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voltage controlled resistor jfet

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liuman51086

New Member
Hi all,

I'm looking to build a voltage controlled resistor from a JFET. I know the general operation of fets and how they work.

My application is purely DC current (very small), non audio related or anything of that kind. I just need it to be a variable resistor in a simple circuit (it's actually going to be used basically in a resistor divider circuit). I can't use a pot because I need it to be adjustable without me touching it. I can't use a digital pot because they don't give me the type of precision I need and the low resistance I want.

I'm looking at this site: Using a FET as a Voltage Controlled Resistor

Any recommendations for the resistances of R1 and R2 for the linear behavior resistors?

More importantly though, does anyone have any recommendations for the model of the JFET to use (there's too many out there, I like the vcr4n fet but it only passes AC in the d-s)? I'm looking for a resistance of the d-s to be somewhere like 100-300 Ω. Of course I could add like another resistor in series or something to get those values.

Thanks!
 

Roff

Well-Known Member
How much voltage will appear across drain-source?
What application will this DC attenuator be used in?
Do you realize that you can make it fairly linear for low voltages, but the V-R curve will not be predictable from unit to unit?
 
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MikeMl

Well-Known Member
Most Helpful Member
First try:
 

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