Q.What is drain resistance?

Drain resistance describes the opposition the JFET channel offers to the drain current, and it comes in two forms.

The DC drain resistance is the simple ohmic resistance of the channel, given by the ratio of the drain-source voltage to the drain current at a constant gate-source voltage:

The AC (dynamic) drain resistance $r_d$ describes how the drain current responds to a small change in drain-source voltage, again at constant $V_{GS}$:

Both are measured in ohms. The DC value is used for finding the operating (bias) point, while the AC value is the one that enters small-signal analysis and the relation $\mu = r_d \times g_m$. …