TRANSCONDUCTANCE

Look back for a moment at the discussion of dynamic current amplification for bipolar transistors earlier in this chapter. The JFET analog of this is called dynamic mutual conductance or transconductance.
Refer to Fig. 1. Suppose that E_G is a certain value, with a corresponding I_D that flows as a result. If the gate voltage changes by a small amount ΔE_G, then the drain current also will change by a certain increment ΔI_D. The transconductance is the ratio ΔI_D/ΔE_G. Geometrically, this translates to the slope of a line tangent to the curve of Fig. 1 at some point.

Fig. 1. Relative drain current as a function of gate voltage in a hypothetical n-channel JFET.

The value of ΔI_D/ΔE_G is not the same everywhere along the curve. When the JFET is biased beyond pinchoff, as in the region marked Y in Fig. 1, the slope of the curve is zero. There is no drain current, even if the gate voltage changes. Only when the channel conducts some current will there be a change in I_D when there is a change in EG. The region where the transconductance is the greatest is the region marked X, where the slope of the curve is steepest. This is where the most amplification can be obtained. A small change in E_G produces a large change in I_D, which in turn causes a large variation in a resistive load placed in series with the line connecting the drain to the power supply.