ACWWong
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what one normally would do is to use a pi network of R's in lieu of one R. that is there are three resistors, two of which connect from each source to ground, and one across.
S-Rd-S | | Rs Rs | | gnd gnd
So Rs is used in the biasing mirror to keep the current constant, and now Rd provides the bulk of the lineraity imporvement (at the cost of noise & gain). Typically Rd would be replaced with a spiral L to give the linearity with less noise (now at the cost of area). It often helps to transform the pi to t (star - delta impedance transform) to get a clearer idea as to how the values of Rd and Rs contribute to the linearity, this is especially true if Rs and Rd become comparable. At one extreme, if Rs >>Rd (usually where one aims) then the linearity/IIP3 is now dominated by Itail*Rd (in addition to Vod= vgs-vth).
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