Audio Amplifier with Common Transistors

 Author:   Posted on:   Updated on:  2018-01-01T16:44:14Z
Here is the schematic of a small audio amplifier that can provide up to 300mW to an 8 ohm load and can be used in low power devices like battery powered radios. This circuit can be an alternative to the LM386 IC. Due to the simplicity of the schematic, the circuit can be built also on breadboard, for those of you who want to experiment and learn how an amplifier works.

The design is straightforward. A common small signal NPN transistor (like BC547, 2N2222, 2N3904, S8050) drives a balanced power amplifier made of similar transistors. The output transistor pairs can be BC327 with BC337 or S8050 with S8550. They must handle peak currents of 300-400mA (this is why BC547/BC557 or 2N3904/2N3906 should not be used here).

The amplifier can be powered from a 9V battery or from a 12V power source. The circuit draws a current of about 170mA. The quiescent current is less than 10mA.

Audio amplifier with common transistors build on breadboard
Audio amplifier build on breadboard
I built the amplifier on a breadboard and tested the output stage with BC327/BC337 and S8050/S8550 pairs (the latter are seen in the above picture). If the input signal is too high, smoke may come out of the transistors. This is because the quiescent current is proportional to temperature. To avoid this, the output transistors can be fitted to a small heatsink. Here is the breadboard design in Fritzing (with BC327/BC337 transistors, which have different pinout than S8050/S8550):

Audio amplifier breadboard
Audio amplifier breadboard
Next is the schematic and PCB for this circuit. If you will look carefully, you may notice that on the breadboard I used different values for some of the resistors. If you decide to build the circuit, you should follow part values from the below schematic:

Audio amplifier with common transistors schematic
Audio amplifier schematic
The schematic is given for both channels. Resistors R4 and R8 and diodes D1, D2 and D3, D4 set quiescent current. R4 and R8 can be lowered down to 1.8 kohms (as I did on the breadboard) if powering the circuit from a 9V battery. Amplification is determined by R1 and R3 (R5 and R7). The values of 39 kohms for R1/R3 is suitable for line out audio level of 1 to 1.4Vp-p. If a lower level signal is fed to the circuit, R1/R3 should have smaller values too. You should not alter R3/R2 (R7/R6) because they set the DC operating point of the amplifier.

Audio amplifier with common transistors PCB outline
Audio amplifier PCB outline
The PCB design is for two channels which can be separated. Output transistors footprint is for S8050/S8550. If you decide to use BC327/BC337 rotate them 180 degrees. I have not built the PCB yet, so here is a 3D render in KiCAD:

Audio amplifier 3D prototype
Audio amplifier 3D prototype
If you wish, you can download LTspice simulation file and PCB design as PDF.

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