Bipolar Junction Transistors Questions and Answers

Answer: A

Applying beta and then Kirchhoff's current law at the device:

\[ \begin{aligned} I_{C} &= 100 \times 20\mathrm{\mu} = 2 \ \mathrm{mA} \\ I_{E} &= 2 + 0.02 = 2.02 \ \mathrm{mA} \end{aligned} \]

so 2 mA and 2.02 mA. The emitter current is the largest of the three, since it supplies both the others - a check worth making, because an answer with the emitter current smallest has a sign error somewhere.

Answer: B

The emitter current is largest, because it supplies both the collector and the base: the three are related by \( I_{E} = I_{C} + I_{B} \). The collector current is very nearly as large, and the base current is smaller by a factor of beta. They are certainly not equal. Getting the ordering right is a useful check on any analysis, and it follows from current conservation alone rather than from any device parameter.

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.

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.

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.

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.

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.

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.

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.

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.