One Transistor Audio Amp
How it works:
One transistor audio amp
A transistor is a current amplifier. The input signal is coupled into the base-emitter circuit of the transistor via the 0.1 uF capacitor. The transistor must have a bias current following in the same base-emitter circuit for it to work properly -- this is supplied by the 2.2 meg ohm resistor from the base to +9 volts. The emitter-collector current flows through the 3.3K load resistor and is an amplified version of the input current. The 0.22 uF capacitor couples the amplified signal into the headphones. Since this amplifier is capable of amplifying signals well beyond the audio frequencies, which could cause stability problems, a 500 pF bypass capacitor is used to shunt out any radio frequency stray signals at the input.
The current gain of a transistor is the ratio of the emitter-collector current to the base-emitter current. For the 2N2222A, this is about 175. The voltage gain will depend on several factors. The computer program in the next link will calculate voltage gain and several other performance characteristics of a transistor amplifier like this one.
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