Small Amplifier Using Transistors
When audio is detected, the output is push-pull and consumes less than 3mA (with no signal) but drives the earpiece to a very loud level. It’s extremely difficult to set up because the whole circuit is DC coupled. Basically you don’t know where to start with the biasing. 8k2 between the emitter of the first transistor and 0v rail and the 470R resistor are the two most critical component.
The emitter voltage on the BC 547 is set by the 8k2 across the 47u and this turns it on. To call the driver transistor, the collector is directly connected to the base of a BC 557. Current flow through the 1k and 470R resistors so that the voltage developed across each resistor turns on the two output transistors is caused by these transistors and the output of BC 557 are now turned on. The end result is mid-rail voltage on the join of the two emitters.Major negative feedback is provided by 8k2 feedback resistor while the 330p prevents high-frequency oscillations occurring. [Source: Talking Electronics]