I have a project where I need to have audio input into atmega168PA. I've decided to implement it by using an electret microphone which will feed into op amplifier for low-pass filtering and amplification.
Design is not mine, I found it from another thread.
Total gain of the circuit should be around 900 and output should be nicely centered around 2.5VDC. Problem is that measuring with an oscilloscope I see that it doesn't give nearly enough amplification. By tapping pretty strongly on the mic I get only about 500-600mV (shown below) of peak-to-peak amplitude around 2.5V. With a loud clap I get little more than volt. Not nearly enough to register normal voice.
I understand that microphones need a lot of amplification so I added another stage (shown below) with amplification gain of 15 which should have produced gain more than 15000 in total, but still the peak-to-peak amplitude was little under a volt.
So, what am I doing wrong? I used TL074ACN op amps and I also tried LM324N op amp, but the results where same. Is it possible that neither of those amps are suitable for this application?
I also tried a simple circuit (shown below) with a transistor 2N3904 with schematic found from some other thread, which produced very good results allthough as I didn't have correct bias the signal was very distorted. But peak-to-peak amplitude was okay (around 3.6V).
As I said I want to actually feed the output into ADC so I've designed a Sallen-key low-pass filter with cut-off frequency around 5kHz and verified that it works (I'm going to use the same one for DAC output). So my final schematic should look something like this:
Can someone point out whats wrong with my circuit or how to troubleshoot the problem? Or should I just go with transistor for amplification?