[sdiy] CMOS V.C.R. with 1V/Oct Response

[eq927 at freenet.carleton.ca]

Scott and list,

Well.. the audio distortion specs shouldn't be any different from a CMOS 
vcr that doesn't use this servo technique,  but I think you are asking 
about something else...

What this will do is shape the nonlinear MOSFET control voltage vs. 
channel resistance response into something that is close to V/oct or 
V/Hz: eg. a doubling/halving of Rds for equal changes in CV, or an 
increase/decrease in Rds by a fixed number of ohms for a certain change 
in CV. I haven't tried this with a linear current source, but it should 
work, as long as the current does not go to zero microamps. Probably 
transistor leakage currents would keep it running even if the CV reduces 
the control current to a very small value. The MOSFET on it's own won't 
give a linear Vgs vs. Rds, however this circuit will linearize the 
built-in response, so in that sense, the linear range is extended. I 
assume it would work with the Vss pin connected to an op amp virtual 
ground, as long as the op amp can maintain the virtual ground despite 
the changing drain current through the MOSFET.
Mike

> 
>  I have a question:  If I wanted to use
> a linear CV, could I replace the expo converter (sink?) with a
> linear one?  (sure...)  but the real question is would doing so
> extend the usable linear range?
> 

> Also, would there be a problem with using this approach with
> VCRs that have Vss connected to a virtual ground instead of
> real ground?  (I am guessing this would work)...
>