This configuration is shown in the figure. The signal is applied to the gate and taken from the source.
Formulas 1
Common drain gain:
G = Vout / Vin
G = (Gm x Rs) / (1 + gm x Rs)
Where:
Gv is the voltage gain
Vout is the output voltage in volts (V)
Vin is the input voltage in volts (V)
gm is the transconductance in siemens (S)
Rs is the source resistance in ohm (Ω)
Note: The value for of Gv in common applications is in general near 1.
Formula 2
Output impedance:
Rout = Rs / (1 + gm x Rs)
Where:
Rout is the output impedance in ohm (Ω)
Rs is the source resistance in ohm (Ω)
gm is the transconductance in siemens (S)
Application example:
Calculate the voltage gain and the output impedance for a Common-Surce circuit using a JFET with 5 000 uS of transconductance and a 5 000 Ω source resistor.
Data:
Rs = 5 000 Ω = 5 x 103 Ω
gm = 5 000 uS = 5 x 10-6 S
Gv = ?
Rout = ?
a) Calculating Gv (formula 1):
Gv = ( 5 x 10-3 x 5 x 103 ) / (1 + 5 x 10-3 x 5 x 103 )
Gv = 25 / 1 + 25
Gv = 0.961
b) Calculating Rout (formula 2):
Rout = (5 x 103) / ( 1 + 5 x 10-3 x 5 x 103)
Rout = 5000 / 6
Rout = 833.3 Ω
