[sdiy] CMOS as voltage variable resistor

[Tim Ressel]

I have used this technique to form variable delay
elements for a simulator. The gate delay is dependent
on Vcc. You have to deal with the output amplitude
varying. 

--TR

--- Theo <t.hogers at home.nl> wrote:
> 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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