Hi Bernd,

> I use a PIC16C84 based Circuit with Audio (Freqency: 100 - 1000 Hz) Input.
>
> My problem is to construct a AUTOMATIC GAIN CONTROLLED OpAmp
> (with CA3140) using only nonsymmetrical 0V GND and 5.4V Powersupply.
>
> 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).
>
> The Output of this should go after a Smitt Trigger in the PIC to measure the
> Frequency.
>
> Has anyone Ideas? Where I can obtain help with AGC Circuits?

There is an IC called a VOGAD made by Plessey Semiconductors
part number SL6270C and the last price I got was about US$2.00
for 20 of (never got around to ordering them though so have not
used them yet)

Stands for Voice Operated Gain Adjusting Device.  Primarily used in
FM transmitter modulator circuits.

Might work for you, might not.

Cheers
--
Kalle Pihlajasaari   kalle@ip.co.za   http://www.ip.co.za/ip
Interface Products   P O Box 15775, DOORNFONTEIN, 2028, South Africa
+ 27 (11) 402-7750   Fax: 402-7751    http://www.ip.co.za/people/kalle