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Date: Wed, 21 May 2025 16:42:19 -0400
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Subject: Re: Designs of voltage/current controlled resistance
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On 5/21/2025 1:11 PM, john larkin 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.


The LM13700 works fine as a multiplier/gain control for audio, but the 
values given in the datasheet for the resistors driving the linearizing 
diodes are kind of nonsense, it works better when the diodes are 
actively driven a la:

<https://imgur.com/a/ETRFQJv>

Not too many extra parts for a substantial improvement in large-signal 
distortion.