What determines the output impedance of a FET common-source amplifier?

In a common-source FET amplifier, the output is taken from the drain terminal. A FET acts as a voltage-controlled current source and naturally possesses a very high internal output resistance. The effective output impedance of the amplifier circuit is determined by what is connected in parallel with this high internal resistance. In a standard common-source configuration, the external drain resistor is connected between the drain and the power supply. Because the external drain resistor's value is much lower than the FET's internal output resistance, the total parallel resistance is heavily dominated by the drain resistor. Thus, the drain resistor essentially dictates the output impedance of the entire amplifier stage.