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

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

1. A class A power amplifier uses ______________
a) Two transistors
b) One transistor
c) Three transistors
d) Four transistors

Explanation: In Class A amplifier, if the collector current flows all times during the full cycle of the input signal, the power amplifier is known as class A power amplifier. It is less used for higher power output stages, as it has poor efficiency. So it consists of only one transistor.

2. The maximum efficiency of resistance loaded class A power amplifier is ____________
a) 5%
b) 35%
c) 25%
d) 50%

Explanation: The theoretical maximum efficiency of a Class A power amplifier is 50%. In practice, with the capacitive coupling and inductive loads (loudspeakers), the efficiency can decrease as low as 25%. This means 75% of power drawn by the amplifier from the supply line is wasted.

3. The maximum efficiency of transformer coupled class A power amplifier is ____________
a) 50%
b) 25%
c) 30%
d) 5%

Explanation: Using the transformer coupling technique, the efficiency of an amplifier can be enhanced to a great extent. The coupling transformer provides good impedance matching between the load and output, and it is the main reason behind the improved efficiency. Therefore, Its efficiency is 50%.

4. Power amplifiers handle ____________ signals compare to voltage amplifiers
a) small
b) large
c) very small
d) equal

Explanation: The Small Signal Amplifier is generally referred to as a “Voltage” amplifier because they usually convert a small input voltage into a much larger output voltage. Although the amplification is high the efficiency of the conversion from the DC power supply input to the AC voltage signal output is usually poor.

5. In class A operation, the operating point is generally located ____________ of the d.c. load line.
a) At cut off point
b) At the middle
c) At saturation point
d) In active region

Explanation: By referring to the output characteristics of the class A amplifier operation, the Q-point is placed exactly at the centre of the AC load line and the transistor conducts for every point in the input waveform. Therefore, it lies in the middle of d.c. load line.
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6. A power amplifier has comparatively ____________ β.
a) Small
b) Large
c) Very large
d) Same

Explanation: A power amplifier has relatively small gain β, As compared to voltage amplifier. Both current and voltage amplifiers have greater gain as compared to power amplifiers. Power amplifies handle large signals as compared to voltage amplifiers. Power amplifiers have the following stages- A, B, C, D, AB.

7. A class A power amplifier is sometimes called ____________ amplifier
a) Reciprocating
b) Single – ended
c) Symmetrical
d) Differential

Explanation: Class-A designs can be simpler than other classes such as class -AB and -B designs require two connected devices in the circuit (push and pull), each to handle one half of the waveform whereas class A can use a single device that’s why it is considered as a single ended amplifier.

8. When no signal is applied, the approximate collector efficiency of class A power amplifier is ____________
a) 25%
b) 10%
c) 0%
d) 50%

Explanation: The efficiency of a class A amplifier is dependent upon the input signal. The relation is direct. Therefore, with no input signal applied(i.e. input signal= 0); the efficiency will be equal to zero.

9. The maximum a.c. power output from a class A power amplifier is 10 W. What should be the minimum power rating of the transistor used?
a) 20W
b) 5 W
c) 25W
d) 50W

Explanation: Maximum AC power is given as;
P0 = (VCEQ . ICEQ)/2
So the minimum power rating used should be,
10W * 2 = 20W.

10. The most costly coupling is ____________ coupling
a) Transformer
b) Impedance
c) RC
d) Direct

Explanation: In Transformer coupling, audio transformers are used. This requires a larger core and more turns, making a more expensive transformer coupling. Transformer coupling is very widely used at radio frequencies, but it is also practical at audio frequencies, say 30 to 30,000 Hz.

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