Hi Everyone,

I have a very low amplitude ac signal (~ 2mV p-p) which I would like to amplifier using BJT's (either 2N3904 or BC548C) to achieve an output of over 1V p-p.
I am currently using a Darlington configuration with a 2N7000 and a 2N3904. Vcc is 20V but the output is not exceeding 140mV p-p.
Circuit diagram is attached.
Suggestions are appreciated.

sandel

 

Suggestion: Go buy a dual or quad op-amp. I think your input impedance needs to be higher, which the op-amp will solve, and your desired gain is 500X. That's pretty tough with discrete transistors. You'll need to chase down sources of noise, so design for good grounding, bypass caps everywhere, shielded wires, short traces, shielding, etc.