Circuit analysis by using a device simulator

In the last few years, numerical-analysis programs for device simulation have progressed to a mature development stage, and some of them include such features as AC analysis or different types of boundary conditions. In this paper we present the analysis of a bipolar differential stage carried out by means of our program HFIELDS [1,2]. The purpose of the simulation is demonstrating how a numerical-analysis program embedding the features mentioned above can be profitably exploited to handle a simple but realistic functional circuit. The result of a steady-state simulation is taken as the starting point for the calculation of the differential and common-mode gains at various operating frequencies. This calculation requires the solution of a linear algebraic system whose dimension is about 6N x 6N, N being the number of unknown nodes. For this, two different techniques have been tested, depending on the position of the operating frequency with respect to the cutoff frequency.