FET is a device that is usually operated in the constant-current portion of its output characteristics. But if it is operated on the region prior to pinch-off (that is where VDS is small, say below 100 mV), it will behave as a voltage-variable resistor (WE). It is due to the fact that in this region drain-to-source resistance RDS can be controlled by varying the bias voltage VGS. In such applications the FET is also referred to as a voltage-variable resistor or volatile dependent resistor. It finds applications in many areas where this property is useful.
Figure shows the drain characteristic curves for a 2N 5951 in the ohmic region (i.e. for low VDS). From the characteristic curve it can be seen that RDS varies with VGS. For example, when VGS = 0, RDS = 133 ohm and when VGS = – 2 V, RDS = 250 ohm. Because of this a JFET operating in the ohmic region with small ac signals acts as a voltage-controlled resistance.
Note that the drain curves shown in figure,extend on both sides of the origin. This means that a JFET can be employed as a voltage-variable resistor for small ac signals, typically those less than 100 mV. When it is employed in this way, it does not require a dc drain voltage from the supply. All that is required is an ac input signal.