Power Amplifiers Questions and Answers

Q: Assertion (A): Assertion: Class B dissipation is greatest at full output. Reason (R): Reason: The output current is largest at full output. Select the correct answer from the options below.

The reason is true but the assertion is false: dissipation is the difference between what the supply delivers and what the load takes, and that difference peaks at roughly two thirds of full swing. At full output most of the power is reaching the load, and at idle almost nothing flows. That the worst thermal case is neither extreme is the single most useful fact for sizing a class B heatsink, and testing only at full power understates the requirement.

Q: Assertion (A): Assertion: Class A dissipation is greatest with no signal applied. Reason (R): Reason: Its quiescent current flows whether or not a signal is present. Select the correct answer from the options below.

Both are true and the second is the reason: the standing current draws constant power from the supply, and whatever is not delivered to the load is dissipated, so idle is the worst case. That is the exact reverse of class B, and it means the two classes are sized from opposite ends of their operating range. Knowing which condition is worst for a given class is what turns a rating into a heatsink.

Q: Consider the following statements about class AB. 1. Both devices conduct for slightly more than half the cycle 2. It removes the crossover distortion of class B 3. Its efficiency exceeds class B's theoretical maximum Which of the statements given above is or are correct?

The first two are correct and the third cannot be: class AB's standing current costs a little efficiency, so it always falls slightly short of class B's 78.5 per cent. It buys much lower distortion for that small loss, which is why nearly every audio output stage is class AB. A claim that a variant beats the ideal it was derived from is worth doubting on that ground alone. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

Q: Statement I: Class B stages produce crossover distortion. Statement II: Class A stages produce crossover distortion. In the light of the above statements, choose the correct answer.

The first is true and the second is false: a class A device never stops conducting, so there is no handover and no gap in the transfer characteristic. Crossover distortion is the price of splitting the cycle between two devices, and class A pays a much larger price in efficiency instead to avoid it. Each class's characteristic defect follows directly from its conduction angle, which is why the angle is the first thing to establish.

Q: Match each class with its maximum efficiency. List I A. Class A, resistive load B. Class A, transformer coupled C. Class B List II P. Twenty-five per cent Q. Fifty per cent R. Seventy-eight and a half per cent Select the correct match.

The pairing is A-P, B-Q, C-R. The first two differ only in their coupling, which shows that an efficiency figure belongs to a class and a coupling arrangement together rather than to the class alone. The third's transcendental value betrays its origin in integrating a half sine, and remembering that makes the odd number easier to recall than a bare figure would be.

Q: Match each protection scheme with the fault it addresses. List I A. V-I limiting B. Output relay C. Emitter ballast List II P. A short across the output Q. Direct voltage reaching the speaker R. Unequal sharing between paralleled devices Select the correct match.

The pairing is A-P, B-Q, C-R. Three schemes addressing three quite different failures, one protecting the amplifier from the user, one protecting the load from the amplifier, and one protecting the devices from each other. A practical output stage carries all three, and recognising which fault each covers explains why none of them can be omitted as redundant.

51.

Assertion (A): Assertion: Class B dissipation is greatest at full output.

Reason (R): Reason: The output current is largest at full output.

Select the correct answer from the options below.

A.Both A and R are true, but R is not the correct explanation of A
B.A is true but R is false
C.A is false but R is true
D.Both A and R are true, and R is the correct explanation of A

Answer: C

The reason is true but the assertion is false: dissipation is the difference between what the supply delivers and what the load takes, and that difference peaks at roughly two thirds of full swing. At full output most of the power is reaching the load, and at idle almost nothing flows. That the worst thermal case is neither extreme is the single most useful fact for sizing a class B heatsink, and testing only at full power understates the requirement.

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52.

Assertion (A): Assertion: Class A dissipation is greatest with no signal applied.

Reason (R): Reason: Its quiescent current flows whether or not a signal is present.

Select the correct answer from the options below.

A.Both A and R are true, but R is not the correct explanation of A
B.A is true but R is false
C.A is false but R is true
D.Both A and R are true, and R is the correct explanation of A

Answer: D

Both are true and the second is the reason: the standing current draws constant power from the supply, and whatever is not delivered to the load is dissipated, so idle is the worst case. That is the exact reverse of class B, and it means the two classes are sized from opposite ends of their operating range. Knowing which condition is worst for a given class is what turns a rating into a heatsink.

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53.

Assertion (A): Assertion: A MOSFET output stage needs emitter ballast resistors.

Reason (R): Reason: Devices in parallel must share current between them.

Select the correct answer from the options below.

A.A is false but R is true
B.Both A and R are true, and R is the correct explanation of A
C.Both A and R are true, but R is not the correct explanation of A
D.A is true but R is false

Answer: A

The reason states a real requirement but the assertion is false for MOSFETs: their on-resistance rises with temperature, so a hot device sheds current to its cooler partners and the sharing corrects itself. Bipolar devices need the ballast because their forward drop falls as they heat, giving positive feedback instead. A requirement that holds for one device family and not another is worth attaching to the mechanism rather than to the arrangement.

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54.

Consider the following statements about class AB.

1. Both devices conduct for slightly more than half the cycle

2. It removes the crossover distortion of class B

3. Its efficiency exceeds class B's theoretical maximum

Which of the statements given above is or are correct?

A.1, 2 and 3
B.1 and 2 only
C.1 and 3 only
D.2 and 3 only

Answer: B

The first two are correct and the third cannot be: class AB's standing current costs a little efficiency, so it always falls slightly short of class B's 78.5 per cent. It buys much lower distortion for that small loss, which is why nearly every audio output stage is class AB. A claim that a variant beats the ideal it was derived from is worth doubting on that ground alone. The quantity that varies and the quantity that does not are worth naming before any substitution is attempted.

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55.

Consider the following statements about thermal design.

1. Junction, interface and heatsink resistances add in series

2. Thermal compound reduces the interface resistance

3. A larger heatsink reduces the junction-to-case resistance

Which of the statements given above is or are correct?

A.2 and 3 only
B.1, 2 and 3
C.1 and 2 only
D.1 and 3 only

Answer: C

The first two are correct and the third confuses the terms: a heatsink reduces only its own resistance to the air, while the junction-to-case path is fixed inside the package. Since the terms add, improving one has diminishing value once it has fallen below the others, which is why an ever larger sink eventually buys nothing. Naming which term a remedy acts on is what makes thermal design systematic.

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56.

Statement I: Class B stages produce crossover distortion.

Statement II: Class A stages produce crossover distortion.

In the light of the above statements, choose the correct answer.

A.Both Statement I and Statement II are correct
B.Both Statement I and Statement II are incorrect
C.Statement I is incorrect but Statement II is correct
D.Statement I is correct but Statement II is incorrect

Answer: D

The first is true and the second is false: a class A device never stops conducting, so there is no handover and no gap in the transfer characteristic. Crossover distortion is the price of splitting the cycle between two devices, and class A pays a much larger price in efficiency instead to avoid it. Each class's characteristic defect follows directly from its conduction angle, which is why the angle is the first thing to establish.

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57.

Match each class with its maximum efficiency.

List I

A. Class A, resistive load

B. Class A, transformer coupled

C. Class B

List II

P. Twenty-five per cent

Q. Fifty per cent

R. Seventy-eight and a half per cent

Select the correct match.

A.A-P, B-Q, C-R
B.A-Q, B-P, C-R
C.A-R, B-Q, C-P
D.A-P, B-R, C-Q

Answer: A

The pairing is A-P, B-Q, C-R. The first two differ only in their coupling, which shows that an efficiency figure belongs to a class and a coupling arrangement together rather than to the class alone. The third's transcendental value betrays its origin in integrating a half sine, and remembering that makes the odd number easier to recall than a bare figure would be.

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58.

Match each protection scheme with the fault it addresses.

List I

A. V-I limiting

B. Output relay

C. Emitter ballast

List II

P. A short across the output

Q. Direct voltage reaching the speaker

R. Unequal sharing between paralleled devices

Select the correct match.

A.A-P, B-R, C-Q
B.A-P, B-Q, C-R
C.A-Q, B-P, C-R
D.A-R, B-Q, C-P

Answer: B

The pairing is A-P, B-Q, C-R. Three schemes addressing three quite different failures, one protecting the amplifier from the user, one protecting the load from the amplifier, and one protecting the devices from each other. A practical output stage carries all three, and recognising which fault each covers explains why none of them can be omitted as redundant.

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59.

Consider the following pairs of class and its characteristic.

Class A - Conducts the whole cycle

Class B - Conducts half the cycle

Class C - Conducts well under half the cycle

Class D - Conducts a quarter of the cycle

How many of the pairs given above are correctly matched?

A.Only two
B.All four
C.Only three
D.Only one

Answer: C

The first three are correctly paired and the fourth is not, so three match. Class D is not described by a conduction angle at all: its devices switch fully on and off at a rate far above the signal frequency, and the information is in the duty cycle. Applying a classification scheme beyond the family it was devised for is the error here, and it is worth noticing that the switching classes needed a new letter precisely because the old description did not fit. A limit that survives making every external component ideal is an intrinsic one, and worth knowing for that reason.

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60.

An amplifier is specified as follows.

Parameter	Value
Supply	plus and minus 35 V
Load	8 ohms
Class	AB

Roughly what output power should be expected?

A.About 150 W
B.About 35 W
C.About 300 W
D.About 70 W

Answer: D

The peak output approaches the rail, less a volt or two of saturation:

\[ P \approx \frac{33^{2}}{16} \approx 68 \ \mathrm{W} \]

so about 70 W into eight ohms. Doubling that would need either twice the supply voltage or half the load impedance, which is why the same amplifier is often quoted at a larger figure into four ohms. Reading a power specification means checking which load it was measured into.

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