Circuit composition of Class B complementary symmetrical power amplifier

Although the amplifier circuit working in Class B has low power consumption and is conducive to improving efficiency, it has serious distortion, which causes half of the waveform of the input signal to be eliminated. How to solve the above contradiction?

(a) Basic complementary symmetry circuit (b) Emitter follower composed of NPN tube (c) Emitter follower composed of PNP tube
Figure XX_01 Let's study the complementary symmetry circuit shown in Figure XX_01a. T1 and T2 are NPN tubes and PNP tubes respectively. The base and emitter of the two tubes are connected together. The signal is input from the base and output from the emitter. RL is the load. Since this circuit has no base bias, vBE1 = vBE2 = vi. When vi = 0, T1 and T2 are both in the cut-off state, so this circuit is a Class B amplifier circuit. This circuit can be regarded as a combination of two emitter followers in Figure XX_01b and c.

Considering that the BJT emitter junction conducts electricity only when it is forward biased, when the signal is in the positive half cycle, vBE1 = vBE2 >0, then T2 is cut off, T1 undertakes the amplification task, and current passes through the load RL; when the signal is in the negative half cycle, vBE1 = vBE2 <0, then T1 is cut off, T2 undertakes the amplification task, and current still passes through the load RL; in this way, one works in the positive half cycle and the other works in the negative half cycle, and the two tubes complement each other's deficiencies, so that a complete waveform is obtained on the load, which is called a complementary circuit.

The complementary circuit solves the contradiction between efficiency and distortion in the class B amplifier circuit. In order to make the waveform obtained on the load have the same size in the positive and negative half cycles, it is also required that the characteristics of the two tubes must be completely consistent, that is, the working performance is symmetrical. Therefore, the circuit shown in Figure XX_01a is usually called a class B complementary symmetrical circuit.