This is the mini audio amplifier which built using inexpensive transistors. The circuit is quite simple, low-cost and it consumes very low energy to work. It will work with 3V battery / power supply circuit
How the Circuit Operates:
The output is push-pull and needs lower than 3mA (without any signal) but drives the earpiece to a very loud level when audio is detected. The entire circuit is DC coupled and this makes it really challenging to set up.
Generally you don't know where to start with the biasing in this amplifier circuit. The two most crucial parts are 8k2 between the emitter of the first transistor and 0v rail and the 470R resistor. The 8k2 across the 47uF sets the emitter voltage on the BC547 and this turns it on. The collector is directly joined to the base of a BC557, known as the driver transistor. Both these transistors are now activated and the output of the BC557 causes current to flow via the 1k and 470R resistors so that the voltage produced across each resistor activates the two output transistors. The end result is mid-rail voltage on the join of the two emitters.
The 8k2 feedback resistor gives significant negative feedback whilst the 330p avoids high-frequency oscillations happening.
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