Equations:

I_DS=

I_DSS(1-V_GS/Vp)²

RS= V_RS/I_DS

VGS is called the gate-source cutoff voltage or cutoff voltage, and is represented by VGS (off). In the case of n-channel JFET, the VGS (off) value has a negative sign. It is difficult to measure the VGS of the actual JFET corresponding to ID = 0. When using a small signal JFET in an amplifier, the VGS that reaches ID = 0.1-10μA is often defined as VGS (off).