Feedback Amplifiers Questions and Answers

Q: An amplifier's slew rate is \( 1 \ \mathrm{V/\mu s} \). The largest undistorted \( 10 \ \mathrm{V} \) peak sine wave it can produce is at about

The steepest slope of a sine wave is at its zero crossing:\[ \begin{aligned} 2\pi f A &\le 10^{6} \\ f &\le \tfrac{10^{6}}{2\pi \times 10} \approx 16 \ \mathrm{kHz} \end{aligned} \]giving about 16 kHz. Above that the output becomes a triangle rather than a sine, however wide the small-signal bandwidth. The limit falls in proportion to amplitude, so the same amplifier reaches ten times the frequency at a tenth of the output.

Q: How does slew limiting differ from clipping in its cause?

Slew limiting is a limit on how fast the output can change, set by an internal current and a capacitance, while clipping is a limit on how far it can go, set by the supply rails. Both occur at the output, neither is a temperature effect, and both affect any waveform. The distinction matters practically because the remedies differ entirely: a faster amplifier for one, a higher supply or a smaller signal for the other. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

Q: Why does a feedback amplifier's distortion rise at high frequencies?

Distortion is divided by one plus the loop gain, and since the open-loop gain falls steadily above the dominant pole, the loop gain and hence the correction fall with it. Device linearity is roughly frequency-independent over the audio range, the network's phase affects stability rather than distortion directly, and the bias current does not fall. A specification quoted at one frequency and assumed at all others is a common source of disappointment, and this is the mechanism behind it.

Q: Why is a feedback amplifier's distortion usually specified across the band?

Because the loop gain falls with frequency, the same amplifier corrects far less at twenty kilohertz than at one, so a single figure would flatter it. Device linearity is roughly constant, load variation is a separate matter that also deserves specifying, and the equipment is adequate. A specification that varies by a factor of ten across the intended band has to be quoted across that band, and a single headline figure should always prompt the question of where it was measured.

Q: Consider the following statements about an internally compensated operational amplifier. 1. Its dominant pole is placed at a few hertz 2. It is stable at any closed-loop gain down to unity 3. Its distortion correction is constant across the audio band Which of the statements given above is or are correct?

The first two are correct and the third is not: the loop gain falls twenty decibels per decade above the dominant pole, so the correction at twenty kilohertz is some sixty times less than at twenty hertz. The first two statements are two views of the same design choice, and the third is the price it charges at the top of the band. A device stable everywhere is necessarily one whose loop gain has been given away at high frequency, and the distortion figure shows it.

111.

An amplifier's slew rate is \( 1 \ \mathrm{V/\mu s} \). The largest undistorted \( 10 \ \mathrm{V} \) peak sine wave it can produce is at about

A.16 kHz
B.160 kHz
C.1.6 kHz
D.100 kHz

Answer: A

The steepest slope of a sine wave is at its zero crossing:

\[ \begin{aligned} 2\pi f A &\le 10^{6} \\ f &\le \tfrac{10^{6}}{2\pi \times 10} \approx 16 \ \mathrm{kHz} \end{aligned} \]

giving about 16 kHz. Above that the output becomes a triangle rather than a sine, however wide the small-signal bandwidth. The limit falls in proportion to amplitude, so the same amplifier reaches ten times the frequency at a tenth of the output.

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112.

Why does slew limiting produce a triangular output?

A.The compensation capacitor resonates with the load
B.The output rises at its maximum rate throughout
C.The devices clip symmetrically at both extremes
D.The feedback loop opens during each transition

Answer: B

Once the demanded slope exceeds what the internal current can deliver, the output simply ramps at its maximum rate until the input reverses, and a succession of constant-slope ramps is a triangle. Amplitude clipping flattens the peaks instead, which is a different and distinguishable distortion. The loop does effectively lose control during the ramp, though that is a description of the symptom rather than its cause. A distortion whose shape identifies its mechanism is worth recognising, since it names the specification being exceeded.

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113.

How does slew limiting differ from clipping in its cause?

A.One depends on frequency, the other on temperature
B.One affects only sine waves, the other only square waves
C.One is a rate limit, the other an amplitude limit
D.One occurs at the input, the other at the output

Answer: C

Slew limiting is a limit on how fast the output can change, set by an internal current and a capacitance, while clipping is a limit on how far it can go, set by the supply rails. Both occur at the output, neither is a temperature effect, and both affect any waveform. The distinction matters practically because the remedies differ entirely: a faster amplifier for one, a higher supply or a smaller signal for the other. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

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114.

Why does a feedback amplifier's distortion rise at high frequencies?

A.The devices become less linear at high frequencies
B.The feedback network's phase shift grows with frequency
C.The output stage's bias current falls with frequency
D.The loop gain falls as the open-loop gain rolls off

Answer: D

Distortion is divided by one plus the loop gain, and since the open-loop gain falls steadily above the dominant pole, the loop gain and hence the correction fall with it. Device linearity is roughly frequency-independent over the audio range, the network's phase affects stability rather than distortion directly, and the bias current does not fall. A specification quoted at one frequency and assumed at all others is a common source of disappointment, and this is the mechanism behind it.

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115.

An amplifier has \( 60 \ \mathrm{dB} \) of loop gain at \( 1 \ \mathrm{kHz} \) and a dominant pole at \( 10 \ \mathrm{Hz} \). Its loop gain at \( 10 \ \mathrm{kHz} \) is about

A.40 dB
B.60 dB
C.20 dB
D.80 dB

Answer: A

Above the dominant pole the gain falls twenty decibels per decade:

\[ 60 - 20 = 40 \ \mathrm{dB} \]

giving about 40 dB, so the distortion correction has fallen by a factor of ten over one decade of frequency. That is why distortion measurements are quoted at several frequencies rather than one, and why a figure at a kilohertz says little about behaviour at twenty.

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116.

Why is a feedback amplifier's distortion usually specified across the band?

A.The measurement equipment is band-limited
B.The correction falls as the loop gain rolls off
C.The devices' linearity varies across the band
D.The load impedance changes with frequency

Answer: B

Because the loop gain falls with frequency, the same amplifier corrects far less at twenty kilohertz than at one, so a single figure would flatter it. Device linearity is roughly constant, load variation is a separate matter that also deserves specifying, and the equipment is adequate. A specification that varies by a factor of ten across the intended band has to be quoted across that band, and a single headline figure should always prompt the question of where it was measured.

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117.

Consider the following statements about an internally compensated operational amplifier.

1. Its dominant pole is placed at a few hertz

2. It is stable at any closed-loop gain down to unity

3. Its distortion correction is constant across the audio band

Which of the statements given above is or are correct?

A.2 and 3 only
B.1, 2 and 3
C.1 and 2 only
D.1 and 3 only

Answer: C

The first two are correct and the third is not: the loop gain falls twenty decibels per decade above the dominant pole, so the correction at twenty kilohertz is some sixty times less than at twenty hertz. The first two statements are two views of the same design choice, and the third is the price it charges at the top of the band. A device stable everywhere is necessarily one whose loop gain has been given away at high frequency, and the distortion figure shows it.

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118.

Consider the following pairs of limit and its cause.

Slew rate - Fixed charging current into the compensation capacitor

Clipping - The supply rails

Small-signal bandwidth - The dominant pole and the closed-loop gain

Rising distortion with frequency - Falling device linearity

How many of the pairs given above are correctly matched?

A.Only one
B.Only two
C.All four
D.Only three

Answer: D

The first three are correctly paired and the fourth is not, so three match. Distortion rises with frequency because the loop gain falls, not because the devices themselves become less linear. Three independent limits on one amplifier, each with its own cause and its own remedy, and attributing a symptom to the wrong one leads to buying a faster amplifier when a larger supply was wanted or the reverse.

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