JFET model

JFETs are modeled using an equivalent circuit. Model parameters should not be mistaken for datasheet parameters. 

See below for details. 

Large signal model

All transistor node voltage references are with respect to the internal nodes in the following equations (that is, the ohmic resistance pin that is connected the inside of the structure.) 

Static equations 

Drain-source current: 

Gate currents: 

Capacitances

Temperature dependent parameters: 

The following parameters are functions of temperature. T is the operating temperate and TNOM is the nominal (or measured) temperature. T and TNOM can be adjusted in a number of ways. 

Noise equations

The device has thermal noise generators,  and  , as a result of the series ohmic resistances, and the shot and flicker noise generators, collectively  , as a result of the PN junction. 

Ohmic resistance noise:

Shot and flicker noise:

References 

1. G. Massobrio and P. Antognetti, Semiconductor Device Modeling with SPICE, 2nd edition, McGraw-Hill, 1993.
   
2. A. Vladimirescu, The SPICE Book, Wiley, 1994.