Interactive System for the Symbolic Analysis of Analog Circuits E. TLELO-CUAUTLE, A. QUINTANAR-R., G. GUTIÉRREZ-P., M. GONZÁLEZ-R., S. FUENTES-GOIZ INAOE - Instituto Tecnológico de Puebla Luis Enrique Erro No. 1. 72000 MEXICO – Av. Tecnológico No. 420. 72000 MEXICO Abstract: - An interactive system called SIASCA which automates the calculation of symbolic expressions (SEs) representing the dominant behavior of analog circuits, is introduced. SIASCA includes a more general class of active devices whose behavior is modeled at different levels of abstraction using nullors in order to enhance the capability of symbolic analysis to calculate simplified SEs. Key-Words: - Symbolic Analysis, Analog Design Automation, Frequency Response, Nullor. 1 Introduction 2 The modeling approach Symbolic analysis is focused on the calculation of simplified SEs [1]-[6], which represent the dominant behavior of an analog circuit. The calculated SEs help the designer to gain insight and get an inference about the behavior of a circuit. Furthermore, it is very much needed that the calculation of SEs be guided towards an interactive design process by generating analytical design equations, useful for synthesis and optimiza- tion procedures [7]-[9]. In this manner, this paper is focused on the development of an interactive system called SIASCA, which calculates simplified SEs of analog circuits. As shown in Fig. 1, SIASCA includes a more general class of devices, namely: opamps [4], OTAs [5], CCII- [7], BJT and MOSFET. The basic set of primitives [2]: controlled sources, independent sources, resistor, inductor and capacitor are selected from the Elements button. The analog ground sets a reference node to (0), as it is usually done by any circuit simulator alike SPICE. SIASCA has been developed under Visual Java++, while the analysis procedures are implemented using MAPLE 8 . TM 0 . To minimize the complexity in computing SEs using SIASCA, the behavior of all active devices are modeled at different levels of abstraction using nullors [5],[6]. This modeling approach is quite useful to formulate a compacted system of equations (CSEs) [1]-[3]. For example: The behavior of the three terminals MOSFET can be modeled at different levels of abstraction, e.g. the generic MOSFET is shown in Fig. 2a. The addition of the transconduc- tance, output conductance and gate-source capacitor can be added as shown in Figs. 2b, 2c, and 2d. Fig. 2. Modeling the MOSFET using the nullor. Fig. 3. Miller Operacional Amplifier. The selection of a correct model which minimizes the computational effort in doing small-signal symbolic- analysis, depends on both the biasing and frequency operating conditions [6]. For example, to calculate Fig. 1 The graphical user interface of SIASCA.