_{CB}/BV_{CBO})^{n}]}\)

Exam Year(s)

UGC NET - 2013,

Answer: D

The avalanche multiplication factor of an npn transistor is given by Miller's empirical formula \(M = \dfrac{1}{1 - (V_{CB}/BV_{CBO})^{n}}\), the fourth option. As \(V_{CB}\) approaches the breakdown voltage \(BV_{CBO}\), the denominator tends to zero and M grows without limit, which is the onset of avalanche breakdown.

The first option has a plus sign in the denominator, so M would fall below 1 and never rise. The second and third options give M as a power of a bracket and do not become infinite at \(V_{CB} = BV_{CBO}\), so they cannot represent breakdown.

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126.

Consider the following four common type of transistors :

1. Point Contact Transistor
2. Bipolar Junction Transistor
3. MOS Field Effect Transistor
4. Junction Field Effect Transistor Correct arrangement of these transistors in the increasing order of input impedance is

A.1, 2, 4, 3
B.1, 2, 3, 4
C.2, 1, 3, 4
D.2, 1, 4, 3

Exam Year(s)

UGC NET - 2013,

Answer: D

The key gives the order 2, 1, 4, 3, the fourth option. The BJT has the lowest input impedance, since its base-emitter junction is forward biased, then the point contact transistor, then the junction field effect transistor with a reverse biased gate junction, and the MOSFET has the highest because its gate is insulated by an oxide layer.

The other options place the MOSFET or the JFET lower in the sequence, which does not fit the key.

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127.

List - I	List - II
a. BJT	i. Pinch off effect
b. FET	ii. Controlled rectification
c. SCR	iii. Negative Resistance Characteristics
d. Tunnel diode	iv. Punch through effect

Codes : a b c d

A.a-i, b-iii, c-ii, d-iv
B.a-i, b-ii, c-iii, d-iv
C.a-iv, b-i, c-ii, d-iii
D.a-i, b-iv, c-iii, d-ii

Exam Year(s)

UGC NET - 2013,

Answer: C

A BJT is subject to the punch-through effect, where the base-width is reduced to zero by the collector depletion layer (iv); a FET shows the pinch-off effect of the channel (i); an SCR is used for controlled rectification (ii); and a tunnel diode shows negative resistance (iii). So the code is a-iv, b-i, c-ii, d-iii. A BJT's punch-through is the collapse of the base region at high collector voltage, a FET's pinch-off is the narrowing of its channel at the drain end, an SCR's main use is controlled rectification in power circuits, and tunnel diode negative resistance comes from band-to-band tunnelling. The answer is a-iv, b-i, c-ii, d-iii.

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128.

List - I (Devices)	List - II (Characteristics)
a. BJT	i. Voltage controlled negative resistance
b. MOSFET	ii. High current gain
c. Tunnel diode	iii. Voltage regulation
d. Zener diode	iv. High input impedance

A.a-ii, b-iv, c-i, d-iii
B.a-i, b-iv, c-ii, d-iii
C.a-ii, b-iii, c-i, d-iv
D.a-i, b-iii, c-ii, d-iv

Exam Year(s)

UGC NET - 2014,

Answer: A

The correct matching is a-ii, b-iv, c-i, d-iii. A BJT is valued for its high current gain. A MOSFET has an insulated gate, so its input impedance is very high. A tunnel diode shows a negative resistance region controlled by voltage. A Zener diode holds a nearly constant voltage in breakdown, so it is used for voltage regulation.

The other options give the high input impedance to the BJT or the voltage regulation to the MOSFET, which do not suit those devices.

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129.

Match the following :

List - I (Biasing of BJT)	List - II (Functions)
a. E-B junction forward bias and C-B junction reverse bias	i. Very low gain amplifier
b. Both E-B and C-B junctions forward bias	ii. Saturation condition
c. E-B junction reverse bias and C-B junction forward bias	iii. High gain amplifier
d. Both E-B and C-B junctions reverse bias	iv. Cut-off condition

Codes : a b c d

A.ii iii i iv
B.iii ii i iv
C.iii ii iv i
D.ii iii iv i

Exam Year(s)

UGC NET - 2014,

Answer: B

The match is a-iii, b-ii, c-i, d-iv. With the emitter-base junction forward biased and the collector-base junction reverse biased, the transistor is in the active region and works as a high gain amplifier. With both junctions forward biased it is in saturation. With the emitter junction reverse biased and the collector junction forward biased (the inverse active mode) it gives very low gain. With both junctions reverse biased it is cut off.

The other options swap the saturation and cut-off conditions or the high-gain and low-gain modes.

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