What is the formula for Common-Source JFET Gain?

The Common-Source JFET Gain formula is Av = −gm × Rd, where Av—Voltage gain (negative = inverting); gm—Transconductance (S); Rd—Drain load resistance (Ω).

What does Av mean in the Common-Source JFET Gain formula?

Av is Voltage gain (negative = inverting).

Common-Source JFET Gain

Q: How do you calculate Common-Source JFET Gain?

Use the formula Av = −gm × Rd. The voltage gain of a common-source FET amplifier is the device transconductance times the drain load resistance, inverting. Enter the values you know in the calculator above and it solves for the remaining variable.

Av = −gm · Rd

Calculator

Voltage Gain

Transconductance

Drain Resistor

Result

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Formula

Av = −gm × Rd

Description

The voltage gain of a common-source FET amplifier is the device transconductance times the drain load resistance, inverting. Because a FET’s gm is lower than a comparable BJT’s, common-source gains are modest, but the very high input impedance of the gate makes FET stages ideal as low-loading front ends and high-impedance buffers.

Variables

Av — Voltage gain (negative = inverting)
gm — Transconductance (S)
Rd — Drain load resistance (Ω)

Practical Notes

With a source degeneration resistor Rs, Av = −gm·Rd/(1 + gm·Rs), which linearises the stage. FET gm depends on bias current, so gain varies with operating point.

Related Concepts

All Discrete Amplifiers formulas →

Thermal Voltage Small-Signal Emitter Resistance Transistor Transconductance Diode Dynamic Resistance Common-Emitter Voltage Gain BJT Alpha–Beta Relationship Darlington Pair Current Gain Emitter-Follower Input Impedance

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