Differential Amplifiers Questions and Answers

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.