[sdiy] CMOS as voltage variable resistor

[Happy Harry]

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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