[sdiy] CMOS as voltage variable resistor

[Happy Harry]

OTOH... the basic principle of war is to get the OTHER
guy to die for your country...

inline ;^P


>From: Grant Richter <grichter at asapnet.net>
>To: <harrybissell at prodigy.net>, <synth-diy at dropmix.xs4all.nl>
>Subject: Re: [sdiy] CMOS as voltage variable resistor
>Date: Wed, 01 May 2002 01:37:21 -0500
>
>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?

A series might help, agreed. If there are TWO stages in series
before the control element... you are hosed. I think there are
even in the 4016A...

yep... hosed.  The 4016 has an inverter driving the pmos fet
series element... the nmos does not have the inverter
4016B has two inverters so they are both buffered.


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.

You can only use CMOS inverters that are the UB series... they have
the gates of both pmos and nmos available at the package pin
(no buffers)... the 4049 would work (though it is not symmetrical)
but the 4050 will NOT work....

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?


I think you could if the jfets were well matched...

H^) harry





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