[sdiy] CMOS as voltage variable resistor

[Grant Richter]

I am willing to fight to the death for your right to disagree with me ;^)

It could be useful, so here goes,

> 
> I don't think so.  The control pin is buffered so you will have
> almost NO control of the "linear" state of the switch...

Perhaps it is still possible to get A series 4016s rather than buffered B
series? That would give more linear range. The idea is usually shown with
matched photoresistors, but other types of voltage variable resistors
(MOSFET or JFET) may be able to use the same technique.
> 
> Its a switch, on or off... no linear range at all. Think of it like a
> comparator with no feedback... try and make that stay at exactly
> zero volts... no way!

I believe the purpose of the B series is to increase the overall voltage
gain, but there must still be some linear region. Otherwise you could not
make op-amps out of CMOS inverters, which you can do according to the
National Databook. I would expect the linear region only to be about a volt
in width, but our friend the op-amp might manage that.
> 
> OTOH... Grant is correct in that putting one TG in the feedback of
> an opamp can linearize the ON RESISTANCE... in fact it can cancel
> both the on resistance and any voltage dependence of on resistance as well.
> 
That would be very useful, wouldn't it? Perhaps we should do a little bread
boarding. Being able to reduce the distortion in a JFET variable resistor
would also be very useful. Could the idea work for that?