Gilbert Mixer overdrive problem

hi all,

I am confused about the overdirve voltage about second stage of Gilbert Mixer(the four cmos cross-couple stage).

I wanna make the overdrive voltage small( Vgs-Vth) and input is Vin, and the signal from the first stage is Vs then, Vgs= Vin -Vs.

THe problem is Vin and Vs are both periodic signal, they change all the time.

How can i keep Vin - Vs = Vgs stable and small ?


Best Wishes
Gang