Voltage Amplification

The graph in Fig. 1 shows the drain (channel) current, I_D as a function of the gate bias voltage E_G for a hypothetical n-channel JFET when no signal is applied to the gate electrode. The drain voltage E_D is assumed to be constant. When E_G is fairly large and negative, the JFET is pinched off, and no current flows through the channel. As E_G gets less negative, the channel opens up, and current begins flowing. As E_G gets still less negative, the channel gets wider, and the current I_D increases. As E_G approaches the point where the source-gate (S-G) junction is at forward breakover, the channel conducts as well as it possibly can. If E_G becomes positive enough so that the S-G junction conducts, the JFET no longer works properly. Some of the current in the channel is shunted through the gate. This is like a garden hose springing a leak.
The best amplification for weak signals is obtained when E_G is such that the slope of the curve in Fig. 1 is steepest. This is shown roughly by the range marked X in the graph. For power amplification, results are often best when the JFET is biased at or beyond pinchoff, in the range marked Y.

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