[sdiy] CMOS as voltage variable resistor

[jhaible]

> Perhaps it is still possible to get A series 4016s rather than buffered B
> series?
[...]
> 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.

The 4016 uses complementary FETs for the switch. One of these needs
an extra inverter for CV, which will also be in your CV-feedback loop.
I see two problems:
More than one inverter stage will be difficult to compensate / stabilize
in closed loop configuration.
Even though you have a *tiny* region of fairly linear operation (if they
haven't added a hysteresis), I doubt this tiny region will be matched
for two devices on one chip. Matching error will be amplified with
each further inverter gain stage in your loop.

> Perhaps we should do a little bread

Certainly !

JH.