Feedback Amplifiers Questions and Answers

Answer: C

A returned voltage adds in series with the input source, while a returned current adds in shunt at the input node - and the two produce opposite effects on the input impedance. The amplifier's own impedance and the output sampling are independent choices, and the fraction's size is irrelevant. Since input and output arrangements are chosen independently, the four topologies really are four rather than a smaller set in disguise. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: D

A series-returned voltage opposes the source, so less current flows for the same applied voltage and the input impedance rises by the factor of one plus the loop gain. Shunt return would lower it instead. Nothing is left unchanged by feedback, and the feedback resistance does not set the impedance directly. Series raises and shunt lowers at the input, which is half of the rule that covers all four topologies. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: A

Sampling the output voltage makes the amplifier work to hold that voltage constant whatever the load draws, which is exactly a low output impedance - divided by one plus the loop gain. Sampling the current would raise it instead. The rule at the output is that whichever quantity is sampled is stabilised, and stabilising a voltage means a low impedance while stabilising a current means a high one.

Answer: B

Voltage series gives a high input impedance and a low output impedance, so it loads its source lightly and holds its output voltage steady as the load changes - the specification of a voltage amplifier exactly. The other three suit a transconductance, a transresistance and a current amplifier respectively. Each topology produces one of the four possible impedance combinations, so naming the amplifier wanted names the topology. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: C

Current shunt lowers the input impedance and raises the output impedance, which is what a current amplifier needs: it accepts a current from a source without loading it in voltage terms and delivers a current independent of the load. The pattern is consistent - shunt at the input lowers, current sampling at the output raises - so the topology can be deduced from the wanted impedances rather than recalled.

Answer: D

Voltage shunt feedback lowers both impedances: low at the input so it accepts a current with negligible voltage, and low at the output so it delivers a voltage regardless of load. That is the transimpedance amplifier, and it is how a photodiode's current is turned into a usable voltage. Reading the two words as instructions - sample the voltage, return in shunt - gives both impedance effects without any memorisation. Naming the cost of a choice alongside its benefit is what turns a fact into a design decision.

Answer: A

Current series feedback raises both impedances: high at the input so it senses a voltage without loading, and high at the output so it delivers a current independent of load. That is a transconductance amplifier, and an emitter resistor left unbypassed is its commonest form. The four topologies cover the four possible input-output conversions exactly, which is why there are neither more nor fewer of them.

Answer: B

The pairing is A-P, B-Q, C-R, and every entry follows from two independent rules: series at the input raises and shunt lowers, while voltage sampling at the output lowers and current sampling raises. Two rules generate all four topologies' effects, so the table can be reconstructed rather than learned - which is far more reliable when a circuit has to be identified under pressure.

Answer: C

The resistor carries the emitter current, so it samples the output current, and its voltage appears in series with the base-emitter input - current series feedback. That explains everything degeneration does: the gain falls, the input impedance rises, and the output impedance rises. Recognising a familiar circuit as one of the four topologies means every general result about that topology applies to it at once.

Answer: D

The whole output voltage is returned in series with the input, so the follower is voltage series feedback with a fraction of one - which is why its gain is so close to unity, its input impedance so high and its output impedance so low. All three properties follow from the topology rather than needing separate derivation. A circuit with complete feedback of one kind is the extreme case of that kind, and it makes a good check on the general results.