Source follower stage (also called as “common-drain” stage) can operate as voltage buffer since its voltage gain is almost equal to 1. The source follower senses the signal at the gate and drives the load at the source, allowing the source potential to “follow” the gate voltage.

When Vin < VTh, M1 is OFF. So no current will flow through device. ∴ Vout = 0

As Vin crosses VTh, M1 turns ON in saturation. (for typical values of VDD). As Vin increases, ID increases.

So Vout will increase. Vout follows Vin with a difference equal to VGS.

To calculate Av, differentiate (1) w.r.t. Vin

We know that,

This is the expression for Voltage gain of source follower stage. Now, expression for output resistance Ro is given as –

We write,