Power Amplifiers Questions and Answers

Answer: A

Sensitivity says how loud one watt makes the speaker at a metre, so a speaker three decibels more sensitive needs half the amplifier power for the same level. Power handling, impedance and frequency range are three separate specifications. Since amplifier power is expensive and sensitivity is free once the speaker is chosen, comparing sensitivities is usually the more effective way to reach a required sound level.

Answer: B

Headroom is how far the output can rise above the average programme level before it clips, and since music has peaks ten or more decibels above its average, an amplifier needs several times the average power in reserve. The other options describe internal voltage margins, impedances and load tolerance. A specification expressed as a margin rather than an absolute value is one that depends on the signal as much as on the equipment, which is why programme material has to be named when it is quoted.

Answer: C

A sine wave's peak is a fixed root two above its mean, while music routinely peaks ten to twenty decibels above its average, so an amplifier that handles a continuous sine at its rated power will clip on music at a far lower average level. Bandwidth, impedance variation and measurement difficulty are real differences but not the reason. A rating measured with the least demanding waveform will always flatter equipment used with a more demanding one.

Answer: D

The figure is the root sum of the harmonic components divided by the fundamental, expressed as a percentage or in decibels. Noise, frequency response and phase are three other specifications. Because the figure depends on level and on frequency, a single number is nearly meaningless without both - which is why a reputable specification quotes distortion at a stated power across a stated band.

Answer: A

Crossover distortion dominates at low levels and clipping at high, so the curve is a shallow U and a figure quoted at one point can flatter or condemn the amplifier depending on which point was chosen. Equipment accuracy, load variation and supply sag are secondary. A specification that varies non-monotonically with an operating condition has to be quoted as a curve or at several points, and reducing it to one number is what makes some published figures worthless. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Answer: B

Two tones through a non-linearity produce sum and difference products that can land well away from either tone, in parts of the band where the ear is sensitive and where no harmonic of either would appear. Clipping level, low-frequency response and noise are separate measurements. A test that generates products at frequencies unrelated to the input reveals non-linearity that a harmonic test can miss entirely, which is why both are specified. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: C

An amplifier's small-signal bandwidth can be far wider than the frequency at which it can still deliver full output, because the rate at which the output can change is limited independently of the small-signal response. Load impedance, stability and device temperature are separate matters. Two independent limits govern the same amplifier, one for small signals and one for large, and a bandwidth figure that does not say which is being described is incomplete. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: D

An output device passing amperes at a current gain of fifty needs tens of milliamps of drive, far more than a small-signal voltage amplifier can deliver, so a driver stage stands between them. Voltage gain comes from earlier stages, bias from the spreader, and feedback from the output node. Each stage in a chain exists because the one before it cannot meet the demands of the one after, and naming that mismatch explains why the stage is there at all. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Answer: A

A driver passing a hundred milliamps with twenty volts across it dissipates a couple of watts, which is more than a small package can shed unaided. Temperature tracking is the bias spreader's job, the drivers dissipate far less than the output devices, and their temperature limits are ordinary. A stage that looks small-signal on a schematic can still need cooling, and checking the current-voltage product of every device is what catches it. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: B

As the output approaches the positive rail the upper driver runs out of headroom, and a capacitor from the output to the driver's collector load carries its supply upward with the signal, letting the output swing closer to the rail. Coupling, biasing and filtering are other functions. Feeding a stage from a supply that moves with its own output is the same bootstrapping idea used to raise an input impedance, applied here to recover swing rather than impedance. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.