This is the schematic diagram of mini microphone amplifier circuit, which all of the active components are based on transistor. This circuit project is called "mini" since its size is small and the output is small too. It uses surface mount technology.
The output is push-pull and consumes less than 3mA (with no signal) but drives the earpiece to a very loud level when audio is detected.
The whole circuit is DC coupled and this makes it extremely difficult to set up. Basically you don't know where to start with the biasing. The two most critical components are 8k2 between the emitter of the first transistor and 0v rail and the 470R resistor.
The 8K2 resistor across the 47u capacitor sets the emitter voltage on the BC547 and this turns it on. The collector is directly connected to the base of a BC557 transistor, called the driver transistor. Both of these transistors are now activated and the output of the BC557 causes current to flow through the 1k and 470R resistors so that the voltage produced across each resistor turns on the two output transistors. The end result is mid-rail voltage on the join point of the two emitters. The 8K2 feedback resistor provides major negative feedback while the 330p prevents high-frequency oscillations occurring.
This is a video tutorial about how to a very simple audio amplifier based on the LM386 amplifier chip. It can be built for less than $20 (or might be less than $8 in some countries) and used to amplify any low level audio signal including a guitar, bass or mp3 player.
Watch the video: