Power Amplifiers Questions and Answers

Q: Why is a loudspeaker's sensitivity quoted in decibels per watt at one metre?

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.

Q: What is meant by an amplifier's headroom?

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.

Q: Why does music need more headroom than a sine wave test?

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.

Q: What does an amplifier's total harmonic distortion figure measure?

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.

Q: What does intermodulation distortion reveal that harmonic distortion may not?

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.

81.

Why is a loudspeaker's sensitivity quoted in decibels per watt at one metre?

A.It states the sound level a given power produces
B.It states the power the speaker can handle safely
C.It states the impedance the speaker presents
D.It states the frequency range the speaker covers

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.

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82.

What is meant by an amplifier's headroom?

A.The range of load impedances it will tolerate
B.The margin above the average level before clipping
C.The gap between its supply rails and its output devices
D.The difference between its input and output impedances

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.

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83.

Why does music need more headroom than a sine wave test?

A.Its impedance demands vary more with time
B.Its distortion is harder to measure accurately
C.Its peaks are far above its average level
D.Its frequency content is wider than a single tone

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.

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84.

What does an amplifier's total harmonic distortion figure measure?

A.The noise present with no signal applied
B.The variation of gain across the frequency band
C.The phase shift between input and output
D.The harmonics added relative to the fundamental

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.

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85.

Why is distortion usually quoted at several power levels?

A.It varies with level in a way one figure cannot show
B.Measurement equipment is inaccurate at low levels
C.The load impedance changes as the level rises
D.The supply voltage sags at higher power levels

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.

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86.

What does intermodulation distortion reveal that harmonic distortion may not?

A.The noise floor with the input short circuited
B.Products from two tones falling back in band
C.The absolute level at which the amplifier clips
D.The amplifier's response at very low frequencies

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.

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87.

Why is a bandwidth specification quoted with a power level?

A.The feedback loop is unstable at high frequencies
B.The output devices are slower when cold
C.Slew limiting reduces the bandwidth at large outputs
D.The load impedance falls at the band edges

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.

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88.

What does a driver stage supply to a power output stage?

A.The voltage gain the whole amplifier provides
B.The quiescent bias for the output devices
C.The feedback signal for the loop
D.The base current the output devices demand

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.

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89.

Why does a driver stage often have its own small heatsink?

A.It carries appreciable current at a substantial voltage
B.It must track the output devices' temperature
C.It dissipates more power than the output devices
D.Its junction temperature limit is lower than usual

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.

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90.

What is a bootstrap capacitor used for in an output stage?

A.To filter the supply feeding the driver stage
B.To lift the driver's supply above the rail on peaks
C.To couple the signal into the output devices
D.To provide the quiescent bias for the drivers

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.

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