Re: Designs of voltage/current controlled resistance

On 5/21/2025 8:44 AM, RodionGork wrote:

What approaches exist of creating voltage or current-controlled
resistance?

Range of values?  Operating potential?

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.

Volume control can be done with an analog multipler.  Or, you could
use a DAC (if you can tolerate discrete steps in your gain/attenuation).
One of my tape decks uses a CdS cell excited by a lamp (led?) in a
light-tight enclosure.
A lot will depend on where, in the signal path, you want to
adjust the gain/attenuation.

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.

SWMBO's "bookshelf HiFi" uses a motor driven pot to control a set
of pots sharing a single driveshaft.  The advantage it has is:
- you can see the setting (without some additional display)
- you can manually control the setting (without some additional control)
It can also be applied to any control that operates by rotation
(e.g., a variable capacitor, tone controls, etc.)

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).

The motorized pot may be the easiest for them to understand.
But, likely hardest to come by.

May I be missing some fine schematic with a handful BJT or something
like this?