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Using a FET or Transitors as a variable resistor

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walters

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fet variable resistor

How can i use a FET or transitor in a circuit to be acting like a variable resistors?

I think i use DC voltage to change the FET or transitors resistance

But what are these circuits called and where can i find a schematic network that was a FET or transitor that is used as a linear or audio rheostate or voltage divider resistor ?
 

fet as variable resistor

"JFET as variable resistor"



Regards,
IanP
 

fet as a variable resistor

what are these circuit/networks called

In my electronic books i don't have a charpter or page about using a FET,JET,mosFET or transitor as a voltage or current variable resistor where and what are these circuits called please?
 

jfet vcr

It's called a Voltage Controlled Resistance (VCR). Usually an FET is used as opposed to a BJT, as a FET, since it's really resistive, allows current to flow in both directions.

Any book on FET's contains FET curves, and those curves show that with a small Vds, relative to Vgs, the FET is linear and this region is called the Voltage Controlled Resistance region. Vds must be small.

Also, there is a way to get a little more linearity, increase the Vds range, by adding Vds/2 to Vgs.

1
--- =
Rds

2k [ (Vgs - Vt) - Vds/2 ]

becomes

1
---- =
Rds

2k [ (Vgs + Vds/2 - Vt) - Vds/2 ] =

2k [ Vgs - Vt ]

via this trick.

(The above equation is from "The Art of Electronics" by Horowitz and Hill.)

I have used a MOSFET as a VCR, but I cannot stress enough that Vds must be very small. You must use the VCR at a place in your circuit where the voltage is very low -- I used it as the lower resistor in a very top-heavy voltage divider. Also, I found that I needed to parallel another resistor across Rds for best results. (I used a VCR as a gain control for a sine wave oscillator design a few months back.)
 

n fet as a variable resistor

So are they called VCR circuits or VCR networks
 

using a fet

The shape of the conducting channel in a FET is altered when a potential difference is applied to the gate terminal (potential relative to either source or drain.) In an n-channel "depletion-mode" device, a negative gate voltage causes a depletion region to expand in size and encroach on the channel from the side, narrowing the channel. If the depletion region completely closes the channel, the resistance of the channel becomes very large, and the FET is effectively turned off. Positive gate voltage attracts electrons from the surrounding semiconductor next to the gate, forming a conductive channel. At low source-to-drain voltages, small changes to the gate voltage will alter the channel resistance. In this mode the FET operates like a variable resistor. This mode is not employed when amplification is needed.
 

fet voltage controlled resistor

Thanks for the help

Where do u find the circuit description or theory behind how a VCR circuit or network works?
 

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