A Model-Agnostic Technique for Simulating Per-Element Distortion Contributions

Abstract

The nonlinearity of an element can be altered while retaining the original operating point and first-order terms by appropriately combining multiple instances of the nonlinear element. Each instance of the combination is biased at the same operating point as the original element, but is driven by a scaled version of incremental signal present across the latter. Maintaining the sum of scaling factors over all instances to be unity preserves first-order behavior. Substituting an element in a weakly nonlinear circuit with its composite counterpart changes the contribution of that element to the output distortion. Per-element distortion contributions in a weakly nonlinear circuit can then be determined by substituting the relevant element by the composite element, altering its nonlinear terms by known factors, and simulating the output distortion in each case. This technique can be used in a standard circuit simulator which implements the appropriate nonlinear device models, but, crucially, requires no knowledge of the device model details on the part of the circuit designer. The principle is demonstrated by applying it to a common source amplifier with a nonlinear load, a second order RC filter with nonlinear resistors, and a two stage fully differential opamp.