Hi,
We all know that the main purpose of amplifiers is produce an output signal that is the reproduction of its signal but amplified in magnitude. It would also be assumed that you already understood how MOSFETs work. Just a refresh, for proper operation of the MOSFET you have given, the gate-source voltage must be greater than the threshold voltage of the MOSFET.
A MOSFET may be thought of as a variable resistor whose Drain-Source resistance (typically Rds) is a function of the voltage difference on the Gate-Source pins. If there is no potential difference between the Gate-Source, then the Drain-Source resistance is very high and may be thought of as an open switch — so no current may flow through the Drain-Source pins. When there is a large Gate-Source potential difference, the Drain-Source resistance is very low and may be thought of as a closed switch — current may flow through the Drain-Source pins. Ideally we want Drain-Source resistance to be very high when no current is flowing, and very low when current is flowing.
I would suggest you review some small signal analysis on amplifiers.
