Power Amplifiers MCQs

Practice Power Amplifiers MCQs with four options, answers and explanations for focused Electronics & Telecommunication Engineering revision.

Question 1

Electronic Devices & CircuitsPower Amplifiers

The maximum theoretical collector efficiency of an ideal resistively loaded class-A power amplifier is approximately?

A25%
B50%
C78.5%
D100%

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Correct Answer: A. 25%

Explanation: For a conventional series-fed resistive class-A power amplifier, the maximum theoretical collector efficiency is 25%. Transformer-coupled class-A operation can ideally reach 50%.

Reference: Standard power electronics: class-A amplifier efficiency

Question 2

Electronic Devices & CircuitsPower Amplifiers

The maximum theoretical efficiency of an ideal class-B push-pull power amplifier is approximately?

A25%
B50%
C78.5%
D100%

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Correct Answer: C. 78.5%

Explanation: The ideal maximum efficiency of a class-B push-pull amplifier is pi/4, or approximately 78.5%.

Reference: Standard analog electronics: class-B efficiency

Question 3

Electronic Devices & CircuitsPower Amplifiers

Crossover distortion in a class-B push-pull amplifier occurs mainly near?

AThe zero crossing of the output waveform
BThe positive supply rail only
CThe highest input frequency only
DDC cutoff frequency only

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Correct Answer: A. The zero crossing of the output waveform

Explanation: Near zero crossing, neither transistor may conduct until the input exceeds the required base-emitter threshold, producing a dead zone and crossover distortion.

Reference: Standard analog electronics: class-B crossover distortion

Question 4

Electronic Devices & CircuitsPower Amplifiers

Class-AB operation is commonly used instead of pure class-B operation primarily to?

AReduce crossover distortion
BEliminate all power dissipation
CMake voltage gain exactly zero
DOperate both devices permanently in cutoff

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Correct Answer: A. Reduce crossover distortion

Explanation: Class-AB biases the output devices slightly on around zero crossing, substantially reducing crossover distortion while retaining better efficiency than class A.

Reference: Standard analog electronics: class-AB amplifier

Question 5

Analog ElectronicsPower Amplifiers

The maximum theoretical efficiency of an ideal class-B push-pull amplifier is approximately?

A25%
B50%
C78.5%
D100%

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Correct Answer: C. 78.5%

Explanation: The ideal maximum class-B push-pull efficiency is π/4, approximately 78.5%.

Reference: Standard analog electronics: power amplifiers

Question 6

Analog ElectronicsPower Amplifiers

Compared with class-B operation, class-AB biasing is mainly used to reduce?

AVoltage gain
BCrossover distortion
CInput impedance
DSupply voltage

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Correct Answer: B. Crossover distortion

Explanation: Class-AB keeps the output devices slightly conducting around zero crossing, reducing crossover distortion.

Reference: Standard analog electronics: power amplifiers

Question 7

Analog ElectronicsPower Amplifiers

Which power-amplifier class normally offers the best linearity but relatively poor efficiency?

AClass A
BClass B
CClass C
DSwitching class D

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Correct Answer: A. Class A

Explanation: Class-A operation keeps the device conducting throughout the cycle, providing high linearity at the cost of substantial quiescent power dissipation.

Reference: Standard analog electronics: power amplifiers

Question 8

Analog ElectronicsPower Amplifiers

The purpose of a heat sink attached to a power transistor is primarily to?

AIncrease junction temperature
BReduce thermal resistance from the device to ambient
CIncrease transistor beta exactly
DProvide positive feedback

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Correct Answer: B. Reduce thermal resistance from the device to ambient

Explanation: A heat sink improves heat transfer away from the device, lowering junction temperature for a given power dissipation.

Reference: Standard analog electronics: power amplifiers

Question 9

Analog ElectronicsPower Amplifiers

A transistor dissipates 2 W. If the total junction-to-ambient thermal resistance is 25 degrees C/W and ambient temperature is 30 degrees C, the estimated junction temperature is?

A50 degrees C
B55 degrees C
C80 degrees C
D105 degrees C

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Correct Answer: C. 80 degrees C

Explanation: Temperature rise = P times thermal resistance = 2×25 = 50 degrees C. Junction temperature = 30+50 = 80 degrees C.

Reference: Standard analog electronics: power amplifiers

Question 10

Analog ElectronicsPower Amplifiers

A class-A amplifier conducts collector current for approximately what portion of an input cycle?

A90 degrees
B180 degrees
C270 degrees
D360 degrees

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Correct Answer: D. 360 degrees

Explanation: A class-A output device conducts throughout the entire 360-degree signal cycle.

Reference: Standard analog electronics: power amplifiers

Question 11

Analog ElectronicsPower Amplifiers

In ideal class-B operation, each transistor in a push-pull pair conducts for approximately?

A90 degrees
B180 degrees
C270 degrees
D360 degrees

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Correct Answer: B. 180 degrees

Explanation: Each class-B device conducts for one half-cycle, corresponding to approximately 180 degrees.

Reference: Standard analog electronics: power amplifiers

Question 12

Analog ElectronicsPower Amplifiers

A class-C amplifier has a conduction angle that is typically?

ALess than 180 degrees
BExactly 180 degrees
CExactly 270 degrees
DExactly 360 degrees

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Correct Answer: A. Less than 180 degrees

Explanation: Class-C devices conduct for less than half of each input cycle and are commonly used with tuned loads in RF applications.

Reference: Standard analog electronics: power amplifiers

Question 13

Electronic Devices & CircuitsPower Amplifiers

In a class-B complementary push-pull output stage, crossover distortion occurs mainly near?

AThe zero crossing where both transistors may be insufficiently conducting
BThe positive supply rail only
CThe maximum power point only
DThe transistor breakdown voltage only

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Correct Answer: A. The zero crossing where both transistors may be insufficiently conducting

Explanation: Near zero input, neither transistor may conduct until its base-emitter threshold is reached, producing a dead zone and crossover distortion.

Reference: Standard analog electronics: class-B crossover distortion

Question 14

Electronic Devices & CircuitsPower Amplifiers

Biasing a complementary push-pull output stage slightly into conduction changes class B toward class AB mainly to?

AReduce crossover distortion
BEliminate all power dissipation
CForce both devices permanently into cutoff
DCreate negative resistance

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Correct Answer: A. Reduce crossover distortion

Explanation: Class-AB bias keeps both output devices slightly conducting around zero crossing, reducing the dead zone responsible for crossover distortion.

Reference: Standard analog electronics: class-AB operation

Question 15

Electronic Devices & CircuitsPower Amplifiers

The maximum theoretical efficiency of an ideal class-B push-pull power amplifier with a sinusoidal output is approximately?

A25%
B50%
C78.5%
D100%

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Correct Answer: C. 78.5%

Explanation: For an ideal class-B push-pull stage delivering a sinusoid at maximum undistorted swing, the theoretical maximum efficiency is π/4 ≈ 78.5%.

Reference: Standard analog electronics: class-B efficiency

Question 16

Electronic Devices & CircuitsPower Amplifiers

In a class-A power amplifier, the active device ideally conducts for what portion of the input cycle?

A90 degrees
B180 degrees
C270 degrees
D360 degrees

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Correct Answer: D. 360 degrees

Explanation: Class-A operation keeps the active device conducting throughout the complete 360-degree signal cycle.

Reference: Standard analog electronics: amplifier classes

Question 17

Electronic Devices & CircuitsPower Amplifiers

A class-C amplifier conducts for a portion of the input cycle that is typically?

AGreater than 360 degrees
BExactly 360 degrees
CLess than 180 degrees
DExactly 180 degrees

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Correct Answer: C. Less than 180 degrees

Explanation: Class-C bias allows device current for less than half of each cycle. Tuned circuits are commonly used to recover a sinusoidal output in RF applications.

Reference: Standard analog electronics: class-C operation

Question 18

Analog ElectronicsPower Amplifiers

In an ideal class-B push-pull amplifier delivering a sinusoid, the maximum theoretical efficiency is approximately?

A50%
B66.7%
C78.5%
D100%

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Correct Answer: C. 78.5%

Explanation: For ideal class-B push-pull operation at maximum sinusoidal output, eta_max=pi/4≈0.785 or 78.5%.

Reference: Standard analog electronics: class-B efficiency

Question 19

Analog ElectronicsPower Amplifiers

A class-AB output stage is biased so that each output device conducts for?

ALess than 180 degrees
BExactly 180 degrees
CMore than 180 degrees but less than 360 degrees
DExactly 360 degrees in every design

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Correct Answer: C. More than 180 degrees but less than 360 degrees

Explanation: Class-AB operation lies between class A and class B; each device conducts for more than half a cycle but less than the full cycle.

Reference: Standard analog electronics: class-AB conduction angle

Question 20

Analog ElectronicsPower Amplifiers

For a transistor dissipating 3 W with total junction-to-ambient thermal resistance of 20 degrees C/W at an ambient temperature of 30 degrees C, the estimated junction temperature is?

A50 degrees C
B60 degrees C
C90 degrees C
D110 degrees C

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Correct Answer: C. 90 degrees C

Explanation: The temperature rise is P×theta=3×20=60 degrees C. Adding 30 degrees C ambient gives an estimated junction temperature of 90 degrees C.

Reference: Standard analog electronics: thermal calculation