Feedback Amplifiers Questions and Answers

Answer: A

Loop gain is the open-loop gain multiplied by the fraction fed back, and it is the single quantity that decides how much every benefit of feedback amounts to. The gain with the loop broken is the open-loop gain, and the returned fraction is the feedback factor alone. Because desensitivity, impedance change, distortion reduction and bandwidth extension all scale with one plus the loop gain, computing that one number first tells you how much the feedback is doing. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: B

The feedback equation gives the open-loop gain divided by one plus the loop gain, which is the whole of the subject in one expression. Multiplying by the fraction gives the loop gain itself, and the remaining options describe nothing standard. Every consequence of feedback follows from this division, and it is worth being able to write it down before anything else in the topic is attempted.

Answer: C

With a large loop gain the one in the denominator becomes negligible and the closed-loop gain tends to one over the feedback fraction, which is set by passive components alone. That is the central result: the amplifier's own gain drops out of the answer entirely. It does not approach unity unless the fraction happens to be one, as in a follower. Arranging for a performance figure to depend on resistors rather than on devices is what feedback is for.

Answer: D

Applying the feedback equation:

\[ A_{f} = \frac{10000}{1 + 100} \approx 99 \]

giving about 99, within one per cent of the ideal hundred that the reciprocal of the fraction predicts. That the approximation is good to one per cent when the loop gain is a hundred is exactly the trade being made: a little accuracy for a great deal of predictability.

Answer: A

Recomputing with the reduced open-loop gain:

\[ A_{f} = \frac{5000}{1 + 50} \approx 98 \]

so about 98 - a fall of one per cent for a halving of the amplifier's own gain. That is desensitivity in a single calculation, and it is why a feedback amplifier can be built from devices whose parameters spread threefold and still meet a one per cent specification.

Answer: B

Desensitivity is the factor of one plus the loop gain by which a change in open-loop gain is attenuated at the output - the property that makes feedback worth its cost in gain. The closed-loop gain does fall, but that is the price rather than the benefit. Input impedance changes and bandwidth rises rather than falling. Naming the benefit separately from the price is what makes the trade discussable.

Answer: C

The sensitivity is reduced by one plus the loop gain:

\[ \frac{10\%}{200} = 0.05\% \]

giving about 0.05 per cent. Ten per cent would be the figure with no feedback at all. A loop gain of two hundred divides every open-loop imperfection by two hundred, which is why designers spend gain so freely on feedback.

Answer: D

The fed-back fraction subtracts from the input, so the device sees a smaller error signal and the overall gain falls - which is the same mechanism that produces every other benefit. The network's attenuation sets the fraction rather than causing the loss, the operating current is unchanged, and the bandwidth extension is a consequence rather than a cause. One mechanism producing one cost and several benefits is why feedback is so nearly universal. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Answer: A

There are four, formed by sampling either the output voltage or the output current and returning it either in series or in shunt with the input. Two choices at each end give four combinations, and each one raises or lowers a different pair of impedances. Enumerating the possibilities from the two independent choices, rather than learning four names, is what makes the set easy to hold and easy to reconstruct when a circuit has to be identified. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: B

A network connected across the load samples the voltage there, while one in series with the load carries the load current and so samples that. Opposition is what makes the feedback negative rather than deciding what is sampled, inversion is a property of the amplifier, and the resistor value sets the fraction. Reading a diagram means asking which quantity the network is physically in a position to measure, and the connection settles it. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.