Bernd,

You asked:

>I need this because the input Voltage is a sinodial Wave with a variable
>Amplitude of only 3 to 10 milli Volts. I want to obtain a stabilized Wave
with a
>Amplitude of 2 Volts whitch has about +2.5V DC Offset (will say: the Voltage
>varies between +0.5 and +4.5 Volt like a +2.5V shifted Sinewave).

Perhaps I'm oversimplifying the problem, but I don't see whay you need AGC
at all.

Your main problem will be getting teh milivolt level signal out to the
electronics, without picking up noise etc.  Use a balanced line, with
shielded cable. If the environment is very bad, put a voltage amp right at
hte source to drive the cable.

Use whatever op amp you like, but for single supply applications, the good
ol' MC 3403 (quad op amp) would be where I'd start.  Biasing the signal the
way you want should be a piece of cake.  Once you've popped the signal to a
decent level and gotten it out of the noise floor, you shouldn't have to
worry about leveling it before feeding to a Schmidt Trigger.

Hope this helps,
.... Gregg