The power amplifier circuit is shown in the figure. The operating voltage (90 14) of the input electrode base is equal to the midpoint voltage of the two output transistors, which is generally half of the power supply voltage. The stability of this voltage is controlled by two diodes at the base of the output transistor. A 3.3 ohm resistor is connected in series with the emitter of the output transistor to stabilize the bias current. To reduce the influence of ambient temperature and parameter differences of different devices (such as diodes and output transistors) on the circuit. When the bias current increases, the voltage between the emitter and the base of the output transistor will decrease to reduce the bias current. The input impedance of the circuit is 500 ohms, and the voltage gain is 5 when an 8 ohm speaker is used.

When the circuit outputs 50mW power without distortion, there is about 2V voltage swing on the speaker. Increasing the power supply voltage can improve the output power, but at this time, we should pay attention to the heat dissipation of the output transistor. When the power supply voltage is 9V, the power consumption of the circuit is about 30mA. Note that the magnification of the two output power tubes should be close. Other equipment parameters can be selected with reference to the figure. This circuit is suitable for making earphone amplifiers or other small power amplifiers.