Bipolar Junction Transistors Questions and Answers

Q: What is punch-through in a bipolar transistor?

Punch-through occurs when the collector-base depletion region widens until it meets the emitter, so the base disappears and current flows uncontrolled. Emitter reverse breakdown, current crowding and base current reversal are separate phenomena. It is the extreme case of the same base-narrowing that produces the Early effect, which is a good reminder that a gentle effect and a catastrophic one are often the same mechanism at different magnitudes.

Q: What is secondary breakdown?

Secondary breakdown is a thermal instability in which current concentrates into a hot spot that then conducts still better, destroying the device - a runaway rather than an avalanche. It is why the safe operating area is bounded by more than a simple power hyperbola. The other options describe an avalanche region, a junction rating and normal leakage. A failure mechanism driven by positive feedback needs its own boundary on a rating chart, because no average calculation predicts it.

Q: What does a safe operating area diagram bound?

The diagram bounds which combinations of collector current and collector-emitter voltage the device may occupy, limited by maximum current, maximum voltage, maximum power and secondary breakdown. Beta temperature range, gain-bandwidth and stability are all specified elsewhere. Four separate limits drawn on one pair of axes is far more useful than four numbers, because a design point can then be checked against all of them at a glance. Most errors on this topic come from carrying a result across from a neighbouring scheme where it does not hold.

Q: What do the common-emitter input characteristics plot?

The input characteristics show base current against base-emitter voltage, and they look like an ordinary diode curve because that is what the input is. Collector current against collector voltage is the output family, and the direct current transfer is a third plot. Recognising that a transistor's input is simply a forward-biased junction explains why base voltage cannot be used to set the operating point with any precision. Most errors on this topic come from carrying a result across from a neighbouring scheme where it does not hold.

Q: Why are the common-emitter output characteristics not quite horizontal?

The Early effect narrows the effective base as collector voltage rises, so beta creeps up and the characteristic slopes gently. Base current is held constant along each curve by definition, leakage is a temperature matter, and the fall in beta at high current bends the curves at the top rather than tilting them. That small slope is the whole of a transistor's output resistance, and it sets the maximum gain a single stage can reach. The shared properties explain why these schemes form a family, and the differences explain why more than one exists.

Q: A transistor operates at \( I_{C} = 1 \ \mathrm{mA} \). Its transconductance is about

Dividing the collector current by the thermal voltage:\[ \begin{aligned} g_{m} &= \frac{1 \ \mathrm{mA}}{26 \ \mathrm{mV}} \\ &\approx 40 \ \mathrm{mS} \end{aligned} \]giving about 40 millisiemens, or equivalently a resistance of 26 ohms seen looking into the emitter. Twenty-six millisiemens confuses the thermal voltage's numerical value with the answer. The rule of forty millisiemens per milliamp is worth memorising, since it makes most small-signal gain estimates a matter of mental arithmetic.

Q: An emitter resistor of \( 100 \ \Omega \) is now added without bypassing. The gain becomes about

The emitter resistance adds to the intrinsic 26 ohms:\[ A_{v} \approx -\frac{5000}{100+26} \approx -40 \]so about -40, five times less than before. What has been bought is predictability: the gain now depends on two resistors rather than on the operating current, so it barely moves with temperature or device spread. Trading gain for stability through local feedback is the single most common move in analogue design.

21.

Extrapolating the output characteristics backwards, the intercept on the voltage axis is

A.Where the extrapolated output characteristics meet
B.The voltage at which the device enters saturation
C.The collector-base breakdown voltage of the device
D.The base-emitter drop at the rated collector current

Answer: A

Extrapolating the output characteristics backwards, they converge on the negative voltage axis, and that intercept is the Early voltage - a large value meaning flat characteristics and high output resistance. Saturation voltage, breakdown voltage and the base drop are all separate parameters. A figure defined by an extrapolated intercept is a convenient way to summarise a slope, and it appears the same way in several device models. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

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22.

What is punch-through in a bipolar transistor?

A.The base current reversing direction at high voltage
B.The depletion region reaching across the whole base
C.The emitter junction breaking down under reverse bias
D.The collector current concentrating in a small area

Answer: B

Punch-through occurs when the collector-base depletion region widens until it meets the emitter, so the base disappears and current flows uncontrolled. Emitter reverse breakdown, current crowding and base current reversal are separate phenomena. It is the extreme case of the same base-narrowing that produces the Early effect, which is a good reminder that a gentle effect and a catastrophic one are often the same mechanism at different magnitudes.

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23.

What is secondary breakdown?

A.The base-emitter junction breaking down at low voltage
B.Leakage current doubling for every ten degrees of rise
C.Localised current concentration destroying the device
D.A second avalanche region beyond the first breakdown

Answer: C

Secondary breakdown is a thermal instability in which current concentrates into a hot spot that then conducts still better, destroying the device - a runaway rather than an avalanche. It is why the safe operating area is bounded by more than a simple power hyperbola. The other options describe an avalanche region, a junction rating and normal leakage. A failure mechanism driven by positive feedback needs its own boundary on a rating chart, because no average calculation predicts it.

Discuss in Forum

24.

What does a safe operating area diagram bound?

A.The temperature range over which beta is specified
B.The frequency range in which gain exceeds unity
C.The load impedances that keep the stage stable
D.The combinations of current and voltage permitted

Answer: D

The diagram bounds which combinations of collector current and collector-emitter voltage the device may occupy, limited by maximum current, maximum voltage, maximum power and secondary breakdown. Beta temperature range, gain-bandwidth and stability are all specified elsewhere. Four separate limits drawn on one pair of axes is far more useful than four numbers, because a design point can then be checked against all of them at a glance. Most errors on this topic come from carrying a result across from a neighbouring scheme where it does not hold.

Discuss in Forum

25.

What do the common-emitter input characteristics plot?

A.Base current against base-emitter voltage
B.Collector current against collector-emitter voltage
C.Collector current against base current directly
D.Emitter current against base-emitter voltage

Answer: A

The input characteristics show base current against base-emitter voltage, and they look like an ordinary diode curve because that is what the input is. Collector current against collector voltage is the output family, and the direct current transfer is a third plot. Recognising that a transistor's input is simply a forward-biased junction explains why base voltage cannot be used to set the operating point with any precision. Most errors on this topic come from carrying a result across from a neighbouring scheme where it does not hold.

Discuss in Forum

26.

Why are the common-emitter output characteristics not quite horizontal?

A.Beta falls at high collector current densities
B.The Early effect widens the depletion region
C.The base current rises with collector voltage
D.Leakage doubles for every ten degrees of rise

Answer: B

The Early effect narrows the effective base as collector voltage rises, so beta creeps up and the characteristic slopes gently. Base current is held constant along each curve by definition, leakage is a temperature matter, and the fall in beta at high current bends the curves at the top rather than tilting them. That small slope is the whole of a transistor's output resistance, and it sets the maximum gain a single stage can reach. The shared properties explain why these schemes form a family, and the differences explain why more than one exists.

Discuss in Forum

27.

What does the transconductance of a bipolar transistor depend on?

A.Its base-emitter voltage and its base resistance
B.Its collector-emitter voltage and the Early voltage
C.Its collector current alone, at a given temperature
D.Its beta and its collector resistor together

Answer: C

Transconductance is the collector current divided by the thermal voltage, so it depends only on the operating current, at about 40 millisiemens per milliamp. Beta, resistors and voltages do not enter into it. That independence from beta is enormously useful, because it means a well-biased stage's gain can be predicted even though beta itself may vary three to one between devices.

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28.

A transistor operates at \( I_{C} = 1 \ \mathrm{mA} \). Its transconductance is about

A.26 mS
B.1 mS
C.400 mS
D.40 mS

Answer: D

Dividing the collector current by the thermal voltage:

\[ \begin{aligned} g_{m} &= \frac{1 \ \mathrm{mA}}{26 \ \mathrm{mV}} \\ &\approx 40 \ \mathrm{mS} \end{aligned} \]

giving about 40 millisiemens, or equivalently a resistance of 26 ohms seen looking into the emitter. Twenty-six millisiemens confuses the thermal voltage's numerical value with the answer. The rule of forty millisiemens per milliamp is worth memorising, since it makes most small-signal gain estimates a matter of mental arithmetic.

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29.

That transistor drives a \( 5 \ \mathrm{k\Omega} \) collector load. Its voltage gain is about

A.-200
B.-40
C.-5000
D.-26

Answer: A

Gain is transconductance times load resistance, with the sign for inversion:

\[ A_{v} = -g_{m}R_{C} = -0.04 \times 5000 = -200 \]

so about -200. The remaining options drop the resistance or the transconductance. Since the gain works out as the collector resistor divided by the emitter's own 26 ohms, it can also be read straight off as a ratio of resistances, which is often the quicker route.

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30.

An emitter resistor of \( 100 \ \Omega \) is now added without bypassing. The gain becomes about

A.-400
B.-40
C.-200
D.-2

Answer: B

The emitter resistance adds to the intrinsic 26 ohms:

\[ A_{v} \approx -\frac{5000}{100+26} \approx -40 \]

so about -40, five times less than before. What has been bought is predictability: the gain now depends on two resistors rather than on the operating current, so it barely moves with temperature or device spread. Trading gain for stability through local feedback is the single most common move in analogue design.

Discuss in Forum

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