# Analog Circuits Questions and Answers – Push-Pull Class B Amplifier

This set of Analog Circuits Multiple Choice Questions & Answers focuses on “Push-Pull Class B Amplifier”.

1. Which of these is not true for a class B amplifier?
a) It has zero DC bias
b) They have an efficiency less than that of class A amplifiers
c) The quiescent power dissipation is zero
d) The conduction angle is only 180°

Explanation: The class B amplifier has zero DC bias as the transistors are biased at cut-off only. Each transistor conducts when the input is greater than the base-emitter voltage. The conduction angle is only 180° for this amplifier. They have higher efficiency than class A amplifiers.

2. What is the output of a class B amplifier for sinusoidal input?
a) Sinusoidal amplifier
b) Half-sinusoidal
c) Sinusoidal with higher frequency
d) Square wave

Explanation: If Q-point is in cut-off, then IC varies only in the positive direction, for saturation, it varies in the negative direction. So the output of Class B amplifier is half sinusoidal. There is no effect in the shape or the frequency of the wave.

3. How do we obtain sinusoidal output out of a class B amplifier?
a) By using non-sinusoidal inputs
b) By utilizing two transistors
c) By biasing it in the active region
d) By adding a capacitor to the output

Explanation: To obtain sinusoidal output from a class B amplifier, two transistors must be used. Such a circuit is a class B push-pull amplifier, used in unturned power amplifiers and audio frequency power amplifiers.

4. In a class B amplifier, it is found that DC power is 25W, find the ac power.
a) 10 W
b) 62.5 W
c) 25 W
d) 50 W

Explanation: For a class B amplifier, figure of merit = 0.4 = dc power/ac power
Thus AC power = DC power/0.4 = 25/0.4 = 62.5 W.

5. When is the maximum efficiency of class B amplifier achieved?
a) When VMAX = VCC
b) When two transistors are used
c) When VMIN = 0
d) Efficiency is always constant

Explanation: Efficiency % = [1-VMIN/VCC] x 78.5
Maximum efficiency occurs when VMIN=0 and efficiency is 78.5%.
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6. What is the disadvantage of a class B push-pull amplifier?
a) The efficiency reduces
b) The figure of merit increases
c) The cross-over distortion occurs
d) The Q-power dissipation is very large

Explanation: A class B amplifier helps increase efficiency, and the figure of merit reduces. The q power dissipation reduces and cross over distortion increases. Due to two transistors, when one transistor turns off the other does not begin conduction immediately, hence output current is zero for a short interval.

7. Read statements and select the correct option below.

```A: A push-pull amplifier decreases harmonic distortion
B: Output has half-wave symmetry
```

a) A and B are both correct and B is the correct reason for A
b) A is correct and B is incorrect
c) Both A and B are correct but B is not the correct reason for A
d) Both A and B are incorrect

Explanation: A push-pull amplifier reduced harmonic distortion as it cancels even harmonic frequencies. Net current in load I=K(I1-I2)
Due to this, even harmonics cancel out and decrease harmonic distortion. Thus output has half-wave symmetry.

8. Why does no DC current flow in the primary winding of the output transformer of class B push-pull amplifier?
a) Because DC currents from both transistors flow in opposite directions
b) Because the net impedance is very high to allow flow of current
c) The winding only allows AC current to flow
d) Current only flows in secondary winding due to the presence of load at that side

Explanation: The net DC current in the primary winding of the output transformer is zero because DC collector currents of both transistors being used flow in opposite directions and hence transformer saturation doesn’t occur.

9. Which of these is incorrect for complementary symmetry push-pull amplifiers?
a) During positive cycle NPN transistor conducts
b) It is easier to fabricate on IC
c) Size of the transformer required reduces
d) Efficiency and figure of merit are same as transformer coupled push-pull amplifier

Explanation: The complementary symmetry push-pull amplifier uses one NPN and one PNP transistor to conduct in positive and negative cycles respectively. It does not affect efficiency or figure of merit, but since no transformer is being used, it is easier to fabricate on ICs.

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