Class B - Push-Pull Amplifier

 Class B  (Push Pull)

Push-Pull Amplifier is a power amplifier which is used to supply high power to the load. 
It consists of two transistors in which one is NPN and another is PNP. 
One transistor, NPN (Not Point iN),  pushes the output on positive half cycle and other PNP (Point iN pulls on negative half cycle, this is why it is known as Push-Pull Amplifier. 

The advantage of Push-Pull amplifier is that there is no power dissipated in output transistor when signal is not present. 
There are three classification of Push-Pull Amplifier but generally Class B Amplifier is considered as Push Pull Amplifier.

Class B generally suffers from an effect known as Crossover Distortion in which signal get distorted at 0V. We know that, a transistor requires 0.7v at its base-emitter junction to turn it on. 
So when AC input voltage is applied to push-pull amplifier, it starts increasing from 0 and until it reaches to 0.7v, transistor remains in OFF state and we don’t get any output. 

Same thing happen with PNP transistor in negative half cycle of AC wave, this is called Dead Zone. 
To overcome this problem, diodes are used for biasing, and then the amplifier is known as Class AB Amplifier.
Class AB Amplifier

A common method to remove that crossover distortion in Class B amplifier is to bias both the transistor at a point slightly above then the cut-off point of transistor. Then this circuit is known as Class AB amplifier circuit. Crossover distortion is later explained in this article.