Charge Based Modeling In State Variable Based Simulator.

Abstract

A new parameterized nonlinear device model formulation is described that enables the same computer code to be used in any circuit analysis (ie. Harmonic Balance, Transient and D.C) routine with no charge conservation issues. The parametric description provides great flexibility for the design of nonlinear device models. The number of parameters or state variables required is the minimum number necessary and can be chosen to achieve robust numerical characteristics. An example illustrates charge conservation problems that can occur in the transient simulation of microwave circuits if the models are not correctly formulated. Implementation of the BJT and MESFET in ƒREEDA™ using both the Universal Modeling algorithm (Charge as state variable) and the conventional algorithm (Voltage as state variable) is described. The above concept allows the modeling of thermal effects to be included in the simulations of electronic circuits, by viewing thermal sub-systems as sub-circuits. All these developments are implemented in a circuit simulator program, called ƒREEDA™. This program provides unprecedented flexibility for the addition of new device models or circuit analysis algorithms. ƒREEDA™ was applied to the two tone harmonic balance simulation of a MESFET amplifier. Distortion in MESFET models was studied and the importance of the time delay parameter tau is discussed with the aid of IM3 plots.