[sdiy] Simple VCA?

[Tim Stinchcombe]

> The interesting thing about that is that although they 
> specifiy the JFET used 
> there they don't seem to specify any particular kind of 
> opamp.  The feedback 
> network is about as simple as you can get.

Just found the circuit in the back of an old Teledyne Semiconductor databook
('JFET Design and Application Manual', 1980), which has a small section of
application notes at the end - unfortunately it is very sparse too: the op
amp is shown as a 741; the JFET either 2N4302 or 2N4220; input resistor (on
-ve in) and resistor to ground (on +ve in) are both 510 ohms; feedback
resistor 'RF' not given (useful huh?). No equations or anything, but the
figure caption says: "Electronic gain control utilizing the low offset
voltage of the 741, the JFET will have linear resistance over several
decades".

I'm sure I have seen this somewhere else, but a quick skim of my FET books
didn't uncover it - maybe I'll have another more leisurely peruse.

I agree with Achim that it must be acting as some sort of subtractor, but my
feeble attempt so far at analyzing it hasn't made much sense!

Tim
__________________________________________________________
Tim Stinchcombe 

Cheltenham, Glos, UK
email: tim102 at tstinchcombe.freeserve.co.uk