200mW Amplifier

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

250 mW amplifier circuit 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.

output graph THD graph 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.

bode plot 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

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