Feedback Amplifiers Questions and Answers

Q: What does hysteresis in a comparator prevent?

A little positive feedback moves the threshold after each transition, so a noisy input crossing slowly cannot cause the output to chatter back and forth. Output current, offset drift and input saturation are addressed by other means. The amount of hysteresis needed is set by the expected noise amplitude, which makes it a rare case of a design parameter chosen directly from a measurement of the environment.

Q: Statement I: Negative feedback reduces gain and increases bandwidth. Statement II: Positive feedback increases gain and increases bandwidth. In the light of the above statements, choose the correct answer.

The first is true and the second is half wrong: positive feedback increases gain but narrows bandwidth, since every effect of negative feedback reverses. That reversal is consistent across all its consequences - distortion grows, impedances move the other way, and sensitivity to the open-loop gain increases. Learning the negative case thoroughly gives the positive case by inspection, provided the reversal is applied to every term rather than just the gain.

Q: Consider the following statements about the feedback equation. 1. The denominator is one plus the loop gain for negative feedback 2. The closed-loop gain tends to the reciprocal of the fraction at high loop gain 3. The denominator is one minus the loop gain for positive feedback Which of the statements given above is or are correct?

All three are correct, and between them they cover both signs of feedback and the limiting behaviour that makes negative feedback useful. The change of sign in the denominator is the whole difference between an amplifier and an oscillator, and the limiting form is why a closed-loop gain can be trusted to a resistor's tolerance. One equation, read three ways, is the foundation of the entire topic.

Q: Match each feedback effect with its factor. List I A. Closed-loop gain B. Bandwidth C. Internal distortion List II P. Divided by one plus loop gain Q. Multiplied by one plus loop gain R. Divided by one plus loop gain Select the correct match.

The pairing is A-P, B-Q, C-R: gain and distortion are both divided while bandwidth is multiplied, all by the same factor. That one number governs the single cost and every benefit is why computing the loop gain first answers every question about a feedback circuit, and why designers speak of a loop's quality in terms of it rather than of the components.

Q: The product of the open-loop gain and the fraction returned is the ______ gain.

This is the loop gain, and it is the single quantity that decides how much every benefit of feedback amounts to. Closed-loop gain is the result rather than the driver, and differential and common-mode gains belong to a differential stage's description. Because desensitivity, bandwidth extension, distortion reduction and both impedance changes all scale with one plus it, computing it first is always the right first step.

Q: Which of the following is not improved by negative feedback? The amplifier's input-referred noise The gain's stability against device spread The internal distortion of the output stage The bandwidth at a given closed-loop gain

Input-referred noise is amplified along with the signal, so the ratio between them is untouched - feedback acts only on what arises inside the loop. The other three all improve by one plus the loop gain. Knowing which single specification feedback cannot help with is what keeps a low-noise design honest, and it is why the first device's own noise still dominates however much loop gain is applied.

Q: Which of the following does negative feedback not reduce? The closed-loop gain's dependence on the feedback resistors The dependence on the open-loop gain The internal distortion of the amplifier The output impedance under voltage sampling

The dependence on the feedback resistors is if anything increased: at high loop gain the closed-loop gain is set by them alone, so their tolerance appears undiminished in the result. The other three are all reduced by one plus the loop gain. Feedback does not remove an accuracy requirement but relocates it, from the devices to a few passive components - which is worth having only because those components can be made good. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

71.

What does hysteresis in a comparator prevent?

A.Offset drift with temperature
B.Saturation of the input differential pair
C.Multiple transitions on a noisy slow input
D.Excessive current in the output stage

Answer: C

A little positive feedback moves the threshold after each transition, so a noisy input crossing slowly cannot cause the output to chatter back and forth. Output current, offset drift and input saturation are addressed by other means. The amount of hysteresis needed is set by the expected noise amplitude, which makes it a rare case of a design parameter chosen directly from a measurement of the environment.

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72.

Statement I: Negative feedback reduces gain and increases bandwidth.

Statement II: Positive feedback increases gain and increases bandwidth.

In the light of the above statements, choose the correct answer.

A.Both Statement I and Statement II are correct
B.Both Statement I and Statement II are incorrect
C.Statement I is incorrect but Statement II is correct
D.Statement I is correct but Statement II is incorrect

Answer: D

The first is true and the second is half wrong: positive feedback increases gain but narrows bandwidth, since every effect of negative feedback reverses. That reversal is consistent across all its consequences - distortion grows, impedances move the other way, and sensitivity to the open-loop gain increases. Learning the negative case thoroughly gives the positive case by inspection, provided the reversal is applied to every term rather than just the gain.

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73.

Consider the following statements about the feedback equation.

1. The denominator is one plus the loop gain for negative feedback

2. The closed-loop gain tends to the reciprocal of the fraction at high loop gain

3. The denominator is one minus the loop gain for positive feedback

Which of the statements given above is or are correct?

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

Answer: A

All three are correct, and between them they cover both signs of feedback and the limiting behaviour that makes negative feedback useful. The change of sign in the denominator is the whole difference between an amplifier and an oscillator, and the limiting form is why a closed-loop gain can be trusted to a resistor's tolerance. One equation, read three ways, is the foundation of the entire topic.

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74.

Match each feedback effect with its factor.

List I

A. Closed-loop gain

B. Bandwidth

C. Internal distortion

List II

P. Divided by one plus loop gain

Q. Multiplied by one plus loop gain

R. Divided by one plus loop gain

Select the correct match.

A.A-P, B-R, C-Q
B.A-P, B-Q, C-R
C.A-Q, B-P, C-R
D.A-R, B-Q, C-P

Answer: B

The pairing is A-P, B-Q, C-R: gain and distortion are both divided while bandwidth is multiplied, all by the same factor. That one number governs the single cost and every benefit is why computing the loop gain first answers every question about a feedback circuit, and why designers speak of a loop's quality in terms of it rather than of the components.

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75.

Consider the following pairs of topology and the amplifier it makes.

Voltage series - Voltage amplifier

Current series - Transconductance amplifier

Voltage shunt - Transimpedance amplifier

Current shunt - Transresistance amplifier

How many of the pairs given above are correctly matched?

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

Answer: C

The first three are correctly paired and the fourth is not, so three match. Current shunt feedback makes a current amplifier, low in at the input and high at the output; the transresistance amplifier is the voltage shunt case, which converts current to voltage. Since the four topologies map exactly onto the four possible conversions, getting one pairing wrong means two are wrong, which is a useful check.

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76.

An amplifier's measurements are as follows.

Parameter	Open loop	Closed loop
Gain	2000	100
Input impedance	5 k	95 k
Output impedance	800	42

Which topology has been applied?

A.Voltage shunt
B.Current series
C.Current shunt
D.Voltage series

Answer: D

The input impedance has risen and the output impedance fallen, both by a factor of about nineteen, so the return is in series at the input and the sampling is of voltage at the output - voltage series feedback. That factor also matches the gain reduction, as it must. Reading the two impedance directions off a measurement table identifies the topology without any knowledge of the circuit, which is often the quickest route when only measurements are available.

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77.

The product of the open-loop gain and the fraction returned is the ______ gain.

A.loop
B.closed
C.differential
D.common-mode

Answer: A

This is the loop gain, and it is the single quantity that decides how much every benefit of feedback amounts to. Closed-loop gain is the result rather than the driver, and differential and common-mode gains belong to a differential stage's description. Because desensitivity, bandwidth extension, distortion reduction and both impedance changes all scale with one plus it, computing it first is always the right first step.

Discuss in Forum

78.

Which of the following is not improved by negative feedback?

The amplifier's input-referred noise

The gain's stability against device spread

The internal distortion of the output stage

The bandwidth at a given closed-loop gain

A.The bandwidth at a given closed-loop gain
B.The amplifier's input-referred noise
C.The gain's stability against device spread
D.The internal distortion of the output stage

Answer: B

Input-referred noise is amplified along with the signal, so the ratio between them is untouched - feedback acts only on what arises inside the loop. The other three all improve by one plus the loop gain. Knowing which single specification feedback cannot help with is what keeps a low-noise design honest, and it is why the first device's own noise still dominates however much loop gain is applied.

Discuss in Forum

79.

Which of the following does negative feedback not reduce?

The closed-loop gain's dependence on the feedback resistors

The dependence on the open-loop gain

The internal distortion of the amplifier

The output impedance under voltage sampling

A.The internal distortion of the amplifier
B.The output impedance under voltage sampling
C.The closed-loop gain's dependence on the feedback resistors
D.The dependence on the open-loop gain

Answer: C

The dependence on the feedback resistors is if anything increased: at high loop gain the closed-loop gain is set by them alone, so their tolerance appears undiminished in the result. The other three are all reduced by one plus the loop gain. Feedback does not remove an accuracy requirement but relocates it, from the devices to a few passive components - which is worth having only because those components can be made good. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

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80.

Why is the feedback network usually made from resistors rather than active components?

A.Active components would draw too much supply current
B.Resistors have a wider bandwidth than any device
C.Resistors introduce no phase shift at any frequency
D.Its accuracy sets the closed-loop gain directly

Answer: D

Since the closed-loop gain is the reciprocal of the fraction, the network's own accuracy appears in the answer with no reduction, so it must be built from the most predictable components available. Supply current is trivial either way, and neither the bandwidth nor the phase claim is strictly true, since stray reactances affect resistors too. The whole point of feedback is to transfer accuracy to the network, and that is only worthwhile if the network deserves the trust.

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