Characteristics of Common Drain Amplifier
Figure: Basic N-channel JFET source follower circuit (neglecting biasing details).
AV = Uout / Uin = gmRs/gmRs + 1 ≈ (gmRs >> 1)
Ai = ∞
rin = ∞
(The parallel notation A||B points out the impedance of components A and B that are connected in parallel)
rout = RS || 1/ gm
= (Rs/ gm) / (Rs + (1/ gm))
= Rs / gm Rs+1 ≈ 1/ gm (gm Rs >> 1)
The variable gm that is not listed in Figure is the transconductance of the device (generally given in units of siemens).
Figure: Basic N-channel MOSFET common-source amplifier with active load ID.
Figure: Small-signal circuit for N-channel MOSFET common-source amplifier.