500 mW 3 Transistors Audio Amplifier
This small 3 transistors amplifier circuit will be suitable for small battery powered devices, and the circuit is really simple. This tiny power amplifier use same typology as the more complicated version of discrete amplifier in our previous circuit, but with here is in the simplest form.
This circuit is a very good start as a DIY amplifier because its simplicity. Since this audio amplifier process only audio signal (not a dc amplifier), both the input and the output are dc-blocked using capacitors. You can see 3.3 ohm resistors connected to the emittters of PNP-NPN transistor couple, and the purpose of this resistors is to stabilize the transistor gain, so the temperature change won’t affect the performance much. Any PNP-NPN transistor couple with identical performance that capable of handling 100mA collector current should be suitable to replace the final transistors. The voltage swing of the output will be 2 Volt at maximum (at 9V DC supply), so the current at 8 ohm speaker will be 0.25A, and the maximum power would be the 2V*0.25A=0.5Watts. [Source: Bill Bowden’s Circuits Collection]