Unlike its BJT cousin, the MOS transistor is a square-law device; therefore, you would not expect it to provide a linear amplification except over a narrow operating range. Indeed, by exploiting the square-law dependency of the drain current on the gate-to-source voltage, you can design mixers and frequency doublers. This is our conviction of a MOSFET whose drain is connected to a supply voltage through a resistor. But nothing prevents me from replacing the drain resistor with another MOSFET. Then, I get a linear voltage amplification if I force the MOSFET placed between the first transistor drain and the supply voltage into saturation. Bingo!