An equivalent circuit which simulates the low‐frequency dispersion in the transconductance and the output resistance of an ion‐implanted InP JFET is presented. The dispersion consists of reductions in the transconductance and the output resistance occurring between 100 Hz and 1 MHz and involves two dominant time constants in each parameter. The equivalent circuit can be used for the design of monolithic fiber‐optic receiver preamplifiers and other small‐signal circuits. © 1993 John Wiley & sons, Inc.