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voltage controlled resistors

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owenpauljames

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hi all this is my first post (i think, i have been using this forum for reference for a while),

I was wondering if there is a way of using a voltage, set by a potentiometer, to control the resistance of 2 or more resistors in 2 active filters one band pass and one notch, the resistors set the center frequency of the filters. I have had a go at using n channel jfets, but it seems that getting it to work correctly is very tricky, and as soon as i stop using simulations and get into using real world components with varying gains and tolerences this method just wont work, especially given the control voltage for fets seems to be from 0 to -0.5.

Does anyone have any othere suggestions, e.g. are there ic VCR's? Or if the fet method is the best one does anyone have any tips on how to implement it?

thank you
 
You may want to look into "switched capacitor filters" for what you are doing. You change the frequency of the filter by changing the speed of it's clock. Makes tuning easy.
 
Thank you,

could someone post an example of the mosfet version using the mosfet as a vcr to control the gain in an inverting op amp amplifier, a simple example but its just to demonstrate the principle to me.
 
Thank you,

could someone post an example of the mosfet version using the mosfet as a vcr to control the gain in an inverting op amp amplifier, a simple example but its just to demonstrate the principle to me.
MOSFETs will have the same unit-to-unit problems as JFETs.
I designed a circuit many years ago using an LED/dual LDR optocoupler, in a feedback loop. Limitations are that one end of the pot has to grounded, and the response time to voltage change is tens of milliseconds. Another problem is the only source I can find is Allied Electronics, and they cost $7 - $10.

After all that, if you want to see a schematic that might work, I think I can draw one up for you. It won't be simple. It will also include an op amp and a few resistors and caps and transistors.:(
 
I was just thinking of mabey using optocouplers actually. It seems odd that the world of electronics hasn't really produced a good 3 pin IC that is a linear voltage dependent resistor. One of the problems i was having when simulating a fet in the feedback loop was that the voltage aplied to the gate could sometimes appear on the output of the op amp.

I have yet to try with a mosfet yet, i suppose the metal oxide plate is completely seperate from the silicon channel so there shouldn't be any gate drain current should there? there must be a tiny amount in a jfet which was sometimes big enough to cause problems. Am I assessing that right or am i way off?
 
I was just thinking of mabey using optocouplers actually.
The common optocoupler with NPN output won't work.
It seems odd that the world of electronics hasn't really produced a good 3 pin IC that is a linear voltage dependent resistor. One of the problems i was having when simulating a fet in the feedback loop was that the voltage aplied to the gate could sometimes appear on the output of the op amp.

I have yet to try with a mosfet yet, i suppose the metal oxide plate is completely seperate from the silicon channel so there shouldn't be any gate drain current should there? there must be a tiny amount in a jfet which was sometimes big enough to cause problems. Am I assessing that right or am i way off?
A JFET won't have significant gate current as long as the gate is reverse-biased relative to the source and drain.
The MOSFET gate is indeed insulated, but the resistance will be nonlinear (as is a JFET), although there is a technique for linearizing it. You will still have variations in resistance vs voltage from unit to unit.
 
It seems odd that the world of electronics hasn't really produced a good 3 pin IC that is a linear voltage dependent resistor. One of the problems i was having when simulating a fet in the feedback loop was that the voltage aplied to the gate could sometimes appear on the output of the op amp.

As would it with that mythic 3-terminal voltage dependant resistor - the feedback resistor in an active filter isn't referenced to ground or Vcc.

So you're going to goof around with non-linear optos and mosfets instead of using the switched capacitor filter, the thing that was invented to solve this problem?
 
As would it with that mythic 3-terminal voltage dependant resistor - the feedback resistor in an active filter isn't referenced to ground or Vcc.

So you're going to goof around with non-linear optos and mosfets instead of using the switched capacitor filter, the thing that was invented to solve this problem?
I agree that a switched capacitor filter is probably what he needs. You might call it a frequency-controlled filter, as opposed to a voltage-controlled resistor.
 
switched capacitor. Would I need to make a voltage contolled oscillator then amplify that loads to get a square wave, then use that squarewave to alternate the on offs of two mosfets (acting as the switched part of the switched capacitor). Even then the circuit still isn't linear, the equivilent resistance is 1/Cf where f is the switching frequency, so you get an inverse law. Linearity isn't necessecary though. Am I on the right track or is there a simpler way of using it?
 
switched capacitor. Would I need to make a voltage contolled oscillator then amplify that loads to get a square wave, then use that squarewave to alternate the on offs of two mosfets (acting as the switched part of the switched capacitor). Even then the circuit still isn't linear, the equivilent resistance is 1/Cf where f is the switching frequency, so you get an inverse law. Linearity isn't necessecary though. Am I on the right track or is there a simpler way of using it?

I think you just need to make an adjustable frequency squarewave. Can be done with a pic or by analog. Look at this data sheet.
**broken link removed**

Old part but you can get the idea.
 
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