Mono Preamp Mic based BC108 Transistor

The microphone preamp based transistor described here is powered by 9 V and less than 1 mA current consumption, it will allows you to use a small 9 V rectangular conventional battery for several hours.

It is designed for a dynamic microphone which output impedance is between 50 and 200 ohms. It is possible to connect a microphone which the output impedance is higher, but at the cost of a slight decrease in gain. Use with an electret microphone will be discussed in other article. The -3 dB bandwidth is 30 Hz to 100 kHz. With the above  scheme, the gain is about 35 dB to 40 dB (40 dB = amplification by a factor of 100).

This transistor microphone preamp circuit is a classic, implementing an NPN common emitter (common emitter simply means that the emitter of the transistor is common to input and output of the preamplifier stage, the base forming the entry and collector delivers the output signal). The gain is mainly determined by the value of the resistors R3 and R4. R3 contributes to the temperature stability of the assembly, and its value fairly high given the location of the component, reduces the gain significantly. Therefore there branch parallel capacitor C2, which “bypasses” the resistance from the moment or is this a sound input (if no audio signal input, the capacitor sees only one component continuous and performs as an insulator of high value – it acts here in the presence of AC signals). By removing this capacitor C2, the preamp still works, but with a gain of about 15 dB below. The capacitor C4 mounted between collector and base of the transistor allows to limit the bandwidth up to a reasonable value (around 100 KHz), and limits the risk of developing an unwanted parasitic oscillation.