Basic Nmos amplifier working
Rb is biasing the NMOS into saturation (a good thing). 

Rb is also a bootstrap Resistor (maybe good, likely not). Based on the size of Rb, this can do a few things. It can make the NMOS, look like:


A capacitive input
A pure resistive input (nice, but not practical due to tolerances)
An inductive input


It could also turn the circuit into an oscillator by destroying the phase margin. Bootstrapping is a fun thing.