Self-Biased Common-Source Stage
If you didn't have \$R_F\$ (i.e. it were shorted out), the input impedance to the gate would be \$R_D\$ and, the gain from gate to drain would be unity. Not much of a circuit without \$R_F\$ really.
In the circuit above I've added \$R_{IN}\$ and the gain will be approximately: -
$$\dfrac{V_D}{V_{IN}} = -\dfrac{R_F}{R_{IN}}$$