Q. Explain working of Common drain amplifie?

When an ac signal drives the gate of JFET, it produces an ac drain current. It flows through Rs and produces an ac output voltage that is approximately equal to the input voltage and is in phase with it.

fig.(a)  COMMON DRAIN AMPLIFIER

In the figure, R₃ = R_s.

From the figure:

V_in = V_gs+g_mV_gsRs

V_in = (1+g_mR_s) V_gs

V_out = g_mV_gsR_s

V_out/V_in = g_mV_gsR_s/ (1+g_mR_s) V_gs = R_s/(1/g_m)+R_s

R_s> = 1/g

V_out = V_in R_s/(R_s+(1/gm))

Zin depends on the type of biasing circuit used. In case of voltage divider biasing circuit, Zin = R₁ and R₂ in parallel.

The common drain amplifier with high input impedance and near-unity voltage gain is used as a buffer amplifier. The common drain amplifier is also known as "SOURCE FOLLOWER".