voltage controlled resistor

[xmat]

Hello.

I need to make up a schematic with a voltage controlled resistor for an adaptive (state variable) low pass filter..

It is a bit confusing to explain thoroughly, but I can say the following:

Is there a way to have a circuit component (such as a JFET) acting as a voltage controlled resistor, for a WIDE dynamic range of signals/voltages (i.e 0-6 Volts) ?
So far I have come up with solutions that work with signals that vary from 1 to 2 volts, yet I need a more "flexible" VCR..

So, are there any suggestions on where to search or of any schematics??

Your help is really appreciated,

xmat.

 

[audioguru]

You could pulse-width-modulate a 1k resistor with a Cmos CD4066 transmission gate in series with it at a frequency much higher than your circuit. The PWM can be varied from zero where the resistor appears as having an extremely high value, through 50-50 where the resistor is on for half the time and appears as a 2k resistor, to 100% where the resistor appears as its 1k value.
The PWM can be applied to many stages and they will operate exactly the same, without any matching problems.
Since the PWM swutching frequency is very high, it is easy to filter out.
A voltage controlled PWM circuit is an integrator and Schmitt trigger making a triangle wave. Then a comparator is used to give its PWM output when it compares the triangle wave with a DC voltage. :lol:

 

[Oznog]

Sure, here's a JFET designed for the job:
https://www.goldmine-elec-products.com/prodinfo.asp?number=G4257&variation=&aitem=24&mitem=30
And here's an optocoupler. Note it's not a current output but a resistance.
https://www.goldmine-elec-products.com/prodinfo.asp?number=G15396&variation=&aitem=2&mitem=2

But PWM is the more professional way to do it. It's also essentially distortion free. Note the optocoupler only has a linear output when it's fed by a constant current source for example.

 

[xmat]

You could pulse-width-modulate a 1k resistor with a Cmos CD4066 transmission gate in series with it at a frequency much higher than your circuit. The PWM can be varied from zero where the resistor appears as having an extremely high value, through 50-50 where the resistor is on for half the time and appears as a 2k resistor, to 100% where the resistor appears as its 1k value.
The PWM can be applied to many stages and they will operate exactly the same, without any matching problems.
Since the PWM swutching frequency is very high, it is easy to filter out.
A voltage controlled PWM circuit is an integrator and Schmitt trigger making a triangle wave. Then a comparator is used to give its PWM output when it compares the triangle wave with a DC voltage. :lol:

Respect....

Great. Thanks.

Any VC PWM schematic??For a reference..

xmat.

 

[audioguru]

Hi Xmat,
This circuit provides voltage controlled PWM. Select the speed of the opamps, comparator and oscillator frequency to be high enough for your application.

 

[xmat]

Many thanks

Ok, will check how that could best serve my application.

One last question on this:
What do you think of this IC called "LM13700" of National Semiconductors?

Is it fast enough for "variable filtering" of audio signals of a dynamic range of 0-6Volts?

Many thanks for your responses, they have been really helpfull.

xmat.

 

[audioguru]

The LM13700 is also good for your application. :lol:

 

[xmat]

Ok, thanks!!

 

[julien]

Hello all,
I'm very interested in your application, i also have the same request like xmat... but I don't understand the purpose Can you explain in more details the aim of the PWM and switch for building a VCR ?

thanks a lot and so sorry for my poor english...

 

[ericgibbs]

Hello all,
I'm very interested in your application, i also have the same request like xmat... but I don't understand the purpose Can you explain in more details the aim of the PWM and switch for building a VCR ?

thanks a lot and so sorry for my poor english...

hi,
This thread is 4 years old...

 

[julien]

yes I know, but i'm very instersting in this application and I don't understand well how it could work ...

 

[julien]

nobody to help me ... ?

 

[audioguru]

You forgot to tell us what the Voltage Controlled Resistor will be used for.
Maybe An LM13700 will work.
Maybe a FET will work.
Maybe a transistor or diode will work.
Maybe a CD4066 driven with pulse-width-modulation will work.

 

[julien]

hello audioguru and thanks for your reply,
I just would want to understand how the PWM solution works with the CD4066 ? you drive the switch with the PWM... ?

can you tell me more please ?

 

[audioguru]

If a CD4066 is in series with a resistor and is turned on and turned off over and over at a very high frequency then the duty-cycle of the PWM determines the average value of the resistor.

Example, a 10k resistor:
1) If the duty-cycle (onff) is 50:50 then its average value is 20k.
2) If the duty-cycle is 5:95 then its average value is 950k.
3) If the duty-cycle is 95:5 then its average value is 10.5k.

A phase-shift oscillator has 3 resistors that must be changed the same amount at the same time to change the frequency. Three parts of the CD4066 can be used. Then a simple lowpass filter at the output removes the high frequency PWM signal.

 

[julien]

thanks for your reply, but I don't understand why it could done it ... you place the resistor on the pin 1 (for example) of the CD4066 ? not in parallel ? why the high frequency of the MOS switch can permit to have a variable resistor ?

thanks a lot

 

[audioguru]

You connect a 10k resistor in series with pin 1. Then when the switch is conducting there is 10k ohms from the resistor to pin 2. When the switch is turned off then there is infinite resistance.

The duty-cycle of the switching determines the average value of the resistance because the switching is done at a high frequency.
The duty-cycle is caused by the PWM.

The resistor is not connected in parallel with the CD4066 switch because the switch has a resistance when it is turned on of about 90 ohms. The range would be limited and would be different with different ICs.

 

[julien]

Ok, thanks a lot, it's more clear !