Bipolar Junction Transistors Questions and Answers

Q: Why is collector leakage more troublesome in the common-emitter configuration?

In common emitter the leakage appears in the base circuit and is amplified by beta, so a microamp of junction leakage becomes a hundred microamps of collector current. Its direction is the same as the signal current, it does not appear across the input, and the voltage dependence is weak. Any unwanted quantity that enters at the input of an amplifier is multiplied by the gain, which is the general reason input-referred imperfections matter most. Working one numerical case through both schemes side by side fixes the distinction better than any rule of thumb.

Q: Which configuration has a current gain slightly less than unity?

Common base has a current gain of alpha, just under one, but it makes up for that with a high voltage gain and an excellent high-frequency response. Common emitter gives beta and common collector gives beta plus one, both large. The configuration is chosen for its bandwidth and its low input impedance rather than for gain, which is why it appears as the upper device in a cascode and in radio-frequency stages.

Q: In which arrangement does the output follow the input with no amplitude increase?

The common collector, or emitter follower, gives a voltage gain just below one while offering a high input impedance and a low output impedance - which is exactly what a buffer needs. Common base and common emitter both give substantial voltage gain. A stage with no voltage gain can still be indispensable, because impedance transformation is a service in its own right.

Q: Which configuration offers both current and voltage gain?

Common emitter gives beta in current and a large voltage gain as well, so its power gain is the highest of the three - which is why it is the default choice for a general amplifier stage. Common base gives voltage gain without current gain, and common collector the reverse. Ranking the three by what each multiplies is the compact way to remember which to reach for.

Q: Which configuration inverts the signal?

Only common emitter inverts: rising base current increases collector current, which increases the drop across the collector resistor and lowers the output. Common base and common collector are both non-inverting. The inversion matters when stages are cascaded and when feedback is applied, since the sign decides whether the feedback is negative or positive, and getting it wrong turns an amplifier into an oscillator.

Q: Which configuration has the highest input impedance?

The common collector's input impedance is beta times the emitter load, so it is the highest of the three by a wide margin. Common emitter is moderate, at a few kilohms, and common base is very low, at tens of ohms. That high input impedance is precisely why an emitter follower is used to drive a low-impedance load from a high-impedance source without loading it.

Q: Driving a stage directly at its emitter presents an impedance of only tens of ohms. Which is it?

Common base presents the emitter junction directly, so its input impedance is the small-signal emitter resistance, tens of ohms at a milliamp. Common emitter multiplies that by beta and common collector multiplies the load by beta. A low input impedance is a good match to a transmission line or a current source, which is one reason common base survives in high-frequency work.

Q: Which configuration is preferred as an output buffer?

The common collector's low output impedance and high input impedance make it the natural buffer, isolating a delicate source from a demanding load at the cost of no voltage gain. Common emitter has a high output impedance and common base an even higher one. Choosing a configuration means asking what impedance transformation the interface needs, not only how much gain is wanted.

Q: What is the Early effect?

The Early effect is the slight rise in collector current with collector-emitter voltage, caused by the depletion region widening and narrowing the effective base. It gives the output characteristics their finite slope and hence the transistor its finite output resistance. The other three describe temperature effects, high-current fall-off and saturation, all real but distinct. A characteristic that ought to be flat and is not usually reveals a geometry that changes with bias.

11.

What is the leakage current between collector and base with the emitter open?

A.A current equal to the collector current divided by beta
B.A current that falls as the collector voltage rises
C.A reverse saturation current that doubles with temperature
D.A forward current set by the base-emitter drop

Answer: C

With the emitter out of circuit the collector-base junction is simply a reverse-biased diode, so its leakage is a reverse saturation current, roughly doubling every ten degrees. It is not set by the base-emitter drop, is unrelated to beta, and grows slightly rather than falling with voltage. This leakage is amplified by beta in the common-emitter configuration, which is why thermal stability is such a preoccupation in transistor biasing. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

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12.

Why is collector leakage more troublesome in the common-emitter configuration?

A.It flows in the opposite direction to the signal
B.It appears directly across the input terminals
C.It rises with the collector-emitter voltage applied
D.It is multiplied by the current gain

Answer: D

In common emitter the leakage appears in the base circuit and is amplified by beta, so a microamp of junction leakage becomes a hundred microamps of collector current. Its direction is the same as the signal current, it does not appear across the input, and the voltage dependence is weak. Any unwanted quantity that enters at the input of an amplifier is multiplied by the gain, which is the general reason input-referred imperfections matter most. Working one numerical case through both schemes side by side fixes the distinction better than any rule of thumb.

Discuss in Forum

13.

Which configuration has a current gain slightly less than unity?

A.Common base
B.Common emitter
C.Common collector
D.None of the three

Answer: A

Common base has a current gain of alpha, just under one, but it makes up for that with a high voltage gain and an excellent high-frequency response. Common emitter gives beta and common collector gives beta plus one, both large. The configuration is chosen for its bandwidth and its low input impedance rather than for gain, which is why it appears as the upper device in a cascode and in radio-frequency stages.

Discuss in Forum

14.

In which arrangement does the output follow the input with no amplitude increase?

A.All three equally
B.Common collector
C.Common base
D.Common emitter

Answer: B

The common collector, or emitter follower, gives a voltage gain just below one while offering a high input impedance and a low output impedance - which is exactly what a buffer needs. Common base and common emitter both give substantial voltage gain. A stage with no voltage gain can still be indispensable, because impedance transformation is a service in its own right.

Discuss in Forum

15.

Which configuration offers both current and voltage gain?

A.Common collector
B.None of the three
C.Common emitter
D.Common base

Answer: C

Common emitter gives beta in current and a large voltage gain as well, so its power gain is the highest of the three - which is why it is the default choice for a general amplifier stage. Common base gives voltage gain without current gain, and common collector the reverse. Ranking the three by what each multiplies is the compact way to remember which to reach for.

Discuss in Forum

16.

Which configuration inverts the signal?

A.Common base
B.Common collector
C.All three invert
D.Common emitter

Answer: D

Only common emitter inverts: rising base current increases collector current, which increases the drop across the collector resistor and lowers the output. Common base and common collector are both non-inverting. The inversion matters when stages are cascaded and when feedback is applied, since the sign decides whether the feedback is negative or positive, and getting it wrong turns an amplifier into an oscillator.

Discuss in Forum

17.

Which configuration has the highest input impedance?

A.Common collector
B.Common emitter
C.Common base
D.They are all comparable

Answer: A

The common collector's input impedance is beta times the emitter load, so it is the highest of the three by a wide margin. Common emitter is moderate, at a few kilohms, and common base is very low, at tens of ohms. That high input impedance is precisely why an emitter follower is used to drive a low-impedance load from a high-impedance source without loading it.

Discuss in Forum

18.

Driving a stage directly at its emitter presents an impedance of only tens of ohms. Which is it?

A.They are all comparable
B.Common base
C.Common emitter
D.Common collector

Answer: B

Common base presents the emitter junction directly, so its input impedance is the small-signal emitter resistance, tens of ohms at a milliamp. Common emitter multiplies that by beta and common collector multiplies the load by beta. A low input impedance is a good match to a transmission line or a current source, which is one reason common base survives in high-frequency work.

Discuss in Forum

19.

Which configuration is preferred as an output buffer?

A.Common emitter
B.Any of the three equally
C.Common collector
D.Common base

Answer: C

The common collector's low output impedance and high input impedance make it the natural buffer, isolating a delicate source from a demanding load at the cost of no voltage gain. Common emitter has a high output impedance and common base an even higher one. Choosing a configuration means asking what impedance transformation the interface needs, not only how much gain is wanted.

Discuss in Forum

20.

What is the Early effect?

A.Base current falling as the temperature rises
B.Emitter injection efficiency falling at high current
C.Collector-emitter voltage collapsing at saturation
D.Collector current rising with collector voltage

Answer: D

The Early effect is the slight rise in collector current with collector-emitter voltage, caused by the depletion region widening and narrowing the effective base. It gives the output characteristics their finite slope and hence the transistor its finite output resistance. The other three describe temperature effects, high-current fall-off and saturation, all real but distinct. A characteristic that ought to be flat and is not usually reveals a geometry that changes with bias.

Discuss in Forum

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