Dumitru Stama wrote:

> Is there another preamplifier i could use ? a much better one than
> opamp solution adopted by the schematic i attached in my previous
> message ?

What Olin is saying is that you can improve your circuit, in mainly two
respects:

1- Increase the amplification. I'd start with the last stage (because of
the problem with the DC coupling). You do that basically by increasing the
feedback resistor (the one from the output to the inverting input).
Increasing this resistor up to 100k in the last two stages shouldn't be a
problem -- at least not to see whether you get /anything/ useful.

This may work already. It also may be that the signal at the output of the
3rd amplifier has a significant DC offset; that is, it is not centered
around 2.5V (which it should be without DC offset). This in itself doesn't
hurt, but it reduces the maximum amplitude. For that Olin suggested a
second improvement:

2- AC-couple the individual stages. This means basically that you connect
the output of one stage to a capacitor, and that capacitor to the input
resistor of the following stage. The capacitor needs to be big enough so
that the cutoff frequency of this highpass (the cap and the input impedance
of the next stage form a highpass) is lower than the lowest frequency you
want to amplify.


You probably should read up a bit on basic theory of operational
amplifiers. If you find nothing else, National Semiconductors has a number
of quite good application notes, about basic theory, advanced issues, and a
lot of example circuits.

Gerhard

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