How FET is used voltage variable resisitor?

Question

Fet's input is connected with low impedance input then how it is used as voltage variable resitors?

Answer 1

Refer to the characteristic curve on the following page (about 1/2 way down the page):
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/fet.html

In the region of Vds from 0 to about 2.5 volts, notice that each of the ascending lines is completely linear. What is the function of Vds with respect to Id? V = I * R. R is the slope of one of those lines (between 0 and 2.5V), and R varies as a function of Vgs, hence you have a voltage variable resistor.

For example, with Vgs = -1.5V, Vds is 3V when Id is 5 mA, so 3V = R * 5mA; Rds = 600 Ohms

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