Bipolar Junction Transistors Questions and Answers

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.

Answer: C

The output device's base sits at the driver's emitter, so its collector cannot go below one base-emitter drop plus the driver's own saturation voltage, leaving perhaps 0.8 V rather than 0.2. Drive current is ample by construction, differing temperatures are a secondary matter, and the gain fall-off is a separate effect. Tracing a limit to the topology rather than to either device explains why no better transistor would fix it. Working one numerical case through both schemes side by side fixes the distinction better than any rule of thumb.

Answer: D

A cascode puts a common-base stage above a common-emitter stage, so the lower device sees almost no voltage swing and its Miller capacitance is not multiplied. The result is the gain of a common emitter with the bandwidth of a common base. The other options describe cascades, followers and parallel devices. Arranging two stages so that each supplies what the other lacks is the essence of a composite, and the cascode is the cleanest example in electronics.

Answer: A

With the upper device holding its collector nearly fixed, the lower device's voltage gain is about one, so the Miller multiplication of its collector-base capacitance almost disappears. The upper device's low input impedance is the mechanism by which that happens rather than the reason for the bandwidth. Capacitances are not halved, and the current gain is beta. Removing the multiplication rather than the capacitance is what makes the arrangement effective.

Answer: B

A mirror uses matched devices sharing a base-emitter voltage so that one branch's current reproduces another's, which is how bias currents are distributed across an integrated circuit. Voltage copying is a follower's job, gain multiplication a Darlington's, and differential-to-single conversion is done by an active load - though a mirror often serves as that load. Matched devices sharing one controlling voltage is the foundation of nearly every integrated analogue building block.

Answer: C

Mirrors depend on two devices having the same base-emitter voltage for the same current, and devices made side by side on one chip at one temperature match far better than two separately packaged parts. Gain, connection and capacitance are not the issue. Any circuit whose accuracy rests on matching rather than on absolute values is an integrated-circuit technique, and recognising that division explains a great deal about why the two design styles differ.

Answer: D

Both are true and the second is the mechanism: the input capacitance is no longer multiplied by the stage gain, so the bandwidth is set by the device rather than by the Miller effect. The overall voltage gain is still taken, but at the upper device where it does no harm. Separating where the gain is taken from where the capacitance is seen is the whole trick, and it recurs in every wideband amplifier topology.

Answer: A

Both statements are true but the second does not explain the first: the low input current follows from the multiplied current gain, while the series junctions explain the higher input voltage requirement - a different consequence entirely. Two true statements about one device need not stand in a causal relation, and separating the two effects of cascading is exactly what this item tests.

Answer: B

All three are correct, and together they describe the follower completely: no voltage gain, impedance multiplied looking in, impedance divided looking out. The last two are the same reflection rule applied in opposite directions, which is why the device transforms impedance in the way a transformer does but without any winding. A stage worth using despite having no gain is one whose value lies entirely in those two impedances. The parameter that varies and the parameter that does not are worth naming before any substitution is attempted.

Answer: C

The pairing is A-P, B-Q, C-R. Each composite exists because one device could not do the job: the Darlington for gain, the cascode for bandwidth at gain, and the mirror to replicate a current accurately. Asking what a composite buys, and what it costs elsewhere, is the way to decide whether the extra device is worth adding.