Q. What do you mean by Transconductance?

The  control that the gate voltage has over the drain current is measured by transconductance and is similar to the transconductance of the tube.It may be defined as follows:

It is is the ratio of the change in drain current (ΔId) to the change in gate source voltage(Vgs) at constant drain source voltage that is

Transconductance = I_d / V_gs at constant Vds.

The transconductance of a JFET is usually  expressed either in mA or microamperes.

Amplification factor(µ)

It is  the ratio of change in drain source voltage to the change in gate source voltage at constant drain current

Amplification factor= ΔV_ds/ ΔV_gs at constant Id

Amplification factor of a JFET indicates how much more control the gate voltage has over drain current than has the drain voltage.

Relation among JFET parameters.

The relationship among JFET parametes can be established as under:

We know µ=V_ds/ V_gs

Hence

µ = rd*gfs.

Amplification factor = a.c drain resistance * transconductance.