Feedback Amplifiers Questions and Answers

Q: What does the feedback fraction describe?

The fraction, usually written as beta, is the proportion of the output that the network returns to the input, and its reciprocal is the closed-loop gain a large loop gain approaches. The ratio of the two gains is one over one plus the loop gain, a related but distinct quantity. Keeping the fraction, the loop gain and the gain ratio separate is essential, because all three are dimensionless numbers describing the same circuit and they are easily interchanged.

Q: What is meant by the return ratio of a loop?

The return ratio is the loop gain under another name, measured by breaking the loop, injecting a signal and observing what comes back. It is worth knowing because the measured quantity and the calculated one should agree, and a discrepancy points at a path not accounted for. The remaining options describe impedance ratios and bandwidth ratios that have their own names. Alternative names for one quantity are a nuisance, and recognising them saves treating one result as two.

Q: Why is the loop gain measured by breaking the loop rather than computed?

Stray coupling, supply impedance and layout all add paths that no calculation from the schematic would include, so a measured return ratio is the only trustworthy figure in a marginal case. The open-loop gain can be measured with care, the fraction is set by components, and the closed-loop gain is measured directly anyway. That a real loop contains more than the drawn loop is why stability problems so often surprise, and why measurement rather than analysis settles them.

Q: What is meant by conditional stability?

A conditionally stable loop is stable as built but would oscillate if its loop gain were reduced, because the phase dips past inversion at a frequency where the gain is still above unity. The Bode margins can look adequate while the Nyquist plot reveals the problem, which is exactly the case simple rules of thumb fail on. Since gain falls as a circuit warms or ages, such a loop can become unstable in service.

Q: Why is a Nyquist plot needed for a conditionally stable loop?

The Nyquist test asks whether the locus encircles the critical point, which settles cases where the phase crosses inversion more than once and a single margin figure is misleading. It is no more accurate about phase margin, covers the same frequencies, and shows only the loop gain. A general criterion is worth reaching for precisely in the cases where the rules derived from it break down, and conditional stability is the standard example. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Q: A loop's gain is \( 40 \ \mathrm{dB} \) at the frequency where its phase reaches inversion. Is it stable?

Stability requires the gain to be below unity where the phase inverts, and forty decibels is a factor of a hundred above it - so the loop will oscillate. A positive gain margin means the gain has fallen below one, which is a negative figure in decibels. Reading the sign of a decibel margin correctly is the whole of this item, and getting it backwards turns a certain oscillator into a claimed amplifier.

Q: What is a sensible minimum phase margin for a well-behaved amplifier?

Forty-five degrees is the usual minimum, giving a step response with modest overshoot and no sustained ringing; sixty degrees is preferred where the response must be clean. Ten degrees would ring badly, ninety is achievable only at considerable cost in bandwidth, and one hundred and eighty is the whole budget rather than a margin. A specification chosen as a compromise between speed and cleanliness is worth knowing as a number rather than derived each time.

Q: What does an amplifier described as unity-gain stable guarantee?

Unity-gain stable means the internal compensation gives an adequate margin in the worst case, with a feedback fraction of one and hence the largest possible loop gain. It says nothing about the accuracy of a follower's gain, does not guarantee any particular bandwidth figure, and certainly does not cover any load - a capacitive load can destabilise the most carefully compensated device. A guarantee's exact scope matters, and this one covers the feedback fraction rather than the load.

Q: Why can a capacitive load destabilise a feedback amplifier?

The load works against the output impedance to form an extra pole, and since the output is inside the feedback loop that pole's phase is added to the loop's own. It does not change the low-frequency loop gain or the fraction, and the input is unaffected. Anything that adds phase shift inside a loop eats into its margin, whether it was part of the design or attached afterwards, which is why load conditions belong in a stability specification. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Q: What is the usual remedy for a feedback amplifier driving a capacitive load?

Placing a small resistor between the output and the load, with the feedback taken from inside it, keeps the load's pole outside the loop where its phase does no harm. A capacitor across the feedback resistor is a useful lead network for a different problem, a larger feedback resistor changes the gain, and headroom is unrelated. Moving a troublesome element outside a loop is often easier than compensating for it inside, and it costs only a little output impedance.

81.

What does the feedback fraction describe?

A.The part of the output returned to the input
B.The part of the input that reaches the output
C.The ratio of closed-loop to open-loop gain
D.The reduction in gain caused by the feedback

Answer: A

The fraction, usually written as beta, is the proportion of the output that the network returns to the input, and its reciprocal is the closed-loop gain a large loop gain approaches. The ratio of the two gains is one over one plus the loop gain, a related but distinct quantity. Keeping the fraction, the loop gain and the gain ratio separate is essential, because all three are dimensionless numbers describing the same circuit and they are easily interchanged.

Discuss in Forum

82.

What is meant by the return ratio of a loop?

A.The ratio of open-loop to closed-loop bandwidth
B.Another name for the loop gain
C.The ratio of the two impedances at the input
D.The fraction of the output current returned

Answer: B

The return ratio is the loop gain under another name, measured by breaking the loop, injecting a signal and observing what comes back. It is worth knowing because the measured quantity and the calculated one should agree, and a discrepancy points at a path not accounted for. The remaining options describe impedance ratios and bandwidth ratios that have their own names. Alternative names for one quantity are a nuisance, and recognising them saves treating one result as two.

Discuss in Forum

83.

Why is the loop gain measured by breaking the loop rather than computed?

A.The feedback fraction changes when the loop is closed
B.Measurement gives the closed-loop gain more accurately
C.Unintended paths contribute and are hard to predict
D.The open-loop gain cannot be measured directly at all

Answer: C

Stray coupling, supply impedance and layout all add paths that no calculation from the schematic would include, so a measured return ratio is the only trustworthy figure in a marginal case. The open-loop gain can be measured with care, the fraction is set by components, and the closed-loop gain is measured directly anyway. That a real loop contains more than the drawn loop is why stability problems so often surprise, and why measurement rather than analysis settles them.

Discuss in Forum

84.

What is meant by conditional stability?

A.A loop that oscillates only above a certain temperature
B.A loop whose stability depends on the supply voltage
C.A loop stable only when its input is within range
D.A loop stable at its design gain but not at all lower gains

Answer: D

A conditionally stable loop is stable as built but would oscillate if its loop gain were reduced, because the phase dips past inversion at a frequency where the gain is still above unity. The Bode margins can look adequate while the Nyquist plot reveals the problem, which is exactly the case simple rules of thumb fail on. Since gain falls as a circuit warms or ages, such a loop can become unstable in service.

Discuss in Forum

85.

Why is a Nyquist plot needed for a conditionally stable loop?

A.It shows encirclement that margins alone can miss
B.It gives the phase margin more accurately than a Bode plot
C.It works at frequencies a Bode plot cannot represent
D.It shows the closed-loop gain as well as the loop gain

Answer: A

The Nyquist test asks whether the locus encircles the critical point, which settles cases where the phase crosses inversion more than once and a single margin figure is misleading. It is no more accurate about phase margin, covers the same frequencies, and shows only the loop gain. A general criterion is worth reaching for precisely in the cases where the rules derived from it break down, and conditional stability is the standard example. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Discuss in Forum

86.

A loop's gain is \( 40 \ \mathrm{dB} \) at the frequency where its phase reaches inversion. Is it stable?

A.It cannot be determined without the closed-loop gain
B.No, the gain is far above unity there
C.Yes, provided the phase margin exceeds forty-five degrees
D.Yes, since forty decibels is a large gain margin

Answer: B

Stability requires the gain to be below unity where the phase inverts, and forty decibels is a factor of a hundred above it - so the loop will oscillate. A positive gain margin means the gain has fallen below one, which is a negative figure in decibels. Reading the sign of a decibel margin correctly is the whole of this item, and getting it backwards turns a certain oscillator into a claimed amplifier.

Discuss in Forum

87.

What is a sensible minimum phase margin for a well-behaved amplifier?

A.Ninety degrees
B.One hundred and eighty degrees
C.Forty-five degrees
D.Ten degrees

Answer: C

Forty-five degrees is the usual minimum, giving a step response with modest overshoot and no sustained ringing; sixty degrees is preferred where the response must be clean. Ten degrees would ring badly, ninety is achievable only at considerable cost in bandwidth, and one hundred and eighty is the whole budget rather than a margin. A specification chosen as a compromise between speed and cleanliness is worth knowing as a number rather than derived each time.

Discuss in Forum

88.

What does an amplifier described as unity-gain stable guarantee?

A.A closed-loop gain of exactly one when connected as a follower
B.A bandwidth equal to its gain-bandwidth product
C.Freedom from oscillation with any load whatever
D.Adequate margin with the whole output fed back

Answer: D

Unity-gain stable means the internal compensation gives an adequate margin in the worst case, with a feedback fraction of one and hence the largest possible loop gain. It says nothing about the accuracy of a follower's gain, does not guarantee any particular bandwidth figure, and certainly does not cover any load - a capacitive load can destabilise the most carefully compensated device. A guarantee's exact scope matters, and this one covers the feedback fraction rather than the load.

Discuss in Forum

89.

Why can a capacitive load destabilise a feedback amplifier?

A.It adds a pole inside the loop
B.It reduces the loop gain at low frequency
C.It raises the feedback fraction near crossover
D.It increases the amplifier's input capacitance

Answer: A

The load works against the output impedance to form an extra pole, and since the output is inside the feedback loop that pole's phase is added to the loop's own. It does not change the low-frequency loop gain or the fraction, and the input is unaffected. Anything that adds phase shift inside a loop eats into its margin, whether it was part of the design or attached afterwards, which is why load conditions belong in a stability specification. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Discuss in Forum

90.

What is the usual remedy for a feedback amplifier driving a capacitive load?

A.A higher supply voltage for more headroom
B.A small series resistor outside the loop
C.A larger feedback resistor to reduce the fraction
D.A capacitor across the feedback resistor

Answer: B

Placing a small resistor between the output and the load, with the feedback taken from inside it, keeps the load's pole outside the loop where its phase does no harm. A capacitor across the feedback resistor is a useful lead network for a different problem, a larger feedback resistor changes the gain, and headroom is unrelated. Moving a troublesome element outside a loop is often easier than compensating for it inside, and it costs only a little output impedance.

Discuss in Forum

Loading…

Topics

Other Subjects

Tags: Feedback Amplifiers Questions and Answers || Feedback Amplifiers MCQ Questions and Answers || Feedback Amplifiers GK Questions and Answers || Feedback Amplifiers GK MCQ Questions || Feedback Amplifiers Multiple Choice Questions and Answers || Feedback Amplifiers GK || GK on Feedback Amplifiers || Analog Electronics Questions and Answers || Analog Electronics MCQ Questions and Answers || Analog Electronics GK Questions and Answers || GK on Analog Electronics

Feedback Amplifiers Questions and Answers

Topics

Other Subjects

About Us

Stay informed about all important Dates & Notifications