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 email me substitute github with gmail please

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

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.

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

RodionGork <rodiongork@github.com> wrote:

Hi Friends!

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

[...]

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

There are also thermal devices like tungsten-filament lamps,
iron-hydrogen barretters and thermistors. These have a relatively slow
reponse but are less likely to distort the waveform. They have fallen
out of popularity in recent years and may not suit your application
because of the power they consume and the difficulty of obtaining the particular type you may need.

They are two-terminal devices, so the heating power has to be fed into
the same terminals as the signal. It can sometimes be difficult to
prevent interaction between them.

Photoconductive cells have a faster (but slightly strange) response and
can be illuminated by LEDs, tungsten lamps or gas-discharge tubes.
Their linearity is quite good enough for domestic audio purposes. They
are disliked by environmental campaigners because many of them contain
cadmium (in insignificant quantities).

Another method is to use a fast-acting CMOS switch in series with a
resistor. By controlling the on/off ratio of the switch, the resistor
can appear to be a higher value. This is particularly useful for making resistors in different parts of the circuit track accurately together,
such as in an audio filter. The disadvantage is the need for a
switching frquency which is much higher than the highest frequency
signal and the necessity for a filter on the output to remove that
switching frequency from the signal.


--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

"RodionGork" <rodiongork@github.com> wrote in message news:d469c760bbcd14a1f148c0e1b29653f7@www.novabbs.com...

Hi Friends!

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

Usually opto devices or FETs.
It has been recently demonstrated here that there are other approaches but it depends on what you're trying to achieve.

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).

If it's for school students I'd have them play with one of these and a variable light source in a closed box.

https://www.google.com/search?q=ORP12&udm=2

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

Depends on exactly what you want to achieve.

thank you in advance,
sincerely yours,
Rodion

--
to email me substitute github with gmail please

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

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?

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

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.

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

"John R Walliker" <jrwalliker@gmail.com> wrote in message news:100l974$2nno5$1@dont-email.me...

On 21/05/2025 17:26, Edward Rawde wrote:

"RodionGork" <rodiongork@github.com> wrote in message news:d469c760bbcd14a1f148c0e1b29653f7@www.novabbs.com...

Hi Friends!

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

Usually opto devices or FETs.
It has been recently demonstrated here that there are other approaches but it depends on what you're trying to achieve.

To be more specific, have a look at the H11F1 which is an LED
coupled with a bidirectional light sensitive FET. This has been
around for many decades but is still available.
John

Thanks. Interesting device, and LTSpice models exist.

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On Wed, 21 May 2025 16:03:54 -0400, "Edward Rawde"
<invalid@invalid.invalid> wrote:

"John R Walliker" <jrwalliker@gmail.com> wrote in message news:100l974$2nno5$1@dont-email.me...

On 21/05/2025 17:26, Edward Rawde wrote:

"RodionGork" <rodiongork@github.com> wrote in message news:d469c760bbcd14a1f148c0e1b29653f7@www.novabbs.com...

Hi Friends!

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

Usually opto devices or FETs.
It has been recently demonstrated here that there are other approaches but it depends on what you're trying to achieve.

To be more specific, have a look at the H11F1 which is an LED
coupled with a bidirectional light sensitive FET. This has been
around for many decades but is still available.
John

Thanks. Interesting device, and LTSpice models exist.

Interesting, but that looks linear to maybe 20 millivolts.

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

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.

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)

On 22/05/2025 6:14 am, john larkin wrote:

On Wed, 21 May 2025 16:03:54 -0400, "Edward Rawde"
<invalid@invalid.invalid> wrote:

"John R Walliker" <jrwalliker@gmail.com> wrote in message news:100l974$2nno5$1@dont-email.me...

On 21/05/2025 17:26, Edward Rawde wrote:

"RodionGork" <rodiongork@github.com> wrote in message news:d469c760bbcd14a1f148c0e1b29653f7@www.novabbs.com...

Hi Friends!

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

Usually opto devices or FETs.
It has been recently demonstrated here that there are other approaches but it depends on what you're trying to achieve.

To be more specific, have a look at the H11F1 which is an LED
coupled with a bidirectional light sensitive FET. This has been
around for many decades but is still available.
John

Thanks. Interesting device, and LTSpice models exist.

Interesting, but that looks linear to maybe 20 millivolts.

We are talking about the linearity of the light-sensitive FET

https://www.onsemi.com/products/interfaces/high-performance-optocouplers/specific-function-optocouplers/h11f1m

which is shown in Figure 2 of the data sheet. Like all FETs the channel
turns into a constant current diode when there's enough voltage drop
across it - here at more than about +/-100mV, but the range over which
it is linear enough is going to depend on the application.

--
Bill Sloman, Sydney

--- SoupGate-Win32 v1.05
* Origin: fsxNet Usenet Gateway (21:1/5)