Bipolar Junction Transistors Questions and Answers

Q: Why does impedance reflect upwards by beta looking into the base?

For a given voltage across the emitter resistor, the base supplies only a beta-th of the current, so the apparent resistance at the base is beta times larger. The junction resistance is small, the collector current does not flow in the base, and bypassing would remove the effect entirely. Whenever a terminal handles a fraction of the current at the same voltage, the impedance seen there is scaled by the reciprocal of that fraction - a rule that covers transformers and followers alike.

Q: A transistor's data sheet gives the following. ParameterValue hFE at 2 mA100 to 300VCEO45 VfT300 MHz What does the hFE range imply for a design?

A three-to-one spread means any circuit whose gain or bias depends on beta will vary threefold between devices, so the design must be arranged so that it does not - through emitter degeneration or overall feedback. The current figure is a measurement condition rather than a limit, and neither the frequency nor the voltage rating tracks the gain spread. A data sheet range is a design constraint rather than a nuisance, and reading it that way changes what circuit gets drawn.

Q: The thin, lightly doped region between emitter and collector is the ______.

This is the base, and both its thinness and its light doping exist so that injected carriers cross it without recombining. A channel belongs to a field effect transistor, a substrate is the wafer the device is built on, and a depletion layer is the carrier-free zone at a junction. Every one of the base's properties is chosen to serve transistor action, which is why the region is so much harder to make than either of the others. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Q: The rise in collector current with collector voltage, caused by base narrowing, is the ______ effect.

This is the Early effect, and it is what gives the output characteristics their slope and the device its finite output resistance. The Miller effect is the apparent multiplication of a bridging capacitance by voltage gain, the Zener effect is a tunnelling breakdown mechanism, and the Hall effect is the transverse voltage in a magnetic field. Effects named after people are worth attaching firmly to their mechanisms, since the names carry no clue.

Q: The frequency at which the common-emitter current gain falls to unity is the ______ frequency.

This is the transition frequency, and it equals the low-frequency beta times the beta cut-off frequency, which is the quite different point where the gain has fallen by 3 dB. A corner frequency is a general term for a filter's break point and resonance belongs to tuned circuits. Two frequencies describing the same rolling-off gain, separated by a factor of beta, is exactly the kind of pair worth keeping distinct.

Q: Which stage arrangement approaches the intrinsic gain limit?

Replacing the collector resistor with a current source, whose impedance is very high, lets the device's own output resistance set the gain - which is how the input stage of an operational amplifier reaches its enormous gain. A large emitter resistor reduces the gain deliberately, a follower has none to speak of, and common base has the same intrinsic limit but no current gain. Removing the element that dominates a limit is the way to reach the limit underneath it.

Q: What is a Darlington pair?

A Darlington feeds one transistor's emitter into the next one's base, so the overall current gain is roughly the product of the two - thousands rather than hundreds. Parallel devices share current, a complementary pair forms a push-pull stage, and cross-coupling makes a bistable. Cascading to multiply a gain is the obvious move once a single device's gain is insufficient, and the penalty is always somewhere else.

Q: What is the chief penalty of a Darlington arrangement?

The input now requires about 1.4 V rather than 0.7, and the saturation voltage is worse because one device's collector cannot fall below the other's base. The gain varies more rather than less, since two spreads compound. Breakdown voltage is unaffected and saturation is possible, if poorer. A composite that multiplies the wanted property usually adds up the unwanted ones, and checking which is the point of these questions. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

81.

Why does impedance reflect upwards by beta looking into the base?

A.The base carries only a fraction of the emitter current
B.The base-emitter junction has a very high resistance
C.The collector current adds to the base current there
D.The emitter resistor is bypassed at signal frequencies

Answer: A

For a given voltage across the emitter resistor, the base supplies only a beta-th of the current, so the apparent resistance at the base is beta times larger. The junction resistance is small, the collector current does not flow in the base, and bypassing would remove the effect entirely. Whenever a terminal handles a fraction of the current at the same voltage, the impedance seen there is scaled by the reciprocal of that fraction - a rule that covers transformers and followers alike.

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82.

A transistor's data sheet gives the following.

Parameter	Value
hFE at 2 mA	100 to 300
VCEO	45 V
fT	300 MHz

What does the hFE range imply for a design?

A.The collector-emitter rating must be derated by the same factor
B.The gain must not depend on beta
C.The device cannot be used above two milliamps of collector current
D.The transition frequency will vary in the same proportion as the gain

Answer: B

A three-to-one spread means any circuit whose gain or bias depends on beta will vary threefold between devices, so the design must be arranged so that it does not - through emitter degeneration or overall feedback. The current figure is a measurement condition rather than a limit, and neither the frequency nor the voltage rating tracks the gain spread. A data sheet range is a design constraint rather than a nuisance, and reading it that way changes what circuit gets drawn.

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83.

The thin, lightly doped region between emitter and collector is the ______.

A.substrate
B.depletion layer
C.base
D.channel

Answer: C

This is the base, and both its thinness and its light doping exist so that injected carriers cross it without recombining. A channel belongs to a field effect transistor, a substrate is the wafer the device is built on, and a depletion layer is the carrier-free zone at a junction. Every one of the base's properties is chosen to serve transistor action, which is why the region is so much harder to make than either of the others. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

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84.

The rise in collector current with collector voltage, caused by base narrowing, is the ______ effect.

A.Miller
B.Zener
C.Hall
D.Early

Answer: D

This is the Early effect, and it is what gives the output characteristics their slope and the device its finite output resistance. The Miller effect is the apparent multiplication of a bridging capacitance by voltage gain, the Zener effect is a tunnelling breakdown mechanism, and the Hall effect is the transverse voltage in a magnetic field. Effects named after people are worth attaching firmly to their mechanisms, since the names carry no clue.

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85.

The frequency at which the common-emitter current gain falls to unity is the ______ frequency.

A.transition
B.corner
C.resonant
D.beta cut-off

Answer: A

This is the transition frequency, and it equals the low-frequency beta times the beta cut-off frequency, which is the quite different point where the gain has fallen by 3 dB. A corner frequency is a general term for a filter's break point and resonance belongs to tuned circuits. Two frequencies describing the same rolling-off gain, separated by a factor of beta, is exactly the kind of pair worth keeping distinct.

Discuss in Forum

86.

What limits the maximum voltage gain a single common-emitter stage can reach?

A.The base spreading resistance of the device
B.The device's own output resistance
C.The available collector supply voltage
D.The current gain of the device in use

Answer: B

Even with an infinite load resistor the gain saturates at the transconductance times the device's output resistance, which is the Early voltage divided by the thermal voltage - a few thousand. The supply voltage limits the swing rather than the gain, beta does not appear in the expression, and base resistance affects bandwidth and noise. A limit that survives making the external components ideal is an intrinsic device limit, and that is what makes it worth knowing.

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87.

Why is the intrinsic gain limit rarely reached in practice?

A.The device saturates before the gain is reached
B.The base current becomes comparable with the collector current
C.The load resistance is far below the output resistance
D.The transconductance falls as the load rises

Answer: C

A practical collector resistor is a few kilohms while the device's output resistance is hundreds of kilohms, so the load dominates and the gain is set by it. Transconductance depends only on current, saturation is a matter of the operating point, and the base current stays a beta-th of the collector current. When two resistances appear in parallel, the smaller decides the result, which is why the intrinsic limit matters only in stages loaded by current sources.

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88.

Which stage arrangement approaches the intrinsic gain limit?

A.A common emitter with a large emitter resistor
B.An emitter follower driving a light load
C.A common base with a low source impedance
D.A common emitter loaded by a current source

Answer: D

Replacing the collector resistor with a current source, whose impedance is very high, lets the device's own output resistance set the gain - which is how the input stage of an operational amplifier reaches its enormous gain. A large emitter resistor reduces the gain deliberately, a follower has none to speak of, and common base has the same intrinsic limit but no current gain. Removing the element that dominates a limit is the way to reach the limit underneath it.

Discuss in Forum

89.

What is a Darlington pair?

A.Two transistors cascaded so their gains multiply
B.Two transistors in parallel to share the current
C.An NPN and a PNP forming a complementary output
D.Two transistors cross-coupled to form a latch

Answer: A

A Darlington feeds one transistor's emitter into the next one's base, so the overall current gain is roughly the product of the two - thousands rather than hundreds. Parallel devices share current, a complementary pair forms a push-pull stage, and cross-coupling makes a bistable. Cascading to multiply a gain is the obvious move once a single device's gain is insufficient, and the penalty is always somewhere else.

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90.

What is the chief penalty of a Darlington arrangement?

A.An inability to be used in the saturation region
B.Two base-emitter drops in series
C.A current gain that varies less than a single device's
D.A much lower collector-emitter breakdown voltage

Answer: B

The input now requires about 1.4 V rather than 0.7, and the saturation voltage is worse because one device's collector cannot fall below the other's base. The gain varies more rather than less, since two spreads compound. Breakdown voltage is unaffected and saturation is possible, if poorer. A composite that multiplies the wanted property usually adds up the unwanted ones, and checking which is the point of these questions. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Discuss in Forum

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