A circuit-level model for accurately modeling 3rd order nonlinearity in CMOS passive mixers

Many MOSFET models have discontinuities in the 2nd derivative of their drain current with respect to their drain-source voltage. Because these discontinuities occur at V_ds = 0V, they have little effect on simulations of active circuits, but matter when simulating transistors in deep triode, such as CMOS passive mixers. These discontinuities result in qualitatively incorrect simulations of the effects of third order nonlinearity, with the 3rd harmonic behaving proportional to the square of the input signal amplitude, A², instead of A³. In this paper, we present a schematic-level modeling technique that can be easily implemented over any model that fails the Gummel Symmetry Test (e.g. BSIM4) without altering underlying physical equations. We then show how the model performs with respect to IM3 simulations ensuring correct magnitude and slope by comparing our model to measurements from an 8-phase passive mixer manufactured in a deep sub-micron process.