Re: Designs of voltage/current controlled resistance

On Wed, 21 May 2025 15:44:40 +0000, RodionGork <rodiongork@github.com>
wrote:

Hi Friends!
>
What approaches exist of creating voltage or current-controlled
resistance?
>
There is no some specific schematic in which I want it to be plugged in,
but I'm designing a few gadgets related to electronic musical
instruments - and sometimes I want such resistance for volume control
while at other times to be used in certain frequency generator to
control the frequency itself.
>
I vaguely remember there is JFET but care is needed to fit signal into
its small-voltage "ohmic" region and provide suitable bias to the gate.
>
Also at least for some schematics opto-couple with resistor in the
output part seems viable (though I'm not sure about linearity - on the
other hands linearity is not often needed).
>
Next there are motor-driven potentiometers but in my context it looks
ridiculous.
>
Also I vaguely remember tube triode has curious voltage-current
characteristic which probably means it could be utilized in a way
similar to JFET (though it is a bit exotic).
>
And then there are complex and composite devices, digital resistors etc,
but I prefer to work out some more "plain" solutions as they are in part
for educational purposes (e.g. to be constructed by school pupils).
>
May I be missing some fine schematic with a handful BJT or something
like this?
>
thank you in advance,
sincerely yours,
Rodion

To control audio volume, you could use an analog multiplier, a DPOT,
or an MDAC.

You can use a diode or a jfet as a variable resistor, but you'd have
to keep the signal swing low, millivolts, to avoid distortion and then
give up s/n.

A tube or a jfet can be used as a variable gain amp, by adjusting the
DC operating point, but again the price is distortion.

A PWM based VGA would be fun. Not very practical but fun.

If it's just educational, a simple jfet amp with AC coupling in and
out, gain set by DC gate voltage, would be instructive. The distortion
would be instructive too, audible even.

I'll help Spice something if you like. I like to help kids learn
electronics.