Q. What the voltage gains in the FET amplifier?
For common source
Voltage gain=Vo/Vi = - µRd / Rd + rd
where µ is the amplification factor,Rd the drain resistance and rd the resistance series to the drain gate of FET.
For common drain
Voltage gain = µRsVgd/(µ+1)Rs + rd
where µ is the amplification factor,Rs the source resistance, Vgd the input voltage and rd the resistance in series to the drain.