• A power amplifier is referred to as class B amplifier if the output signal is obtained for one half cycle of the ac input signal. Transistor conducts only for the one half cycle of the input signal ,therefore the output current follows only for 180 degrees.

• The Q point of class B amplifier is present at cutoff region that is on X-axis. Thus the transistor will remain off when there is no ac input signal applied. The coordinates of Q points are (VCC, 0) and typically the efficiency of class B power amplifier is 78.5% .

• When we apply an ac input signal to the input terminal of the transistor, the B-E junction of the transistor is in forward bias for only positive half cycle of the input signal. For that half cycle base current starts following. For the next negative half cycle there is no current flows through base hence the output obtained is zero.

• Half cycle is obtained at the output. Therefore the output voltage waveform is distorted. Because it is no more sign wave.

• This distortion can be eliminated by using two transistors for the two alternating half cycles of the input signal. It will produce the complete half cycle of output signal. Each transistor conducts only for one half cycle i.e for 180 degree. Hence it is known as push-pull class B power amplifier. The waveform of class B power amplifier is as shown in the figure.

Cross over distortion

• 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.