# Electronic Devices and Circuits Questions and Answers – Classification of Output Stages

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

1. A class A amplifier with RC = 3.3 k and RE = 1.2 k has a VCC = 20 V. Find IC(sat).
a) 4.4mA
b) 6.1mA
c) 20mA
d) 16.7mA

Explanation: Since, at saturation region, Vcc = 0 V, so Vcc will drop across Rc and RE only.
here, Rc + RE = 3.3 k ohm + 1.2 k ohm,
= 4.5 k ohm,
Ic(sat) = Vcc/(4.5 k ohm),
= 20/(4.5 k ohm) = 4.44 mA.

2. Which of the power amplifiers has the lowest overall efficiency?
a) Class C
b) Class A
c) Class B or AB
d) Class D

Explanation: The efficiency of this type of circuit is very low (less than 30%) and delivers small power outputs for a large drain on the DC power supply. A Class A amplifier stage passes the same load current even when no input signal is applied so large heat sinks are needed for the output transistors.

3. Calculate the efficiency of a class B amplifier for a supply voltage of VCC = 20 V with peak output voltage of VL(p) = 18 V. Assume RL = 16 .
a) 78.54%
b) 75%
c) 70.69%
d) 50%

Explanation: Using Class B efficiency:
Formula:
%n = 78.54 (Vpout / Vcc).
Given
Vcc = 20Vp out.
VL = 18V,
RL = 16V,
%= 78.54( 18V/20V),
Efficiency= 70.69%.

4. For BJT power transistors, the collector terminal is always connected to the transistor’s case________
a) for easy circuit connection
b) to prevent shorts
c) because the collector terminal is the critical terminal for heat dissipation
d) because the collector terminal is located nearest the case

Explanation: Transistor case is used for power transistors as the power dissipated at their collector junction is large. If heat dissipation is not done, this will cause large increases in junction temperature.

5. Which operation class is generally used in radio or communications?
a) A
b) B
c) C
d) D

Explanation: In radio or communications, generally class C is used in case of high level modulation. High level modulation uses class C amplifiers in a broadcast AM transmitter and only the final stage or final two stages are modulated, and all the earlier stages can be driven at a constant level.
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6. Which of the push-pull amplifiers is presently the most popular form of the class B power amplifier?
a) Quasi-complementary
b) Transformer-coupled
c) Complementary-symmetry
d) Symmetry

Explanation: In class B operation, a push-pull connection is obtained using either a transformer coupling or by using complementary (or quasi-complementary) operation with npn and pnp transistors to provide operation on opposite polarity cycles. While transformer operation can provide opposite cycle signals, the transformer itself is quite large in many application.

7. Calculate the harmonic distortion component for an output signal having a fundamental amplitude of 3 V and a second harmonic amplitude of 0.25 V.
a) 3.83%
b) 38.3%
c) 83.3%
d) 8.33%

Explanation: 3 volt is input
0.25 is distortion in input for 3 volt
then (0.25/3)*100 = 8.33%.

8. Which of the power amplifiers is not suitable as audio amplifiers?
a) Class A
b) Class B
c) Class C
d) Class D

Explanation: The transistor biasing gives a much improved efficiency of around 80% to the class C amplifier, it introduces a very heavy distortion of the output signal. Therefore, class C amplifiers are not suitable for use as audio amplifiers.

9. Which type of amplifier uses pulse (digital) signals in its operation?
a) Class A
b) Class B or AB
c) Class C
d) Class D

Explanation: The class D amplifier is nonlinear switching amplifiers or pulse width modulation amplifiers. These amplifiers are called as digital amplifiers.

10. What is the maximum efficiency of a class B circuit?
a) 90%
b) 78.5%
c) 40%
d) 50%

Explanation: The conduction angle is 180° for a Class B amplifier. Since the active device is switched off for half the input cycle, the active device dissipates less power and hence the efficiency is enhanced. Theoretical maximum efficiency of Class B power amplifier is 78.5%.

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