# Electronic Devices and Circuits Questions and Answers – Class B Output Stage

This set of Electronic Devices and Circuits Multiple Choice Questions & Answers (MCQs) focuses on “Class B Output Stage”.

1. The maximum collector efficiency of class B operation is ________
a) 50%
b) 25%
c) 30%
d) 5%

Explanation: Since the active device is switched off for half the input cycle, the active device dissipates less power and hence the efficiency is improved. Theoretical maximum efficiency of Class B power amplifier is 78.5%.

2. A 2-transistor class B power amplifier is commonly called ________ amplifier
a) Dual
b) Single ended
c) Push-pull
d) Differential

Explanation: When Class B amplifier is to be used with a pair of active devices (transistors); it is arranged in a push-pull mode where one transistor conducts one half cycle and the other transistor conducts the other half cycle.

3. When a transistor is in cut off________
a) Maximum voltage appears across transistor
b) Maximum voltage appears across load
c) Maximum current flows
d) Least current flows

Explanation: A transistor in cutoff mode is off – there is no collector current, and therefore no emitter current. It almost looks like an open circuit. Thus Maximum voltage appears across the transistor.

4. The output stage of a multistage amplifier usually employs ________
a) Push-pull amplifier
b) Preamplifier
c) Class A power amplifier
d) Class B power amplifier

Explanation: In push-pull mode; one transistor conducts one half cycle and the other transistor conducts the other half cycle. The output from both transistors are then combined together to get a scaled replica of the input.

5. The size of a power transistor is made considerably large to ________
a) Provide easy handling
b) Dissipate heat
c) Facilitate connections
d) Larger Gain

Explanation: Power amplifiers handle large signals as compared to voltage amplifiers. The size of power transistors is made considerably large to dissipate heat. Low efficiency of power amplifiers results in more battery consumption.
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6. The maximum a.c. power output from a class B power amplifier is 10 W.
What should be the minimum power rating of the transistor used?
a) 10W
b) 15W
c) 4W
d) 20W

Explanation: maximum ac power output voltage = 10W
In class b amplifier, only one half cycle is considered. So the minimum power rating is 4W.

7. The push-pull circuit must use ________ operation.
a) Class A
b) Class B
c) Class C
d) Class AB

Explanation: The basic class B amplifier uses two complimentary transistors (bipolar or FET ) for each half of the waveform, they are arranged in a push-pull form. No DC base bias current in the class B power amplifier.

8. The disadvantage of impedance matching is that it ________
a) Gives distorted output
b) Gives low power output
c) Requires a transformer
d) None of the mentioned

Explanation: The disadvantage of impedance matching is that it gives distorted output. The Power amplifiers generally use transformer coupling because the transformer permits impedance matching.

9. The Power amplifiers generally use transformer coupling because the transformer permits ________
a) Cooling of the circuit
b) Impedance Matching
c) Distortion less Output
d) Good Frequency Response

Explanation: The Power amplifiers generally use transformer coupling because the transformer permits impedance matching. The disadvantage of impedance e matching is that it gives distorted output.

10. The most important consideration in power amplifier is ________
a) Biasing the circuit
b) Impedance Matching
c) Collector Efficiency
d) To keep the transformer cool

Explanation: The most important consideration in power amplifier is Collector Efficiency. The pulsating DC applied to power amplifier causes hum in the circuit. For every stage of power amplifier, the collector efficiency is considered most important.

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