Power Amplifiers Questions and Answers

Answer: A

Class is defined by how much of each input cycle the output device conducts for - the whole cycle in class A, half in class B, rather more than half in class AB, and considerably less than half in class C. Efficiency, impedance and device count all follow from that choice rather than defining it. Because one parameter determines the class and everything else follows, the conduction angle is the first thing to establish about any output stage. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: B

Class B conducts for exactly half the cycle, so two devices working alternately cover both halves. The whole cycle is class A, rather more than half is class AB, and well under half is class C. The half-cycle is what gives class B its high efficiency and also what produces crossover distortion at the handover, so both the virtue and the vice come from the same number.

Answer: C

The quiescent current flows continuously, so the stage dissipates its maximum with no output at all and efficiency rises only as the signal approaches full swing. Dissipation falls rather than rises with output, the conduction angle is the largest rather than the smallest, and no transformer is necessarily present. A stage that consumes the same power whether or not it is doing anything is inefficient by construction, and that is what class B was devised to escape. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: D

A resistively loaded class A stage reaches twenty-five per cent at full swing, because half the supply is dropped across the load resistor at idle and the swing is limited to the quiescent voltage. Transformer coupling raises the ceiling to fifty per cent by removing that standing drop, and seventy-eight and a half is class B's figure. An efficiency limit belongs to a coupling arrangement as much as to a class, so both must be named before a figure means anything. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Answer: A

Class B reaches pi over four, about 78.5 per cent, at full output, because each device draws current only while it is delivering it and nothing flows at idle. Fifty per cent is transformer-coupled class A and twenty-five is the resistive case. That the figure is a transcendental fraction rather than a round number is a clue that it comes from integrating a half sine, which is exactly where it comes from.

Answer: B

Dissipation is the difference between what the supply delivers and what the load takes, and that difference peaks at roughly two thirds of full swing rather than at either extreme. At idle almost nothing flows, and at full output most of the power is reaching the load. That the worst thermal case is neither of the obvious ones is the single most useful fact for sizing a class B heatsink, and testing only at full power will understate the requirement. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: C

Near the zero crossing neither device conducts, because each needs a junction drop before it turns on, so a flat step appears in the output waveform. Saturation clips the peaks instead, supply sag compresses the whole waveform, and core distortion is a transformer effect. Crossover is worst at small signals, where the step is a large fraction of the output, which is the reverse of most distortion mechanisms and is why it is so audible. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: D

Giving each device a small quiescent current so that both conduct near the zero crossing turns class B into class AB and closes the gap in the transfer characteristic. Supply voltage, load impedance and shunt capacitance do nothing about it. The cost is a little standing dissipation and a small loss of efficiency, which is why nearly every practical audio output stage is class AB rather than either pure class.

Answer: A

Biasing the devices into slight conduction costs a small standing current and a few per cent of efficiency, and removes the crossover step that dominates class B's distortion at low levels. The trade runs the other way from the second option. Bandwidth and output impedance are unaffected. A class defined by a conduction angle just over half a cycle exists purely to take that trade, which is why it is the standard choice for audio. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: B

Class C conducts for well under half a cycle and is grossly non-linear, so it is usable only where a tuned circuit restores the sine wave from the pulses it delivers - which means constant-envelope radio transmission. Audio, instrumentation and preamplifier stages all need linearity. That an amplifier can be useful while distorting badly, provided a filter follows it, is worth understanding, since the same idea underlies the switching classes. Checking the units of each quantity catches a wrong formula before the arithmetic begins.