Quasi Complementary Push Pull Amplifier:

In practical power amplifier circuits, it is preferable to employ NPN transistors for both high-current output devices. As the push-pull connection needs complementary devices, a PNP high power transistor is required. A practical means of having complementary operation while using the same, matched NPN transistors for the output is provided by a Quasi complementary push pull power amplifier circuit, as illustrated in Fig. 17.31.

The push-pull operation is obtained by using complementary transistors Q₁ and Q₂ before the matched NPN output transistors Q₃ and Q₄. Here transistors Q₁ and Q₃ form a Darlington pair that provides output from a low-impedance emitter follower. Transistors Q₂ and Q₄ form a feedback pair that provides a low-impedance drive to the load. Variable resistor R₂ is provided for adjusting the dc bias condition so as to minimize crossover distortion. The single input signal applied to the push-pull stage provides full cycle output to the load R_L.

The Quasi-complementary push pull amplifier is considered to be the most popular type of power amplifier used nowadays.