Bipolar Junction Transistors Questions and Answers

Answer: A

The transition frequency summarises the gain-bandwidth product and so decides whether useful gain is available at the working frequency. Saturation voltage matters for switching, dissipation for power stages, and breakdown voltage for high-voltage duty. Each application has one parameter that dominates the choice, and identifying it before opening a data sheet saves comparing a great many irrelevant numbers.

Answer: B

An output stage handles substantial current and voltage together, so the safe operating area, bounding current, voltage, power and secondary breakdown at once, is the decisive specification. Transition frequency is comfortably adequate at audio, base resistance matters for noise, and the base-emitter rating is a protection concern. A composite specification that draws several limits on one chart is exactly what a design point needs to be checked against.

Answer: C

A collector near the supply means little current is flowing, so the device is cut off - no base bias, or an open base circuit. Saturation would put the collector near the emitter instead, a shorted load would also hold it at the supply but is far less likely than a bias fault, and an open bypass capacitor would reduce the gain without moving the operating point so far. Reading the collector voltage first localises the fault to one of three states before any component is suspected.

Answer: D

A collector barely above the emitter is saturation: the base is being driven harder than the load allows, so the device has run out of collector voltage. Cut-off would show the collector at the supply. An open load resistor would leave the collector near the emitter too, but no current would flow, which a measurement across the resistor would reveal at once. Two measurements - collector voltage and the drop across the load - separate every one of these cases.

Answer: A

Without the bypass, the emitter resistor appears in the signal path and the gain becomes the ratio of collector to emitter resistance rather than the much larger ratio to the intrinsic 26 ohms. The operating point is unchanged, since the direct conditions do not involve the capacitor, and the input impedance rises rather than falls. A component that is present for direct current and absent for signals is doing two jobs, and removing it reveals which one was being taken for granted.

Answer: B

Beta rises with temperature and the base drop falls, so a bias arrangement that depends on beta moves the operating point and hence the gain. Transition frequency and capacitance changes are far too small to matter at audio, and saturation voltage is irrelevant in a linear stage. Drift with temperature nearly always traces back to a bias arrangement that fails to cancel a temperature-dependent parameter, which is why the next topic exists. Most errors on this topic come from carrying a result across from a neighbouring scheme where it does not hold.

Answer: C

Beta can vary three to one within one part number, so any stage whose gain or bias depends on beta changes when the device does. The other parameters vary too but affect different aspects of behaviour. A circuit that survives device replacement without adjustment is one whose performance rests on resistor ratios, and getting there is the practical purpose of negative feedback.

Answer: D

If the base drive is too small the device sits in the active region rather than in saturation, so it carries the load current with several volts across it and dissipates heavily. Excess drive causes delay rather than heating, a large load resistor reduces the current, and a low supply reduces the dissipation. A switch that is hot is almost always spending time, or sitting, between its two intended states.

Answer: A

The clamp diverts excess base current once the collector falls near the base, so the device stops just short of deep saturation and turns off promptly. It slightly increases the on-state voltage rather than reducing the loss, offers no base-emitter protection, and does not affect the gain. Trading a little conduction loss for a large reduction in switching delay is worthwhile whenever the switching frequency is high, and the whole Schottky logic family was built on that trade.

Answer: B

The first is true and the second is not: in the active region the collector current is set by the base current times beta and is largely independent of the collector circuit, which is what makes the device a current source. In saturation that control is lost and the load decides. Which quantity is in control changes between regions, and that shift is precisely what distinguishes an amplifier from a switch.