[sdiy] CMOS as voltage variable resistor

[Theo]

On the EDN website: http://www.ednmag.com
there is an article "Inverters form three-phase VCO"
that may be of interest.
The current though the Vdd pin of the 74HCU04 is used
to set the frequency of the oscillator.
This might also work with a 4007 as variable resistor.

Theo


From: Happy Harry <paia2720 at hotmail.com>

> Hey all
>
> I don't think so.  The control pin is buffered so you will have
> almost NO control of the "linear" state of the switch...
>
> 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!
>
> 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.
>
> The only way to use these as variable resistors is to chop them
> (PWM)...
>
> The 4007 has 'some' hopes a a variable resistance device... the
> 4016 / 4066 ... very little (no) hope.
>
> H^) harry
>
>
> >From: Grant Richter <grichter at asapnet.net>
> >To: Synth-DIY <synth-diy at dropmix.xs4all.nl>
> >Subject: [sdiy] CMOS as voltage variable resistor
> >Date: Tue, 30 Apr 2002 18:38:41 -0500
> >
> >Just a thought, I noticed the thread on CMOS with variable resistance.
> >
> >In theory, as the control pin voltage goes from 0 to +V, the transmission
> >gate (TG) resistance changes from very large to quite small. The problem
is
> >the control voltage to resistance curve is deliberately very non-linear.
> >
> >Since 4016/66s should be monolithically matched, you could put one TG
> >inside
> >the feedback loop of an op-amp to linearize the resistance curve. Fed
from
> >the same op-amp output as the linearizing TG, the other sections should
> >show
> >a linear change in resistance to control voltage, within limits of their
> >voltage dependence.
> >
> >This could be useful for building envelope generators. Most envelope
> >generators have a manual resistance in series with a TG switch. Using TGs
> >as
> >voltage variable resistances, you could use the TGs both for current
> >steering and charging rate control into the capacitor.
> >
>
>
>
>
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