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.)