A simple and efficient approximate method to incorporate nonlinear bipolar junction transistor (BJT) into Finite-Difference Time-Domain (FDTD) framework is presented. This method applies Taylor expansion on the nonlinear transport equations of the BJT based on Gummel-Poon model . The results are two coupled one-step explicit finite difference schemes for the electromagnetic fields in the vicinity of the BJT, which can be solved easily. A simulation example is carried out for a power amplifier and the result compares well with the measurement. A two-step simulation scheme is introduced to hasten the process of reaching transient steady state. Finally, brief comments on treating the FDTD framework as a dynamical system is included. This perspective is useful for analyzing the stability of FDTD framework with nonlinear lumped elements.
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