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From: john larkin <jl@glen--canyon.com>
Newsgroups: sci.electronics.design
Subject: Re: Designs of voltage/current controlled resistance
Date: Wed, 21 May 2025 11:29:27 -0700
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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.