How can I have a DC level extracted from AC 'sit' on another DC level

Excuse the crazy title...but imagine an AGC circuit......basically a peak detector, turning the incoming AC signal into a DC representation...that DC is used that control a voltage control amplifier...all well and good.

But what if we want the AGC level to be user setable (therefore maybe the user wants to increase the output of the circuit...but how can this be arranged?

I'm thinking here that the AGC will always try & pull the output back to the median ...how can I effect what that median is? (and if the result of an AGC circuit is a DC representation of the extracted AC, then how can I increase the DC level without it then trying to self contain itself & negate the introduced elevated DC?)

Maybe I just need more medication? "Nurse...over here ...QUICK"

 

You could use a summing amplifier where one of the DC inputs came from a pot so you could adjust it and the other input was the AGC voltage.

 

I think what you are describing here is a quite common type of AGC, or perhaps Automatic Level control (ALC). The fixed level AGC followed by an adjustable gain mentioned above is one possibility.

Another way is to make the AGC itself adjustable. Here a DC level representing the AC output level is produced by a suitable detector. This DC is then compared to a DC reference level by a control amplifier, usually thought of as a difference amplifier, or coud be considered a summing amplifier as mentioned elsewehere, depending on the signs of the signals begun with.

The output level could then be varied either by making the DC reference level user-adjustable, or perhaps by varying the amount of signal reaching the level detector,using an attenuators. The second approach has he advantage of working the detector at constant level, which may be an advantage in very accurate high frequency work, using high quality attenuators.

The DC amplification should be easy enough to arrange with op-amps, but care must be taken to get a stable result as the level detection and gain control systems tend to introduce phase lags in the control loop. Frequency compensation should aim to get a response that settles stably to the required level.

 

Thanks guys...very helpful.