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

Field Effect Transistors (FETs) inherently feature an extremely high input impedance at the gate terminal, often in the megaohms or gigaohms. This is because the gate is electrically isolated from the conducting channel. Because the internal gate impedance is so exceptionally high, the actual input impedance seen by an incoming signal is almost entirely determined by the external resistors used to bias the gate. These external biasing resistors are connected in parallel with the gate's internal resistance. When a very high resistance (the FET gate) is in parallel with a lower resistance (the biasing network), the total effective impedance is approximately equal to the lower resistance. Therefore, the gate biasing network sets the practical input impedance of the amplifier stage.