Voltage controlled resistor

[jhabib]

What are voltage controlled resistors used for?

[anchovie]

Anywhere that requires a control voltage to alter resistance!

If you give us a bit of background to what prompted the question, you might get a more tailored answer.

[~arph]

Err.. your question is a bit generic..

FET's for example can be used AS voltage controlled resistors. For example in tremolo's or phasers.

Or are you referring to the memristor?

[jhabib]

Ha, sorry, sort of new at this!
I was asking about a diode and how letting current only flow in one direction allows for a smaller threshold (I think its called....the boundries that the signal must pass to clip).  In somebody's reply, he mentioned something a forward conduction region that acts as a voltage controlled resistor.  Just wondering what that meant

[anchovie]

Just how it sounds - resistance changes with voltage.

[R.G.]

If your effect circuit does anything except simply amplify a signal or produce a fixed, unchanging filter effect, it will need some kind of electronically changing element to do the work. For instance:
- a tremolo is a circuit which varies the signal's loudness in response to some regularly changing control signal
- a vibrato is a circuit which varies the signal's frequency in response to some regularly changing control signal
- a distortion is a circuit which changes its gain depending on the instantaneous size of the signal
- an compressor is a circuit which changes its gain depending on the *average* size of the signal

The list goes on and on. At its heart, circuits which modify signals often need a control element. It is simplest for most people to generate a control voltage, so voltage *controlled* elements are highly prized, especially voltage controlled resistors, which often do the effect we want directly.

For instance, a voltage controlled resistor (VCR) can do a tremolo directly by placing the VCR in a voltage divider, making what amounts to a voltage controlled volume pot. Multiple voltage controlled resistors made from JFETs are what are used in most phasers.

The voltage controlled resistor itself should not have a "sound". It should pass signal through unchanged except for the change in its resistance, not distorting or filtering on its own as a side effect.

The bad thing about VCRs is that they don't exist as such. There are always side effects, the principle ones being nonlinearity and distortion. By nonlinearity, I mean nonlinearity of resistance versus control signal; if you double the control signal, the resistance doesn't double or half, it changes by maybe the square or square root of the signal. This makes control hard to do well.

But they nearly all have unwanted distortion, at least over some signal range. This is where we get to diodes.

You can measure the incremental resistance of any circuit element by placing some DC voltage/current across it, then changing the DC voltage a tiny amount, and measuring how much the current changed. The ratio of those two is the incremental resistance at that point in the component's operation. For a real, plain vanilla, everyday resistor, the size of the "tiny amount" you change the voltage doesn't matter. Ordinary resistors will have the same computed R = V/I for all regular sized signals, so R is the same if you use a change in voltage of 1mV as it is if you use a change of 1V or 10V, and for any DC voltage across it that doesn't burn it up.

A diode doesn't do that. For a diode, if you put 0V across the diode and then introduce a change of maybe 0.1V, the current doesn't change much. If you do this with 0.2V, 0.3V, etc, nothing much changes until you get to about 0.45V (for silicon diodes!) where the current starts changing more and more. That's if the change is positive. If the change is negative, the diode essentially doesn't conduct at all. So the diode is a very nonlinear resistor. This is why we use it for clipping and distortion of signals which exceed about 0.45V.

However, if you use a tiny signal variation, maybe 0.010V (10mV), and slowly change the DC voltage across the diode, you will see that the computed resistance at 0.1V (which means the diode is getting a voltage of 0.995 to 0.105V when you add the tiny signal variation) may be, for instance, 500K. Then at 0.2V (that is, from 0.195 to 0.205) it may be 400K, 0.3V (from 0.295 to 0.305) 100K, and so on, you'd find that over the range of 0.45V to 0.70V (roughly, depends on the particular diode's ingredients and manufacture) that the incremental resistance would change from mostly nonconducting (about 100K) to almost a short circuit (about 10 ohms). Within that range, the diode is acting as a voltage controlled resistor.

This is only an approximation, of course. The actual diode response always has some distortion of the signal. But the smaller the signal, the less distortion. For signals of under 10-15mV, the distortion is well under 1%. For signals of 100mV, the distortion may be 5-10%, and we all know that distortion can be massive for signals over 0.5V.

There are actually tremolo circuits and filter circuits which use diodes for variable resistors which have been talked about here. The tremolo in the old Thomas Vox amplifiers is based on a set of four diodes which are fed a control current to set their operating point, and a tiny audio signal to be modified. The Steiner filter is a synthesizer filter which uses several diodes to make a voltage controlled filter.

[CynicalMan]

There are actually tremolo circuits and filter circuits which use diodes for variable resistors which have been talked about here. The tremolo in the old Thomas Vox amplifiers is based on a set of four diodes which are fed a control current to set their operating point, and a tiny audio signal to be modified. The Steiner filter is a synthesizer filter which uses several diodes to make a voltage controlled filter.

One really interesting one is Tim Escobedo's Phuncgnosis. It uses 1 to 3 LEDs to change input resistance to ground based on the DC bias applied to the input by an envelope filter. This affects the response of an opamp filter. I've never seen any other circuit like it.

[birt]

If your effect circuit does anything except simply amplify a signal or produce a fixed, unchanging filter effect, it will need some kind of electronically changing element to do the work. For instance:
- a tremolo is a circuit which varies the signal's loudness in response to some regularly changing control signal
- a vibrato is a circuit which varies the signal's frequency in response to some regularly changing control signal
- a distortion is a circuit which changes its gain depending on the instantaneous size of the signal
- an compressor is a circuit which changes its gain depending on the *average* size of the signal

The list goes on and on. At its heart, circuits which modify signals often need a control element. It is simplest for most people to generate a control voltage, so voltage *controlled* elements are highly prized, especially voltage controlled resistors, which often do the effect we want directly.

For instance, a voltage controlled resistor (VCR) can do a tremolo directly by placing the VCR in a voltage divider, making what amounts to a voltage controlled volume pot. Multiple voltage controlled resistors made from JFETs are what are used in most phasers.

The voltage controlled resistor itself should not have a "sound". It should pass signal through unchanged except for the change in its resistance, not distorting or filtering on its own as a side effect.

The bad thing about VCRs is that they don't exist as such. There are always side effects, the principle ones being nonlinearity and distortion. By nonlinearity, I mean nonlinearity of resistance versus control signal; if you double the control signal, the resistance doesn't double or half, it changes by maybe the square or square root of the signal. This makes control hard to do well.

But they nearly all have unwanted distortion, at least over some signal range. This is where we get to diodes.

You can measure the incremental resistance of any circuit element by placing some DC voltage/current across it, then changing the DC voltage a tiny amount, and measuring how much the current changed. The ratio of those two is the incremental resistance at that point in the component's operation. For a real, plain vanilla, everyday resistor, the size of the "tiny amount" you change the voltage doesn't matter. Ordinary resistors will have the same computed R = V/I for all regular sized signals, so R is the same if you use a change in voltage of 1mV as it is if you use a change of 1V or 10V, and for any DC voltage across it that doesn't burn it up.

A diode doesn't do that. For a diode, if you put 0V across the diode and then introduce a change of maybe 0.1V, the current doesn't change much. If you do this with 0.2V, 0.3V, etc, nothing much changes until you get to about 0.45V (for silicon diodes!) where the current starts changing more and more. That's if the change is positive. If the change is negative, the diode essentially doesn't conduct at all. So the diode is a very nonlinear resistor. This is why we use it for clipping and distortion of signals which exceed about 0.45V.

However, if you use a tiny signal variation, maybe 0.010V (10mV), and slowly change the DC voltage across the diode, you will see that the computed resistance at 0.1V (which means the diode is getting a voltage of 0.995 to 0.105V when you add the tiny signal variation) may be, for instance, 500K. Then at 0.2V (that is, from 0.195 to 0.205) it may be 400K, 0.3V (from 0.295 to 0.305) 100K, and so on, you'd find that over the range of 0.45V to 0.70V (roughly, depends on the particular diode's ingredients and manufacture) that the incremental resistance would change from mostly nonconducting (about 100K) to almost a short circuit (about 10 ohms). Within that range, the diode is acting as a voltage controlled resistor.

This is only an approximation, of course. The actual diode response always has some distortion of the signal. But the smaller the signal, the less distortion. For signals of under 10-15mV, the distortion is well under 1%. For signals of 100mV, the distortion may be 5-10%, and we all know that distortion can be massive for signals over 0.5V.

There are actually tremolo circuits and filter circuits which use diodes for variable resistors which have been talked about here. The tremolo in the old Thomas Vox amplifiers is based on a set of four diodes which are fed a control current to set their operating point, and a tiny audio signal to be modified. The Steiner filter is a synthesizer filter which uses several diodes to make a voltage controlled filter.

this seems to be very close to the way the famous magnatone vibrato works with varistors. so i guess if you experimented a bit you could make the same kind of circuit with a diode (maybe a zener?) or even a small neon light?

[R.G.]

this seems to be very close to the way the famous magnatone vibrato works with varistors.

It's not close to, it is *exactly* the way the Magnatone vibrato works with varistors.

so i guess if you experimented a bit you could make the same kind of circuit with a diode

There is a subtlety there. The region over which most of the incremental resistance change happens is referred to as the "knee" or "conduction knee" of the device. The signal you're trying to use the variable resistance on must be much smaller than the conduction knee to avoid distortion of the signal by the fact that some parts of the signal are at a low voltage part of the knee, some at a high voltage part. If the signal is too big, it effectively modulates itself. That might be useful for some things - I've used it before for a skunk-works distortion - but it is in general not what one wants to do.

The knee of a silicon diode is maybe 0.25V wide. The signal levels for which the distortion is neglegible is around 10-15mV, about 200 times smaller than the knee. The knee of the old varistors used in the Magnatones was perhaps 70V wide, so signals of upwards of a volt were not very distorted. Let me intercept your next question: modern varistors have been "improved" by making the knee very sharp so they can be used for voltage protection devices. No one makes the old soft-knee varistors like the Magnatone used. But I have built Maggie simulators out of tubes and LED/LDR pairs.

(maybe a zener?) or even a small neon light?

Zener? Maybe. True zeners, below about 6V, have a softer knee than avalanche-mode zeners of higher voltage. Their knee is comparable to silicon diodes, so there's not much advantage. Avalanche mode zeners have sharp knees; this is a virtue in voltage regulator zeners, so the makers all try to make sharp knees.

Neon bulbs have a negative knee - that is, when the neon ionizes, its resistance suddenly becomes much less and there has to be some external means to limit current. These probably won't work in a variable-resistance setup. Indeed, the negative-resistance nature of neons is how they can be used in neon-bulb oscillators and counters.

[birt]

for a led/ldr pairs the voltage swing of the oscillator in maggies is probably way too big... the variable resistance is in series with the signal so the LDR must be in series and the max resistance has to be pretty high i think. maybe an incandescent bulb is more interesting than a led since it can probably work with the unmodified maggie lfo.

sorry to go OT :p

[sevenisthenumber]

 

[Cliff Schecht]

There are actually tremolo circuits and filter circuits which use diodes for variable resistors which have been talked about here. The tremolo in the old Thomas Vox amplifiers is based on a set of four diodes which are fed a control current to set their operating point, and a tiny audio signal to be modified. The Steiner filter is a synthesizer filter which uses several diodes to make a voltage controlled filter.

One really interesting one is Tim Escobedo's Phuncgnosis. It uses 1 to 3 LEDs to change input resistance to ground based on the DC bias applied to the input by an envelope filter. This affects the response of an opamp filter. I've never seen any other circuit like it.

Ken Stone's recreation of the Steiner filter is a famous example in the synth world. Diodes are used as feedback elements between the collectors of a differential pair and depending on where the signal is inputted, different filter types can be achieved. The voltage that is used to control the diodes comes from the differential pair and blocking caps prevent any of this from affecting the audio. It's a very clever circuit that I've done an extensive amount of tweaking on. PAiA also had a 2700 series lowpass filter that used a diode to ground as the voltage controlled element in the filter. Not the best sounding unit in the world but they did their job and were very inexpensive compared to the alternatives.

[CynicalMan]

There are actually tremolo circuits and filter circuits which use diodes for variable resistors which have been talked about here. The tremolo in the old Thomas Vox amplifiers is based on a set of four diodes which are fed a control current to set their operating point, and a tiny audio signal to be modified. The Steiner filter is a synthesizer filter which uses several diodes to make a voltage controlled filter.

One really interesting one is Tim Escobedo's Phuncgnosis. It uses 1 to 3 LEDs to change input resistance to ground based on the DC bias applied to the input by an envelope filter. This affects the response of an opamp filter. I've never seen any other circuit like it.

Ken Stone's recreation of the Steiner filter is a famous example in the synth world. Diodes are used as feedback elements between the collectors of a differential pair and depending on where the signal is inputted, different filter types can be achieved. The voltage that is used to control the diodes comes from the differential pair and blocking caps prevent any of this from affecting the audio. It's a very clever circuit that I've done an extensive amount of tweaking on. PAiA also had a 2700 series lowpass filter that used a diode to ground as the voltage controlled element in the filter. Not the best sounding unit in the world but they did their job and were very inexpensive compared to the alternatives.

Is this it?


That's a neat circuit. I suppose there are modern VCR alternatives, but that would still be a simple circuit to build into an envelope filter. It would be easy to mod it for high-pass and maybe possible to mod it for band-pass response. It looks like there's a different band-pass circuit, though.
http://www.freeinfosociety.com/electronics/schemview.php?id=389

[R.G.]

for a led/ldr pairs the voltage swing of the oscillator in maggies is probably way too big...

Yeah. I was after the sound, not the exact circuit. I used a MOSFET oscillator putting a variable current from 0-20ma into the LED. Much simpler. The audio path was identical except for the LDRs instead of the varistors. The audio didn't care.

the variable resistance is in series with the signal so the LDR must be in series and the max resistance has to be pretty high i think.

It's the resistor/capacitor combination that matters, not just the resistor since the following input is a tube grid, high impedance. You just have to keep the resistor/cap combination high enough to not load down the plate-to-cathode on the driving tube. And LDRs have quite high max resistances, well into the megohms. Bigger than most varistors.

maybe an incandescent bulb is more interesting than a led since it can probably work with the unmodified maggie lfo.

Actually, a small neon bulb is the ideal replacement.  Neon bulbs have a negative knee for use in signal work, but if the current is limited externally, you can drive them with a current source, like a tube or FET and have their brightness be modulated from almost-dark to burnout bright. This is how the Fender tremolo "roach" works. It's a neon bulb and an LDR.

sorry to go OT

S'OK. My *mind* is off-topic most of the time. 

[R.G.]

One really interesting one is Tim Escobedo's Phuncgnosis. It uses 1 to 3 LEDs to change input resistance to ground based on the DC bias applied to the input by an envelope filter. This affects the response of an opamp filter. I've never seen any other circuit like it.

Diode variable resistors are used a lot in RF stuff for either switches or variable gain.

The problem you get into using a single diode or string of diodes is that the DC offset on the diode must vary by an amount bigger than the signal swing on the diodes to operate well as a non-distorting variable resistor. Then you have to somehow reject the change in DC drive voltage and keep the changed signal. This is why the Thomas tremolo uses a differentially-driven pair of diode strings. The Steiner also does this by driving two diode strings across the collectors of a diffamp pair by unbalancing the diffamp, and taking the signal off the center tap of the diode string where, if you're lucky, the gross diode control signal is (mostly) self cancelled. The Thomas circuit is really the same thing in principle.

I've messed with the old PAIA diode-modulated filters. There was a wah that used a pair of diodes to ground as the variable resistor in a twin-T filter. Cliff, wasn't that the "Synthespin"? I have it somewhere in the stacks.