Power Amplifiers Questions and Answers

Answer: C

The device conducts only for a brief part of the cycle, near the peak of the voltage waveform where the current and voltage overlap least, so very little energy is dissipated in it. The tank does store energy and reconstruct the sine, but that is what makes the class usable rather than what makes it efficient. It is not a switching class, and its conduction angle is smaller. Reducing the overlap of current and voltage is the general route to efficiency in every class.

Answer: D

Push-pull uses one device for the positive half of the waveform and another for the negative, so each need conduct only half the time. Devices in parallel share current within one half, and devices in series form a totem pole. Splitting the cycle between two devices is what makes class B possible at all, and it also cancels the even harmonics that a single-ended stage produces.

Answer: A

The two halves distort symmetrically in opposite directions, so the even harmonics cancel in the output while the odd ones remain. That is a genuine benefit beyond the efficiency, and it is why push-pull was adopted in valve audio long before efficiency mattered much. Any arrangement with two paths of opposite polarity cancels one parity of harmonic, which is the same principle behind balanced mixers and differential pairs. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: B

A complementary pair uses opposite-polarity devices, so each responds naturally to one half of the waveform and no phase splitter is needed. Two devices of the same type need a phase inverter ahead of them, which is the older quasi-complementary arrangement. The availability of well-matched complementary power devices is what made direct-coupled transformerless audio amplifiers practical.

Answer: C

Early power transistors were almost all NPN, so the negative half was handled by an NPN output device driven by a PNP driver, making the pair behave as a composite PNP. Efficiency and device count are unchanged, and drivers are still needed. A workaround adopted because one component did not exist is worth recognising as such, since it explains circuits that would otherwise look perverse.

Answer: D

An output device passing amperes needs tens or hundreds of milliamps of base drive, and a Darlington's multiplied current gain reduces that to a manageable figure. Its saturation voltage is worse rather than better, it dissipates the same or a little more, and its bandwidth is narrower. Cascading to multiply a gain is the obvious move once one device's gain is insufficient, and the penalties land elsewhere as they usually do. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: A

The spreader develops the two junction drops needed between the bases so that both output devices sit at a small quiescent current, and it is usually a transistor whose base divider sets the voltage. Supply division, current limiting and coupling are other functions. Because the required voltage tracks the output devices' own drops, the spreader is normally mounted on the same heatsink so that it tracks their temperature too. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: B

The output devices' base-emitter drops fall about two millivolts per degree as they heat, so unless the spreader falls with them the quiescent current climbs and can run away. Thermal coupling makes the compensation automatic. The spreader dissipates very little, insulation is a separate matter, and capacitance is not the issue. Compensating a temperature-dependent quantity with an identical device at the same temperature works only if the two really do share a temperature.

Answer: C

Excess standing current dissipates power at idle and, since the required bias voltage falls as the devices heat, can feed a runaway unless the spreader tracks them. Crossover distortion improves rather than worsening, and neither output power nor bandwidth is much affected. The quiescent current is squeezed between crossover distortion below and dissipation above, so it has an optimum rather than a direction of improvement. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: D

A bipolar device's forward drop falls as it heats, so the hottest device would take more current and run away; a fraction of an ohm in each emitter develops an opposing voltage and equalises the sharing. They raise the output impedance slightly rather than lowering it, and provide no gain or filtering. Any device whose conduction improves as it heats needs a ballast element when paralleled, which is exactly the property MOSFETs lack and why they parallel freely. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.