Power Amplifiers Questions and Answers

Q: The distortion that appears at the handover between the two devices of a class B stage is called ______ distortion.

This is crossover distortion, worst at small signals where the step is a large fraction of the output. Harmonic and intermodulation distortion are measures of distortion generally rather than mechanisms, and diagonal clipping is a detector defect in an AM receiver. That this distortion grows relatively worse as the signal falls is the reverse of most mechanisms, and it is why class B sounds poor at low volume until the bias is added. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Q: Which of the following is not a reason to use class AB rather than class B? Higher theoretical efficiency Removal of the crossover step Lower distortion at small signal levels A smoother transfer characteristic near zero

Higher theoretical efficiency is not among them - class AB's standing current costs a little efficiency, which is exactly what it pays for the other three benefits. The remaining options are three ways of describing the same improvement. A variant of a class always trades one of the parent's properties for another, and identifying which way the trade runs is the point of the comparison.

Q: An amplifier sounds gritty at low volume but clean when loud. Which fault fits?

Distortion that is relatively worse at small signals is the signature of crossover: the step at the handover is a large fraction of a small output and a negligible one of a large. Mismatched devices, supply sag and a bad connection would all worsen as the level rose. Whether a defect grows or shrinks with signal level is the most informative single observation about an output stage, and it points here unambiguously.

Q: An amplifier's output devices run hot at idle and hotter still with time. What should be suspected?

Rising temperature at idle with no signal is thermal runaway in the bias: as the output devices heat their required bias voltage falls, and if the spreader does not fall with them the quiescent current climbs. Load, supply and input all act only when a signal is present. The usual cause is a spreader that has come loose from the heatsink, which is why its mounting is checked before anything else.

Q: An amplifier clips asymmetrically on a sine wave. What does this indicate?

Clipping on one half before the other means the output has less headroom in one direction, which follows from unequal rails or from an output sitting off centre. Class A biasing would not produce asymmetry, and a reactive load shifts phase rather than clipping one side. A direct offset at the input is a genuine alternative cause, since it moves the output off centre too, but unequal rails is the more common. The shape of a distorted waveform carries information about its cause, and asymmetry points at the operating point.

Q: An amplifier oscillates at high frequency only when a long loudspeaker cable is fitted. Why?

A long cable presents tens of nanofarads, and that capacitance works against the output impedance to add a pole inside the feedback loop, eating the phase margin. Resistance and inductance affect damping and response rather than stability, and interference would appear as noise rather than oscillation. The standard remedy is a small series inductor at the output, keeping the cable's pole outside the loop - which is why one is fitted in almost every commercial amplifier.

Q: What does an amplifier's damping factor describe?

Damping factor is the load impedance divided by the amplifier's own output impedance, so a figure of a hundred into eight ohms means an output impedance of eighty milliohms. Power ratio is gain, settling is a transient specification, and ripple attenuation is supply rejection. A high damping factor means the amplifier controls the loudspeaker's motion rather than letting it ring, and feedback is what makes such low output impedances achievable. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Q: Why does negative feedback improve an amplifier's damping factor?

Voltage sampling at the output makes the amplifier work to hold that voltage steady whatever the load draws, which divides the output impedance by one plus the loop gain. Current capability is set by the devices, the loudspeaker's resonance is its own property, and the load is what it is. That the same feedback which reduces distortion also raises the damping factor is one mechanism producing two apparently unrelated improvements. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Q: An amplifier has an output impedance of \( 0.08 \ \Omega \) into \( 8 \ \Omega \). Its damping factor is

Dividing the load by the output impedance:\[ DF = \frac{8}{0.08} = 100 \]giving 100. Inverting the ratio gives 0.01, which would describe an amplifier with a higher output impedance than its load. Because the figure depends on the load, quoting a damping factor without naming the impedance it was measured into says very little.

61.

The distortion that appears at the handover between the two devices of a class B stage is called ______ distortion.

A.crossover
B.harmonic
C.intermodulation
D.diagonal clipping

Answer: A

This is crossover distortion, worst at small signals where the step is a large fraction of the output. Harmonic and intermodulation distortion are measures of distortion generally rather than mechanisms, and diagonal clipping is a detector defect in an AM receiver. That this distortion grows relatively worse as the signal falls is the reverse of most mechanisms, and it is why class B sounds poor at low volume until the bias is added. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

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62.

Which of the following is not a reason to use class AB rather than class B?

Higher theoretical efficiency

Removal of the crossover step

Lower distortion at small signal levels

A smoother transfer characteristic near zero

A.A smoother transfer characteristic near zero
B.Higher theoretical efficiency
C.Removal of the crossover step
D.Lower distortion at small signal levels

Answer: B

Higher theoretical efficiency is not among them - class AB's standing current costs a little efficiency, which is exactly what it pays for the other three benefits. The remaining options are three ways of describing the same improvement. A variant of a class always trades one of the parent's properties for another, and identifying which way the trade runs is the point of the comparison.

Discuss in Forum

63.

An amplifier sounds gritty at low volume but clean when loud. Which fault fits?

A.A supply rail sagging under heavy load
B.A loudspeaker lead with a poor connection
C.Insufficient quiescent bias in the output stage
D.An output device with a lower gain than its partner

Answer: C

Distortion that is relatively worse at small signals is the signature of crossover: the step at the handover is a large fraction of a small output and a negligible one of a large. Mismatched devices, supply sag and a bad connection would all worsen as the level rose. Whether a defect grows or shrinks with signal level is the most informative single observation about an output stage, and it points here unambiguously.

Discuss in Forum

64.

An amplifier's output devices run hot at idle and hotter still with time. What should be suspected?

A.A load impedance lower than the design intended
B.A supply voltage above its nominal figure
C.An input signal with excessive amplitude
D.A bias spreader not tracking the output devices

Answer: D

Rising temperature at idle with no signal is thermal runaway in the bias: as the output devices heat their required bias voltage falls, and if the spreader does not fall with them the quiescent current climbs. Load, supply and input all act only when a signal is present. The usual cause is a spreader that has come loose from the heatsink, which is why its mounting is checked before anything else.

Discuss in Forum

65.

An amplifier clips asymmetrically on a sine wave. What does this indicate?

A.The two supply rails are not equal
B.The output stage is biased into class A
C.The input signal contains a direct offset
D.The loudspeaker load is reactive rather than resistive

Answer: A

Clipping on one half before the other means the output has less headroom in one direction, which follows from unequal rails or from an output sitting off centre. Class A biasing would not produce asymmetry, and a reactive load shifts phase rather than clipping one side. A direct offset at the input is a genuine alternative cause, since it moves the output off centre too, but unequal rails is the more common. The shape of a distorted waveform carries information about its cause, and asymmetry points at the operating point.

Discuss in Forum

66.

An amplifier oscillates at high frequency only when a long loudspeaker cable is fitted. Why?

A.The cable's inductance raises the output impedance
B.The cable's capacitance adds a pole inside the loop
C.The cable's resistance reduces the damping factor
D.The cable picks up radio-frequency interference

Answer: B

A long cable presents tens of nanofarads, and that capacitance works against the output impedance to add a pole inside the feedback loop, eating the phase margin. Resistance and inductance affect damping and response rather than stability, and interference would appear as noise rather than oscillation. The standard remedy is a small series inductor at the output, keeping the cable's pole outside the loop - which is why one is fitted in almost every commercial amplifier.

Discuss in Forum

67.

What does an output inductor in parallel with a resistor achieve?

A.It matches the amplifier's impedance to the speaker
B.It limits the current on a short circuit
C.It isolates the loop from a capacitive load
D.It filters the switching frequency from the output

Answer: C

A few microhenries in series with the output, damped by a parallel resistor, separates the cable's capacitance from the feedback take-off point so its pole falls outside the loop. Switching filters belong to class D, impedance matching is unnecessary in a voltage-source amplifier, and current limiting is a separate circuit. Moving a troublesome element outside a loop is often easier than compensating for it inside, and it costs only a little output impedance at high frequencies.

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68.

What does an amplifier's damping factor describe?

A.The ratio of output power to input power
B.The rate at which the output settles after a step
C.The attenuation of supply ripple at the output
D.The load impedance divided by the output impedance

Answer: D

Damping factor is the load impedance divided by the amplifier's own output impedance, so a figure of a hundred into eight ohms means an output impedance of eighty milliohms. Power ratio is gain, settling is a transient specification, and ripple attenuation is supply rejection. A high damping factor means the amplifier controls the loudspeaker's motion rather than letting it ring, and feedback is what makes such low output impedances achievable. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Discuss in Forum

69.

Why does negative feedback improve an amplifier's damping factor?

A.It divides the output impedance by the loop gain
B.It increases the current the output stage can deliver
C.It reduces the loudspeaker's own mechanical resonance
D.It raises the load impedance the amplifier sees

Answer: A

Voltage sampling at the output makes the amplifier work to hold that voltage steady whatever the load draws, which divides the output impedance by one plus the loop gain. Current capability is set by the devices, the loudspeaker's resonance is its own property, and the load is what it is. That the same feedback which reduces distortion also raises the damping factor is one mechanism producing two apparently unrelated improvements. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Discuss in Forum

70.

An amplifier has an output impedance of \( 0.08 \ \Omega \) into \( 8 \ \Omega \). Its damping factor is

A.1000
B.100
C.10
D.0.01

Answer: B

Dividing the load by the output impedance:

\[ DF = \frac{8}{0.08} = 100 \]

giving 100. Inverting the ratio gives 0.01, which would describe an amplifier with a higher output impedance than its load. Because the figure depends on the load, quoting a damping factor without naming the impedance it was measured into says very little.

Discuss in Forum

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