Now, we measure the actual device parameters. Here is a circuit with a single voltage source. We vary the value of R1 to get the Id=Id(Vg) curve.
I used a 100 ohm and a 1k ohm resistors for R1. The lord lines are in orange (-2V gives 20mA) and in blue (-4V gives 4mA) in the figure, and the measured values, -0.44V for a 100 ohm resistor and -1.21V for a 1k ohm resistor, are plotted in red.
The Idss classification for the device is GR (6.0-14.0mA), shown in green in the figure, and we conclude that the Idss=7mA and the Vto=-2V is a good guess.
The Beta is obtained from the equation, Beta=Idss/(Vto^2)=7m/(-2)^2=1.75m, to give the device model of;
.model 2SK192A NJF(beta=1.75m vto=-2.0 cgd=1p cgs=2p)
and the following Id=Id(Vg) curve.
If we choose Id to be 3mA, R4 now becomes (6V-(-0.7V))/3mA~2kohm.
In AC analysis, a relatively large stray capacity of 50nF is assumed.
