Equations:

I_DS=

I_DSS(1-V_GS/Vp)²

RS= V_RS/I_DS

VGS is called the gate-source cut-off voltage or cutoff voltage, indicated by VGS (off). In the case of n-channel JFET, the VGS (off) value has negative notation, and because it is difficult to measure the actual JFET corresponding ID =0VGS, when the amplifier uses a small signal JFET, the ID =0.1-10μA VGS define as VGS (off) More