The theoretical limit for the efficiency of class B amplifiers is ______.

• 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.

The power efficiency of class B amplifier:

For the half-sine loop, Idc is

The output of IC is from 0 to π and π to 2 π it is zero

Idc = ( IC (max) ) / π 

Therefore input power Pin for the input voltage of transistor VCC is

Pin = \(2 × \left[ {\dfrac{{{I_C{_{max}}}}}{π } × {V_{CC}}} \right]\)

Here factor 2 is introduced as there are two transistors in the push-pull amplifier.

R.M.S. value of collector current = ( IC )max / √2

R.M.S. value of output voltage = ( VCC )max / √2

Output power Po is

Po = \(\dfrac{{\left( {{I_C{_{max}}}} \right)}}{{\sqrt 2 }} × \dfrac{{{V_{CC}}}}{{\sqrt 2 }} = \dfrac{{({I_C{}_{max}}) × {V_{CC}}}}{2}\)

Overall efficiency η = [( Po ) / ( Pin )] × 100 = 25 π = 78.5%

The maximum conversion efficiency of a class B push-pull amplifier is 78.5%