Class AB Complementary Symmetrical Output Stage

In a Class AB output, a pair of complementary transistors (PNP and NPN) are connected in a push pull configuration forming a symmetrical output.

The transistors are a complementary pair because they have matching electrical parameters. Both types of transistors will have the same current and voltage handling capability. The only difference being that one is a NPN type whilst the other is a PNP.

One transistor handles the positive half of the cycle, whilst the other handles the negative half. Individually, each transistor acts as a Class B amplifier conducting for only half the cycle (180°).

Advantages / Disadvantages

In an emitter follower output, the emitters of the complementary transistors connect directly. Moreover, this is the point where the output signal is expected. This type of circuit does not need a phase combining transformer. In addition, impedance matching transformer is not required, because in this mode the transistors are able to drive any low impedance loads such as a loudspeaker directly. One major disadvantage of this design is the crossover distortion inherent to push-pull configurations.