In the common-source configuration the signal is applied to the gate and taken from the drain as shown by the figure bellow.

**Formulas 1**

Common source gain:

Gv = Vout / Vin

Gv = -gm x RL

**Where: **

Gv is the voltage gain

Vout is the output voltage in volts (V)

Vin is the input voltage in volts

gm is the transconductance in Siemens (S)

RL is the load resistance in ohm (Ω)

**Application example: **

Determine the voltage gain in a circuit using a JFET with 1 000 uS of transconductance and a 10 kΩ load resistance.

**Data: **

gm = 1 000 uS = 10^{-3} S

RL = 10 k = 10 x 10^{3} Ω

**Applying formula 1:**

Gv = -10^{-3} x 10 x 10^{3}

Gv = -10