Re: Designs of voltage/current controlled resistance

On Wed, 21 May 2025 10:11:03 -0700, john larkin <jl@glen--canyon.com>
wrote:

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.
>
>

That would be a cool class. Discuss it, Spice it, build it, listen to
it.

Read some jfet data sheets too.