Q. How can a JFET be used as a constant current source? Explain from the drain characteristics.

As the VDS voltage is increased from 0 to a few volts, the current will increase as determined by ohm's law and the plot of ID versus VDS will appear as shown below.

The relative straightness of the graph reveals that for the region of low voltage level referred to as  VP. The depletion region will widen, causing a noticeable reduction in the channel width. The reduced path of conduction causes the resistance to increase and the curve in the graph to occur. The more horizontal the curve the more the resistance, suggesting that the resistance is approaching infinite ohms. If VDS is increased to a level where it appears that the two depletion regions will touch. A condition known as pinch off will result. The level of VDS that establishes this condition is referred to as pinch off voltage. The term pinch off is a misnomer which actually means that the value of ID has become 0 A. this is hardly the case as the ID still maintains a saturation level defined as IDSS. In reality a very narrow channel still exists, with a current of very high density. The fact that ID does not drop off to 0 at pinch-off and maintains a saturation level is verified by the fact : the absence of the drain current would remove the possibility of different potential levels through out the n channel material to establish the varying levels of reverse bias along the p-n junction. The result would be a loss of depletion region distribution that caused pinch off in the first place.

The fact about the constant current IDSS can be used to make a JFET a constant current source as its current value remain constant. The JFET is thus a very versatile device and can be used  as a current source with much stabler characteristics than a zener diode current regulator.