Make a pocket amplifier with 8550 and 8050

This power amplifier circuit is shown in the figure. The base operating voltage of the input pole (9014) is equal to the midpoint voltage of the two output pole transistors, which is generally half of the power supply voltage. The stability of this voltage is controlled by the two diodes at the base of the output transistor. A 3.3 ohm resistor is connected in series to the emitter of the output transistor to stabilize the bias current. This is to reduce the impact of ambient temperature and the difference in parameters of different devices (such as diodes and output transistors) on the circuit. When the bias current increases, the voltage between the emitter and base of the output transistor will decrease to reduce the bias current. The input impedance of this circuit is 500 ohms. When using an 8 ohm speaker, the voltage gain is 5.

When the circuit outputs 50mW power without distortion, there is a voltage swing of about 2V on the speaker. Increasing the power supply voltage can increase the output power, but at this time, attention should be paid to the heat dissipation of the output transistor. At 9V power supply voltage, the circuit consumes about 30mA. When making it, pay attention to the fact that the amplification factors of the two output power tubes should be close. Other device parameters can be selected by referring to the diagram. This circuit is suitable for making a headphone amplifier or other low-power amplifiers.