How do you calculate Transistor Transconductance?

Use the formula gm = Ic / VT. Transconductance measures how strongly a transistor’s output current responds to its input voltage. Enter the values you know in the calculator above and it solves for the remaining variable.

What is the formula for Transistor Transconductance?

The Transistor Transconductance formula is gm = Ic / VT, where gm—Transconductance (S = A/V); Ic—Collector bias current (A); VT—Thermal voltage (≈25–26 mV).

What does gm mean in the Transistor Transconductance formula?

gm is Transconductance (S = A/V).

Transistor Transconductance

gm = Ic / VT

Calculator

Transconductance

Collector Current

Thermal Voltage

Result

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Formula

gm = Ic / VT

Description

Transconductance measures how strongly a transistor’s output current responds to its input voltage. For a bipolar transistor it equals the collector current divided by the thermal voltage, so it is set purely by bias current. It is the core gain parameter of a stage: voltage gain equals gm times the load resistance, and it sets the bandwidth and noise of the amplifier.

Variables

gm — Transconductance (S = A/V)
Ic — Collector bias current (A)
VT — Thermal voltage (≈25–26 mV)

Practical Notes

At 1 mA, gm ≈ 38 mS. Unlike a FET, a BJT’s gm is set by current alone, giving very predictable gain. Voltage gain Av = −gm·RC.

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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