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



Notes
Circuit : 


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


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