Bipolar Junction Transistors Questions and Answers

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.