Feedback Amplifiers Questions and Answers

Answer: C

The criterion is that a loop with unity gain and zero net phase shift around it sustains an oscillation - which is a design target for an oscillator and a condition to be avoided in an amplifier. The same statement thus appears in two topics with opposite intent. Phase margin and roll-off rate are practical measures derived from it rather than the criterion itself, and recognising the shared root explains why the two subjects use the same mathematics.

Answer: D

Phase margin is how many degrees short of one hundred and eighty the phase has reached at the frequency where the loop gain passes through unity, and forty-five degrees or more is usually required. The second option describes gain margin, measured at a different frequency. Two margins measured at two frequencies, each reporting the loop's distance from oscillation, and quoting one where the other is meant makes a stability claim meaningless. Working one numerical case through both cases side by side fixes the distinction better than any rule of thumb.

Answer: A

Gain margin is how far below unity the loop gain has fallen at the frequency where the phase reaches inversion, quoted in decibels. It is the companion to phase margin, measured at the other of the two critical frequencies. The difference between open and closed-loop gains is the loop gain, and the gain at the break is fixed by definition. A loop can be comfortable on one margin and marginal on the other, which is exactly why both are specified. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: B

One pole gives ninety degrees at most and so is unconditionally stable, while a second pole approaching its own break pushes the phase towards inversion and the margin disappears. Three is worse still, and a high gain makes matters worse rather than better by pushing the unity-gain frequency higher. The whole of dominant-pole compensation is an arrangement to make sure only one pole is encountered before the gain has gone. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Answer: C

More loop gain pushes the frequency at which the gain falls to unity further up the response, where more poles have taken effect and the phase has shifted further. The poles themselves do not move and the phase at a given frequency is unchanged. This is why an amplifier stable at a closed-loop gain of a hundred can oscillate at unity gain, and why data sheets specify the minimum gain at which a device is stable.

Answer: D

With the whole output returned, the feedback fraction is one and the loop gain equals the open-loop gain, so unity loop gain occurs at the highest possible frequency. The network's impedance and the input impedance are not the issue, and the output current depends on the load. A device described as unity-gain stable has been compensated for exactly this worst case, which is why it is slower than an uncompensated one used at higher gain. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: A

The Nyquist test plots the loop gain in the complex plane and asks whether the locus encircles the point corresponding to unity gain and inverted phase. It settles cases that a Bode reading cannot, such as conditionally stable loops. Phase margin and roll-off rate are practical rules derived from it, and closed-loop flatness is a different specification. A general criterion and the rules of thumb drawn from it should not be confused, since the rules fail in exactly the awkward cases.

Answer: B

It deliberately moves one pole down so the gain has fallen to unity before any second pole contributes phase, guaranteeing a wide margin at the cost of bandwidth. Pole-zero cancellation is a different and more delicate approach, poles cannot be moved above the unity-gain frequency at will, and the low-frequency gain is left intact. Making one effect deliberately dominant so the others need not be controlled precisely is a robust strategy well beyond amplifiers.

Answer: C

A lag network adds a pole, pulling the gain down earlier so that unity gain is reached before the amplifier's own poles have shifted the phase - the mechanism of dominant-pole compensation. A lead network is the one that adds a zero and recovers phase. Coincident poles and zeros cancel and do nothing. Two networks with opposite effects, and choosing between them means deciding whether to reduce the crossover frequency or to improve the phase at it.

Answer: D

A lead network adds a zero whose ninety degrees of phase lead offsets a pole's lag, restoring margin without sacrificing crossover frequency. Adding a pole is what a lag network does. It does not raise the low-frequency gain, and input capacitance is unrelated. Because it buys stability without costing bandwidth, a lead network is preferred where speed matters - at the price of needing the zero placed accurately against a pole that may itself move.