| This is a compact transistor audio amplifier circuit designed to deliver approximately 500 milliwatts (0.5W) of output power into an 8-ohm speaker. It's a great solution for small audio applications such as radios, DIY intercoms, or testing signal sources. The circuit operates from a single 9V supply, but it can also run on lower voltages, such as 5V, at reduced output power.. |
Overview |
The amplifier consists of a preamp stage based on a BC558 PNP transistor (Q1), which boosts the input signal. This is followed by a driver stage (Q3 & Q4) and a push-pull output stage using Q2 (PNP) and Q5 (NPN), forming a class AB amplifier. Diodes D1 and D2 provide biasing for the output transistors, helping reduce crossover distortion. A standard signal generator feeds the amplifier through AC coupling capacitors (C2 and C1), and the output is taken from C5. The oscilloscope shows a clean amplified waveform that remains symmetrical and free of clipping at typical input levels. |
 |
Power Output Estimation |
| The output capacitor (C5) feeds into an 8-ohm load. With a clean sine wave output reaching just under 4.5V peak (as seen on the scope), we can estimate the RMS voltage and power at 1.26W |
However, this is open circuit in simulation. In practice, due to voltage drops, losses, and real speaker loading, the usable power is closer to 500mW before distortion sets in. |
Performance on 5V |
| The circuit can operate from a 5V power supply, though the maximum output swing is reduced. This results in lower output power, typically around 100–200mW, depending on the load. Despite that, it's still useful for small, battery-powered projects. |
Final circuit diagram |
 |
I have built this in the real world several times. It works really well ! It will output much more at 12v but at a cost. Those transistors run hot at 12v and will likely shorten their lives.
If you have Labcenter Proteus, you can run this in a simulation. PCB also available. |