Test: Push-Pull Class B Amplifier - Electronics and Communication Engineering (ECE) MCQ

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.

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.

Class B push-pull amplifier employs two identical transistors with input signals applied to two stages that are 180° apart from each other. The transistor to be operated in class B mode is kept at the cut off bias and current in this state will be equal to zero. The maximum efficiency of the class B amplifier is 78.5%.

Derivation:
The input and output waveforms during the class B operation.

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.

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.

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.

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.

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.

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.

Since class B amplifier uses a balanced centre-tapped transformer in its design, making it expensive to construct.