Description
A small battery driven amplifier with 200mW output. This is a suitable amplifier for portable radios and other battery powered equipment.
Notes
In this amplifier, T1 a single common emitter stage drives a complimentary emitter follower (T2 and T3). Direct coupling is used throughout ensuring good low frequency response and compensation for temperature changes. Should temperature and collector current increase in T1, then T3 is biased further into conduction. The dc voltage at junction of R6 and R7 will decrease which is direct coupled via R1 to T1 base; reducing the base voltage and counter-acting the effects of increased temperature.
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 ResistorR2 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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