Q. Gate source cut off voltage?

It is the gate source voltage where the channel is completely cut off and the drain current becomes zero.It is interesting to note that the value of gate source voltage should always have the same magnitude as the pinch off voltage.

The different parameters of JFET are:

a.c drain resistance(rd):

Corresponding to the a.c plate resistance, we have a.c drain resistance in a JFET.It may be defined as follows:

It is the ratio of change in  drain -source voltage(¨Vds) to the change in drain current(Id) at constant gate source voltage ie, a.c drain resistance rd=¨Vds/¨Id at constant Vgs.

Referring to the output characteristics of JFET it is clear that the drain resistance of a JFET has a large value ranging from 10k? to 1k?.