Why we bias MOSFET amplifiers in the saturation region.D. W. ParentWhen designing an amplifier we usually want the magnitude of the gain to be at least 2.• Operating in the saturation regime allows for a magnitude of the common source gain to be greater than 1.• Why is this so?Looking at the small signal equivalent of a CS amplifier (no Rs) one can see ro puts an upper bound on RD, and thus the gain.DAoDoDomVIVrRrRrgAThis equation for ro is only valid in saturation.DSDodVdIr1This equation for valid everywhere.We can see that for an ideal NMOSFET the slope of ID is zero and thus the ro is infiniteIn the linear regime the smallest ro is 1/0.036Ω. Saturation ro goes to infinity.Even adding an Ro to the Model does not increase ro in the linear regime.DoDomVRrRrgAFor “Real” MOSFETs Ro in saturation is not fixed for all VGS! Design away effect of Ro, by setting RD<<RoIf RD<<ro, then the larger ro, the larger the common source
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