Re: voltage controlled resistor

Franc Zabkar (fzabkar@xxxxxxxxxxxxxxxxx) writes:
On 16 May 2007 10:43:47 -0700, cary.niza@xxxxxxxxx put finger to
keyboard and composed:

Does anyone know of an IC chip or discrete component intended to be
used as a variable resistor whose resistance value may be controlled
by varying its voltage input/signal

About 20 years ago I encountered a device consisting of a light source
(LED ?) and a light dependent resistor. IIRC the part came in a
package that was not much bigger than a TO92 transistor. I think
similar schemes are used to provide noiseless volume controls for
musical instruments, eg the expression pedals for pianos and organs.

And the poster hasn't really given enough detail.

Those things, and you could make them yourself if you had the light
dependent resistors, tend to be the simplest solution, because the
existing circuit only sees a variable resistor. I gather though
that they tended to be not so linear vs control voltage, and I don't
recall them coming in a wide range of values.

Since the original poster didn't specify what he was trying to do,
getting a result is not going to be easy. The FETs mentioned by
some, those work, but you have to retrofit them onto the existing
design, and I thought there were linearity issues too, as in adding
distortion to the circuit if not done right. Linearity in the control
voltage versus resistance too, I think it was Siliconix who had a whole
application note about that.

Someone mentioned 4066s. The trick would be to pulse modulate those
to get the varying resistance. The problem then is that you end up
with junk on the signal, unless, as Don Lancaster pointed out decades
ago, you are controlling a low pass filter that takes out the junk.

One could use an attenuator made up of a forward biased diode, I've
seen those but I don't know how well they worked.

One could always change the circuit, so instead of a stage of amplification
and a volume control, you put in a gain control stage. So any sort of
stage intended for AGC or voltage controlled amplificaton would fit in.

Michael

.

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