Hi Paulo.

One R resistor from gate to source on control voltage and another R
(the same value) from gate to FET drain suppose to bring that FET
quite to be linear device. Those Microchip digi-pots use the same
trick. (negative feedback loop to reduce unlinearity)

Try it and let us know the results.

WBR Dmitry.

PS. Try R = 10K and depends what frequency you'll work and what
    capacitance gate to ground is you'll adjust it.


Olin Lathrop wrote:
>
> >  I am interested in using the linear region of the MOSFET as a
> potentiometer.
> >
> >  My target is a tiny 0.2 ohms max.
> >
> >  Looking at the local electronics distributor, decided to choose an
> IRF640,
> > N-channel trenchmos. It has a 180 miliohm Rds (on), Vdss=200V , Id=16 A.
> >
> >  The initial idea is connecting a 10K potentiometer at the gate, and
> allowing
> > the drain and source as the terminals of the variable resistor.
> > This is going to be battery-supplied (4.5 V), so I expect to use minimum
> > current on it.
> >
> >  Is this correct? What other scheme could I use?
>
> The FET should transition thru various resistances, however these will be
> unpredictable and very nonlinear as a function of the gate voltage.  It
> will probably be too "touchy" at an operating point you care about.  Take
> a look at "digital pots" by Microchip and others if you want more
> predictable and repeatable operation.
>
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