Characteristics of Common Drain Amplifier

                               Figure: Basic N-channel JFET source follower circuit (neglecting biasing details).

Voltage gain:

A_V = U_out / U_in = g_mR_s/g_mR_s + 1 ≈ (g_mR_s >> 1)

Current gain:

A_i = ∞

Input impedance:

r_in = ∞

Output impedance: 

(The parallel notation A||B points out the impedance of components A and B that are connected in parallel)

r_out = R_S || 1/ g_m

= (R_s/ g_m) / (R_s + (1/ g_m))

 = R_s / g_m R_s+1 ≈ 1/ g_m (g_m Rs >> 1)

The variable g_m 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 I_D.

                                      Figure: Small-signal circuit for N-channel MOSFET common-source amplifier.