How do you calculate JFET Drain Current (Shockley)?

Use the formula Id = Idss × (1 − Vgs/Vp)². Shockley’s equation gives the drain current of a JFET in saturation as a function of gate–source voltage. Enter the values you know in the calculator above and it solves for the remaining variable.

What is the formula for JFET Drain Current (Shockley)?

The JFET Drain Current (Shockley) formula is Id = Idss × (1 − Vgs/Vp)², where Id—Drain current (A); Idss—Drain current at Vgs = 0 (A); Vgs—Gate-source voltage (V); Vp—Pinch-off / cutoff voltage (V).

JFET Drain Current (Shockley)

Id = Idss · (1 − Vgs/Vp)²

Calculator

Drain Current

Idss

Gate-Source Voltage

Pinch-off Voltage

Result

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Formula

Id = Idss × (1 − Vgs/Vp)²

Description

Shockley’s equation gives the drain current of a JFET in saturation as a function of gate–source voltage. The current is maximum (Idss) at zero gate voltage and falls to zero as Vgs approaches the pinch-off (threshold) voltage. The square-law relationship gives JFETs their characteristic soft, low-distortion transfer curve used in audio and RF front ends.

Variables

Id — Drain current (A)
Idss — Drain current at Vgs = 0 (A)
Vgs — Gate-source voltage (V)
Vp — Pinch-off / cutoff voltage (V)

Practical Notes

For an n-channel JFET both Vgs and Vp are negative. The transconductance is gm = (2·Idss/|Vp|)·(1 − Vgs/Vp), largest near Vgs = 0.

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