bias of a transistor

Question

This is the design of a Pierce oscillator that has to cancel the resistive part of the quartz.

Could someone explain what is the purpose of M5? What does biasing means for this design?

At the points Q1 and Q2 a quartz crystal is connected.
Also, the drain of M1 is connected to a current mirror.

Answer 1

M5 is the DC_bias feedback element, with the FET used as a non-linear resistor.

The full oscillator circuit looks something like this

^{simulate this circuit – Schematic created using CircuitLab}

Given a 1MegOhm resistor is HUGE, the designer choses a low W/L (1/10 or 1/100) ratio MOSFET, possibly a PFET inside a well to provide a cleaner environment for the PFET and thus create a lower-jitter operating point.