The design of transistor amplifiers require us to establish a DC operating point. This process is biasing (or bias design). The idea is that regardless of the variations in parameters from device to device, we can establish a DC current in the drain/collector that is predictable and insensitive to these variations. We also need to establish the operating point in the active region of operation, so that we still get a high voltage gain.
A key thing to keep in mind is that the amplifier topology is designed with the idea that eventually a small-signal voltage will be attached to the circuit. This helps explain the reason we include certain resistors (often larger ones) where they may seem unnecessary.
MOSFETs
There are two main “good” ways to bias a MOSFET. The first is by fixing and connecting a resistor to the source terminal.