Differential Amplifiers Questions and Answers

Q: What does hysteresis added around a comparator provide?

A little positive feedback moves the threshold after each transition, so a noisy input crossing slowly cannot cause the output to chatter. Offset, common-mode range and speed are unaffected, though the switching does become cleaner rather than faster. The amount needed follows from the expected noise amplitude, which makes it one of the few design parameters chosen directly from a measurement of the environment.

Q: Assertion (A): Assertion: A differential pair rejects supply ripple well. Reason (R): Reason: Ripple moves both sides of the pair together and so appears as a common-mode signal. Select the correct answer from the options below.

Both are true and the second is the reason: a symmetrical arrangement discards anything affecting both halves equally, and supply movement does exactly that. It is the same mechanism that rejects interference picked up on a balanced cable. Recognising that one property of the pair accounts for its rejection of shared inputs, of supply noise and of temperature drift explains why the arrangement is so nearly universal in precision circuits.

Q: Assertion (A): Assertion: Replacing the tail resistor with a current source raises the differential gain. Reason (R): Reason: A current source has a much higher dynamic impedance than a resistor. Select the correct answer from the options below.

The reason is true but the assertion is false: the tail impedance appears in the common-mode gain and not in the differential gain, so what improves is the rejection ratio rather than the gain. A differential signal steers current between the two sides without changing the total, so the tail sees nothing. Knowing which gain a component appears in is what prevents an improvement being claimed for the wrong specification.

Q: Assertion (A): Assertion: Trimming a pair's offset to zero removes its drift. Reason (R): Reason: Offset arises from mismatch between the two devices. Select the correct answer from the options below.

The reason is correct but the assertion is false: the trim cancels the mismatch at one temperature, and as the devices heat the mismatch changes, so the cancellation decays. Drift is quoted separately in microvolts per degree precisely because it cannot be trimmed away. A correction applied at one operating point holds only near that point, which is a general limitation of trimming and the reason low-drift parts command a premium.

Q: Consider the following statements about the differential pair. 1. The tail impedance determines the common-mode gain 2. The collector loads and transconductance set the differential gain 3. Offset voltage arises from the tail current source Which of the statements given above is or are correct?

The first two are correct and the third misattributes: offset comes from mismatch between the two devices and their loads, not from the tail source, which affects both sides equally and so cancels. Sorting the pair's specifications by which component determines each is what makes the circuit designable - the tail for rejection, the loads for gain, and matching for offset. Most errors here come from carrying a result across from a neighbouring case where it does not hold. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Q: Consider the following statements about common-mode rejection. 1. A differential output rejects better than a single-ended one 2. An active load recovers most of what a single-ended output loses 3. Rejection improves as the collector loads are made larger Which of the statements given above is or are correct?

The first two are correct and the third is not: the collector load appears in both gains and largely cancels out of the ratio, so making it larger raises the gain without much improving the rejection. The tail impedance is what governs the ratio. A change that moves both terms of a ratio in the same direction cannot much improve it, and spotting that saves proposing it.

Q: Statement I: A differential input steers current from one device to the other. Statement II: A common-mode input steers current from one device to the other. In the light of the above statements, choose the correct answer.

The first is true and the second is false: a common-mode input moves both devices together, and with a good tail source the total current is unchanged and nothing is steered. The distinction is the whole of the pair's behaviour: differential inputs redistribute a fixed current, common-mode inputs try to change it and are prevented from doing so. Every property of the stage follows from those two responses.

Q: Match each specification with the component that limits it. List I A. Common-mode rejection B. Input offset voltage C. Slew rate List II P. The tail source's impedance Q. Matching between the two devices R. The tail current and the compensation capacitance Select the correct match.

The pairing is A-P, B-Q, C-R, and it is a design guide as much as a table: improve the tail source for rejection, improve the matching for offset, and raise the tail current or reduce the compensation for speed. That the tail current source appears in two of the three, for quite different reasons, is why it receives so much attention in an integrated design.

41.

What does hysteresis added around a comparator provide?

A.Clean switching on a slow or noisy input
B.A lower input offset voltage
C.A wider input common-mode range
D.A faster response to a large overdrive

Answer: A

A little positive feedback moves the threshold after each transition, so a noisy input crossing slowly cannot cause the output to chatter. Offset, common-mode range and speed are unaffected, though the switching does become cleaner rather than faster. The amount needed follows from the expected noise amplitude, which makes it one of the few design parameters chosen directly from a measurement of the environment.

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42.

Why is a differential pair used at the front of a balanced audio input?

A.The pair removes the need for a coupling capacitor
B.Interference picked up equally on both lines is rejected
C.The pair provides more voltage gain than a single stage
D.The pair's input impedance is lower and better matched

Answer: B

A long cable picks up interference on both conductors equally, so it arrives as a common-mode signal and the pair discards it while amplifying the wanted difference. The gain is comparable to a single stage per device, impedance is a design choice, and coupling is still needed. Arranging a system so that the interference is common and the signal is differential is what makes balanced working effective, and the amplifier is only half of it. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

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43.

What must a balanced cable do for that rejection to work?

A.Carry the signal on one conductor only
B.Be terminated in its characteristic impedance
C.Expose both conductors to interference equally
D.Present a low impedance to earth at both ends

Answer: C

The rejection depends on the interference being genuinely common, so the two conductors must be twisted together and see the same field, which is what a twisted pair achieves. Low impedance to earth, single-ended carriage and characteristic termination describe other arrangements. A technique that discards what is common only works if the unwanted signal really is common, and the cable is what makes it so.

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44.

Assertion (A): Assertion: A differential pair rejects supply ripple well.

Reason (R): Reason: Ripple moves both sides of the pair together and so appears as a common-mode signal.

Select the correct answer from the options below.

A.Both A and R are true, but R is not the correct explanation of A
B.A is true but R is false
C.A is false but R is true
D.Both A and R are true, and R is the correct explanation of A

Answer: D

Both are true and the second is the reason: a symmetrical arrangement discards anything affecting both halves equally, and supply movement does exactly that. It is the same mechanism that rejects interference picked up on a balanced cable. Recognising that one property of the pair accounts for its rejection of shared inputs, of supply noise and of temperature drift explains why the arrangement is so nearly universal in precision circuits.

Discuss in Forum

45.

Assertion (A): Assertion: Replacing the tail resistor with a current source raises the differential gain.

Reason (R): Reason: A current source has a much higher dynamic impedance than a resistor.

Select the correct answer from the options below.

A.A is false but R is true
B.Both A and R are true, and R is the correct explanation of A
C.Both A and R are true, but R is not the correct explanation of A
D.A is true but R is false

Answer: A

The reason is true but the assertion is false: the tail impedance appears in the common-mode gain and not in the differential gain, so what improves is the rejection ratio rather than the gain. A differential signal steers current between the two sides without changing the total, so the tail sees nothing. Knowing which gain a component appears in is what prevents an improvement being claimed for the wrong specification.

Discuss in Forum

46.

Assertion (A): Assertion: Trimming a pair's offset to zero removes its drift.

Reason (R): Reason: Offset arises from mismatch between the two devices.

Select the correct answer from the options below.

A.A is true but R is false
B.A is false but R is true
C.Both A and R are true, and R is the correct explanation of A
D.Both A and R are true, but R is not the correct explanation of A

Answer: B

The reason is correct but the assertion is false: the trim cancels the mismatch at one temperature, and as the devices heat the mismatch changes, so the cancellation decays. Drift is quoted separately in microvolts per degree precisely because it cannot be trimmed away. A correction applied at one operating point holds only near that point, which is a general limitation of trimming and the reason low-drift parts command a premium.

Discuss in Forum

47.

Consider the following statements about the differential pair.

1. The tail impedance determines the common-mode gain

2. The collector loads and transconductance set the differential gain

3. Offset voltage arises from the tail current source

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 misattributes: offset comes from mismatch between the two devices and their loads, not from the tail source, which affects both sides equally and so cancels. Sorting the pair's specifications by which component determines each is what makes the circuit designable - the tail for rejection, the loads for gain, and matching for offset. Most errors here come from carrying a result across from a neighbouring case where it does not hold. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Discuss in Forum

48.

Consider the following statements about common-mode rejection.

1. A differential output rejects better than a single-ended one

2. An active load recovers most of what a single-ended output loses

3. Rejection improves as the collector loads are made larger

Which of the statements given above is or are correct?

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

Answer: D

The first two are correct and the third is not: the collector load appears in both gains and largely cancels out of the ratio, so making it larger raises the gain without much improving the rejection. The tail impedance is what governs the ratio. A change that moves both terms of a ratio in the same direction cannot much improve it, and spotting that saves proposing it.

Discuss in Forum

49.

Statement I: A differential input steers current from one device to the other.

Statement II: A common-mode input steers current from one device to the other.

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

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

Answer: A

The first is true and the second is false: a common-mode input moves both devices together, and with a good tail source the total current is unchanged and nothing is steered. The distinction is the whole of the pair's behaviour: differential inputs redistribute a fixed current, common-mode inputs try to change it and are prevented from doing so. Every property of the stage follows from those two responses.

Discuss in Forum

50.

Match each specification with the component that limits it.

List I

A. Common-mode rejection

B. Input offset voltage

C. Slew rate

List II

P. The tail source's impedance

Q. Matching between the two devices

R. The tail current and the compensation capacitance

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, and it is a design guide as much as a table: improve the tail source for rejection, improve the matching for offset, and raise the tail current or reduce the compensation for speed. That the tail current source appears in two of the three, for quite different reasons, is why it receives so much attention in an integrated design.

Discuss in Forum

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