Bipolar Junction Transistors Questions and Answers

Answer: B

The assertion is true but the reason is wrong: common base has substantial voltage gain; what it lacks is the inverting gain from input to output across the collector-base capacitance, so there is no Miller multiplication. The input is the emitter and the capacitance in question does not bridge it. A correct conclusion resting on a wrong mechanism is the most instructive error, because it survives every check except tracing the cause. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: C

The first two are correct and the third is not: the inverse active region works but with a much lower gain, because the doping and geometry are optimised for carriers travelling one way. That asymmetry is a direct consequence of the heavily doped emitter and lightly doped collector. A device whose two ends are built differently cannot be expected to behave the same when reversed, which is worth remembering whenever a transistor is found connected backwards.

Answer: D

The pairing is A-P, B-Q, C-R, and it amounts to a selection guide: common emitter where gain is wanted, common base where bandwidth or a low input impedance is wanted, and common collector where impedance transformation is wanted. Since the three configurations use the same device, the choice is entirely about which terminal is held common and what that does to the impedances.

Answer: A

The first three are correctly paired and the fourth is not, so three match. The Miller effect is the apparent multiplication of the collector-base capacitance by the inverting voltage gain; leakage amplified by beta is a separate matter of bias stability. Two of these effects come from base narrowing at different severities and a third from thermal feedback, so grouping them by mechanism makes the set easier to hold than a list of names. The shared properties explain why these schemes form a family, and the differences explain why more than one exists.

Answer: B

The order is 1, 2, 3, 4: charge must arrive before current flows, the voltage falls as the current rises through the load, and the excess charge that causes storage delay accumulates only once the device is already saturated. Tracing a switching sequence in terms of charge rather than voltage explains both the delays and why they differ between turn-on and turn-off.

Answer: C

The letters name the doping of the three regions in order - n-type emitter, p-type base, n-type collector - and everything else follows: the base must be driven positive with respect to the emitter, and the collector sits positive of both. Terminal order, supply polarity and operating regions are all consequences rather than the meaning. Reading a device's name as a description of its structure makes its bias requirements deducible rather than memorised.

Answer: D

A PNP is the mirror image: the base is driven negative of the emitter, the collector sits negative of both, and all the currents flow the other way. Its gain tends to be a little lower in practice because hole mobility is lower than electron mobility, but that is a physical detail rather than a rule of use. Saturation works identically. Complementary devices let a designer choose which supply rail a stage hangs from, which is the whole basis of push-pull output stages.

Answer: A

Hole conduction is a succession of valence electrons shifting between bonds, which is less mobile than an electron moving in the conduction band - roughly a factor of three at room temperature. Holes carry the same magnitude of charge, exist throughout the crystal, and travel usefully far. That mobility difference is why PNP devices are usually a little slower and why n-channel field effect transistors are preferred where speed matters. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: B

The subscript convention marks it as a direct-current parameter, the ratio of collector to base current at a stated operating point, whereas hfe with lower-case letters is the small-signal equivalent. Transconductance and high-frequency figures are quoted separately. The upper and lower case distinction runs through the whole of device notation, and it is worth reading carefully because the two values can differ noticeably. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: C

Beta rises from low current, peaks, and falls again at high current, so a single figure is meaningless without the current it was measured at. The thermal voltage is set by temperature alone, leakage is largely independent of the operating current, and the collector voltage ratings do fall a little with current but for unrelated reasons. Any parameter with a peak has to be quoted with its conditions, and treating a headline figure as universal is how designs come to work only near one operating point.