• In class B power amplifiers the transistors are biased at cutoff.

• These transistors can enter the active regions if and only if their base emitter junction is forward biased.

• To forward bias these junctions the input voltage must be greater than the cut-in voltage of the junction. Thus as long as the input voltage is less than the cut-in voltage, transistor will remain in off state and the output will be zero. The output of signal gets distorted near zero crossings. The distortion is called as cross over distortion. Hence the conduction angle becomes less than 180

• This is major limitation of class B power amplifier.

• This limitation can be overcome by applying some small forward voltage to each transistor to increase the cut-in voltage. Due to this the Q point gets shifted above X-axis and the operation is no more class B one but it is class “AB” operation.

The cross over distortion can be shown by following figure: