Design Methodologies for Nonlinear Circuit Simulation and Optimization

This paper illustrates the methodology for designing a nonlinear circuit using state-of-the-art CAD tools applied to several examples of FET circuits. The parameters needed for characterization of the (nonlinear) FET are extracted using RoMPE, a parameter extraction program based on gradient optimizers which have been enhanced by adjoint sensitivity calculations to provide exact gradients. Measured DC and small-signal S-parameter data are simultaneously fitted to a modified Materka FET model. Simultaneous fitting of the data to both DC and AC parameters is essential to preserve the implicit dependence of the small-signal model parameters upon the bias. Microwave Harmonica, a harmonic-balance based nonlinear circuit simulator, is then used with the model parameters to design a single-gate FET mixer circuit. The use of an optimizer to improve the conversion gain and spectral purity of the mixer, and its use in the design of oscillator circuits is also demonstrated.