Q.Explain the terms r_d and g_m.

Both $r_d$ and $g_m$ are small-signal JFET parameters read from its characteristic curves, one describing the drain side and the other the gate-to-drain control. …

[!TLDR]

$r_d$ is the AC drain resistance $\Delta V_{DS}/\Delta I_D$ (at constant $V_{GS}$); $g_m$ is the transconductance $\Delta I_D/\Delta V_{GS}$ (at constant $V_{DS}$).

The AC (dynamic) drain resistance $r_d$ measures how the drain current responds to a change in drain-to-source voltage while the gate-to-source voltage is held constant. It is defined as $r_d = \Delta V_{DS}/\Delta I_D$ at constant $V_{GS}$ and is expressed in ohm $(\Omega)$; a high $r_d$ is desirable in the saturation region. The transconductance (or forward transconductance) $g_m$ measures how effectively the gate voltage controls the drain current. It is defined as $g_m = \Delta I_D/\Delta V_{GS}$ at constant $V_{DS}$ and is expressed in si …