The common source amplifier, like all MOSFET amplifiers have the characteristic of high input impedance. The value of the input impedance for both amplifiers is basically limited by the biasing resistors R2 and R3. High input impedance is desirable to keep the amplifier from loading the input source. The CS MOSFET amplifier can be compared to the CE BJT amplifier. Like the CE amplifier, the CS amplifier has a negative voltage gain and an output impedance approximately equal to drain resistor.