In the common-gate configuration a FET is wired as shown in the figure. The signal is applied to the source and taken from the drain.
Formula 1
Common gate gain:
Gv = gm X RL
Where:
Gv is the voltage gain
gm is the transconductance in siemens (S)
RL is the load resisance in ohm (Ω)
Formula 2
Input resistante:
Rin = 1 / Gm
Where:
Rin is the input resistance in ohm (Ω)
gm is the transconductance in siemens (S)
Application example:
What is the voltage gain and input impedance of a circuit using a JFET in common gate configuration where the load resistance is 10 kΩ ? The transconductance ofthe used transistor is 1 000 uS.
Data:
RL=10 kΩ
gm = 1 x 10-3 S
Gv = ?
Rin = ?
Calculating Gv: (formula 1)
Gv = 1 x 10-3 x 10 x 103 = 10
Determining Rin (formula 2)
Rin = 1 / 10-3
Rin = 103
Rin = 1000 Ω
