Automatic Gain Control

Hello,

I need a little advise for a little project.

I need to build a smart(with automatic gain control) amplifier with the following characteristic:

When the output voltage swing exceeds an external limit set by the user (Vref), amplifier decreases its gain such that the output voltage swing remains aroud Vref.

Thank you for your help.

Sounds like your talking about an *audio compressor* or *peak limiter*. Try these Google searches:

* audio compressor circuit
* audio peak limiter circuit

Lots of rich material shall result

Though, actually, upon rereading your request, I see that you never mention "audio", so tailor your searches accordingly ;)

Or, embellish your request.

There are a number of ways to do this, here is a very simple one that works and some additional background info to go along with it. You can have a variable soft limit or just a peak limit, that will depend on your limit circuit. A simple peak limiter uses a comparator with some R/C filter. The output of the comparator functions as Vcnt in the schematics.

Again there are many ways to go about this and some are complex in terms of component count etc. For instance a simple non inverting amp with an A/D converter that selects say a 256 bit resolution gain resistor. I could not find a single chip that does that but am sure it exist. This would be a very good way to accomplish the task.

None the less the circuit in this Intersil application note along with the University of Toronto project term paper should help out. In other words this circuit works. You will have to determine the type of AGC you want, peak or soft.

I didn't see that my second file was uploaded. Sorry, this is the Intersil application note. Also note the one J-FET that is probably the best choice has a 6/1 resistance ratio, so it would provide you the best dynamic range._