Source follower
The source-follower circuit linearly transforms a voltage at a high-impedance input terminal into a voltage at a lower-impedance output terminal, such that the output signal is able to drive larger loads than the input signal. It is constructed by connecting a fixed current source to the source of a MOSFET operated in saturation. As we saw in Section 5.1, a current source can easily be implemented using a saturated MOSFET with fixed source and gate voltages.
This arrangement results in a circuit with two MOSFETs of the same type connected in series (Fig. 5.3(a)), where $M_{2}$ is the current source and $M_{1}$ is the input transistor. The input voltage is applied to the gate voltage of $M_{1}$ and the output voltage $Vout$ is the source voltage of $M_{1}$
