mosfet model vdsat
The Vds ≥ Vgs-Vth if MOSFET operate in saturation region.
You should choose a proper operation point, by doing that you need to specify Vgs voltage.
So the Vdsat = Vgs-Vth, this is the small vds vaule for MOSFET operating in saturation region.
If vdsat is too large, the voltage swing will be reduced.
If vdsat is too small, the Vgs-Vth value is small too, the Ids is also small.
May be the current driving ability is not sufficient to the desired Specs(like GBW of opamp.....).
You can look some analog design book, like sansen, razavi, gray........