GROL Amplifier and Oscillator Circuits 3 — Questions and Answers
Question 1: What is the theoretical maximum efficiency of a Class A power amplifier?
- 25%
- 50% (Correct answer)
- 78.5%
- 100%
Correct answer: 50%
A Class A amplifier biased at the center of its load line has a theoretical maximum efficiency of 50%, though practical circuits typically achieve far less.
Question 2: Crossover distortion in a Class B push-pull amplifier is caused by:
- Excessive supply voltage applied to the output stage
- A dead zone near zero crossing where neither transistor conducts (Correct answer)
- Mismatched transistors in the push-pull pair
- Insufficient bandwidth of the output coupling transformer
Correct answer: A dead zone near zero crossing where neither transistor conducts
Crossover distortion occurs because each transistor has a turn-on threshold voltage, creating a brief gap in conduction near the zero crossing of the waveform.
Question 3: Why are Class C amplifiers used for RF power amplification but not audio amplification?
- They amplify both positive and negative half-cycles equally well
- They are the most linear amplifier class available
- Their high efficiency works with a resonant tank circuit that reconstructs the full sine wave (Correct answer)
- They introduce less noise than other amplifier classes
Correct answer: Their high efficiency works with a resonant tank circuit that reconstructs the full sine wave
Class C amplifiers are very efficient because they conduct for less than 180° of the cycle; the resonant tank circuit stores energy and restores the complete sine wave output.
Question 4: The conduction angle of a properly biased Class A amplifier is:
- Less than 90°
- Exactly 180°
- 360° (Correct answer)
- Between 180° and 359°
Correct answer: 360°
Class A amplifiers are biased so the transistor conducts for the full 360° of the input cycle — the transistor never cuts off.
Question 5: Class AB amplifiers are preferred over Class B in high-fidelity audio applications because they:
- Operate at 100% efficiency
- Reduce crossover distortion while maintaining better efficiency than Class A (Correct answer)
- Provide higher output power than any other class
- Eliminate the need for bias circuitry
Correct answer: Reduce crossover distortion while maintaining better efficiency than Class A
Class AB biases the transistors slightly into conduction, eliminating the crossover distortion of Class B while still being more efficient than Class A.
Question 6: Which amplifier class is most appropriate for amplifying a single-sideband (SSB) suppressed-carrier signal?
- Class C, for maximum efficiency
- Class D, for switching operation
- Class A or Class AB, for linear operation (Correct answer)
- Class B only, because of its push-pull configuration
Correct answer: Class A or Class AB, for linear operation
SSB signals carry information in amplitude variations and require linear amplification to preserve the modulation envelope; Class A and Class AB provide this linearity.
Question 7: The conduction angle of a Class C amplifier is:
- 360° — the transistor always conducts
- Between 180° and 360°
- Less than 180° (Correct answer)
- Exactly 180°
Correct answer: Less than 180°
Class C amplifiers are biased beyond cutoff so they conduct only during a small portion of the input cycle, giving a conduction angle less than 180°.
GROL Amplifier and Oscillator Circuits 3
What is the theoretical maximum efficiency of a Class A power amplifier?