A JFET buffer (common drain amplifier) is useful because it has a very high input impedance compared to a transistor.
Equations:
I DS= I DSS(1-V GS /Vp) 2
RS= V RS/I DS
VGS is called the gate-source inter-blocking voltage or cut-off voltage and is shown by VGS (off). In the case of an n-channel JFET, the VGS (off) value has a negative sign. Measuring the actual JFET corresponding to VGS with ID = 0 is difficult. In the small-signal JFET used by the amplifier, VGS with ID = 0.1-10μA will be reached. More often defined as VGS (off)