200mW Stereo Amplifier

Description

A small battery driven amplifier with 200mW output. This is a suitable amplifier for portable radios and other battery powered equipment.

Notes

The output stage T2 and T3 is designed to be biased at half supply voltage, in this case 4.5 Vdc. This allows maximum output swing. This is only a small amplifier and will drive an 8 ohm load to 150 mW with about 7% distortion or 200mW with higher distortion, THD shown above. The output waveform is symmetrical with inputs below 50mV pk-pk and is shown below.

R4 and R3 set quiescent current, which in this circuit is 2 mA. Overall gain is controlled by Q1. Gain is approximately collector load of Q1 / re + R10 or 30dB The response is fairly flat from 40Hz to 60KHz, see bode plot below. 

Parts List

• R1 56k 1/4W Resistor
• R2 15k 1/4W Resistor
• R3 1.5k 1/4W Resistor
• R4 1.5k 1/4W Resistor
• R5 100 1/4W Resistor
• R6 100 1/4W Resistor
• R7 100 1/4W Resistor
• R8  22 1/4W Resistor
• C1 1000u 25V electrolytic capacitor
• C2 10u 25V electrolytic capacitor
• C3 470u 25V electrolytic capacitor
• T1 BC549C NPN transistor
• T2 BC547 NPN transistor
• T3 BC547 PNP transistor