I have tried to design a amplifier for an electret condeser microphone. It is basically just a common emitter NPN amplifier.
The problem is the frequency response of the real circuit. I get the expected full response on frequencies higher than roughly 10kHz (scratching, s-sounds, crumpling paper). Lower frequencies are practically non-existent. The response I actually expect is below:
The -3db point (marked) is at around 10 Hz.
What could be the cause? Is there some major design error in my amplifier? Is it possible the microphone response is at fault or my biasing is wrong?
My schematic is below: (V1, C1 and Q1 are all part of the microphone capsule)
Best Answer
Ratio R(collector)/R(emitter) roughly sets the gain. Without R6+C3 your circuit has very low gain, ~1.5. Make R5 say 300 ohms and put R6+C3 parallel to R4 and set R6 equal to R4. Adjust R1 to have 1/2 Vcc on collector.
The purpose of R6+C3 in the current circuit is to bypass AC and block DC; it necessary to keep circuit output reliably at desirable voltage - no matter what the gain you won't need to fiddle with bias resistors. You may try C3 1000 uF, may help.