Feedback Amplifiers Questions and Answers

Answer: A

When the gain of an amplifier decreases, its bandwidth increases, so the answer is option (A). The gain-bandwidth product of an amplifier stays practically constant, so trading gain away (as negative feedback does) gives a wider bandwidth.

The bandwidth does not decrease (option B), since that would break the constant product. It does not remain zero (option C), which is meaningless for a working amplifier, and it does not remain unaltered (option D), because the product is fixed, not the bandwidth.

Answer: A

With negative feedback the bandwidth is extended by the factor \(1 + A\beta\), where \(A = 100\) and \(\beta = 0.99\):

The upper cut-off frequency rises and the lower one falls by that factor:

This is option (A). Options B, C and D use factors other than 100, so their cut-off frequencies do not follow from the feedback relations.

Answer: C

Negative feedback reduces the voltage gain, reduces the sensitivity of the gain to changes in the components, and reduces the distortion, so statements (i), (ii) and (iv) are correct, option (C).

Statement (iii) is wrong because negative feedback increases the bandwidth, by the factor \(1 + A\beta\), since the gain-bandwidth product stays constant. Options B and D include (iii), and option A leaves out the reduction of distortion that feedback gives.

Answer: C

The correct match is a-iv, b-iii, c-ii, d-i. Voltage series feedback is the feedback of a voltage amplifier (input voltage, output voltage). Current series feedback senses output current and returns a voltage in series, which is the transconductance amplifier. A current amplifier uses current shunt feedback. Voltage shunt feedback senses voltage and mixes current at the input, which is the trans-resistance amplifier.

The options that give voltage series feedback the trans-resistance or transconductance connection mix up the series and shunt ways of connecting the feedback.

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.