Why can we bias MOSFET with current source?
When an NMOS is biased for constant current operation, which can provide enormous gain, the circuit is grounded source, bias on the gate, and the current source in the drain.
And in that case, some operating_point feedback is needed, to set the Vds near VDD/2 for good output voltage swing.
In a properly designed operational amplifiers, the negative feedback achieves that purpose.