Principles of Nonlinear Active Device Modeling for Circuit Simulation

Principles of nonlinear active device modeling for circuit simulation in SPICE and harmonic balance programs are presented, motivated by an examination of three common problems of standard nonlinear GaAs MESFET models. The first problem is that simulations done in large signal analysis don't fit the measured imaginary parts of the Y-parameters versus bias. On the other hand, good fits to the measurements can be obtained in linear (small signal) analysis. The solution is to use nonlinear Voltage Controlled Charge Source (VCQS) elements in place of two-terminal capacitors in the nonlinear model. A VCQS is a reactive analogue of the familiar voltage controlled current source. This improves the accuracy of the large signal reactive FET model. Linearizing the resulting equations produces an accurate small signal model, consistent with the large signal simulations in the appropriate limit. Linearizing a VCQS element produces another unfamiliar quantity, the transcapacitance. A transcapacitance is a reactive analogue of a transconductance. The second problem is that large signal FET models do not simulate time delays of the FET. The transcapacitance associated with a VCQS element between the source and the drain provides a reactive output current proportional to the time derivative of the gate voltage. This current approximates the effect of the delay.