# Biasing arrangement - common gate amplifier

I'm just really confused at the moment with biasing all these different MOSFET amplifier arrangements. Here is one,

This is a common-gate amplifier and I am told that it is biased by a constant current source. And that's what I don't understand. How is that current source biasing the MOSFET? Shouldn't the current that is passing through $$\R_D\$$ be setting the MOSFET in saturation?If so, what is the point of that current source?

Secondly, maybe I am misunderstanding what biasing truly is. I always thought that the biasing arrangement is the circumstances around a transistor that pushes into one region or another.