Analog Simulation

The cover of the June 1998 edition of the IEEE Spectrum shows the ‘father’ of SPICE, Prof. Emer. DONALD O. PEDERSON of the University of California, Berkeley, on the occasion of his award of the IEEE medal of honor. His Simulation Program with Integrated Circuit Emphasis (abbreviated as SPICE) is without doubt the most important analog simulator in the world of microelectronics. The widespread usage of SPICE is owed on the one hand to the competent electro- technical background and, on the other hand, to the admirable attitude of Prof. PEDERSON in giving the program practically for nothing to all who wanted to use it. It was his principle never to apply for a patent. His generosity is still effective to- day: the version of SPICE which runs on Personal Computers (PSPICE) is available practically cost free for interested users as the so called evaluation version (restricted to 10 transistors, 50 circuit nodes) 1).

This cost effectiveness results in fascinating possibilities in engineering education because every student is practically able to install his own simulated laboratory on his PC. On the following pages the PSPICE evaluation version 8 with German symbols for the circuit elements [10.5] will be used to demonstrate some problems of the simulation of analog circuits in an exemplary way. An extensive discussion of SPICE simulation of analog circuits can be found in [10.17], which refers to the outstanding standard book on analog circuits [10.19]. But first an insight into the mathematical and electro-technical fundamentals of SPICE will be given which tend to stay in the background, with the nowadays usual over-emphasis of pro- gram handling problems. This will be more or less a takeover of the introduction to SPICE formulated by the author of this chapter in [10.6].

After that the core question of analog simulation will be dealt with, which is the modeling of semi- conductor circuit elements, especially Bipolar Junction and MOS Field Effect Transistors. As in [10.17], the static behaviour will be emphasized because the dynamics of modern electronics are more or less determined by the connection delays.

An extra section is dedicated to the operational amplifier, the most important component  analog world. The very effective analog behavioral modeling (ABM) technique will be applied and the (erroneous!) macro model of the operational amplifier to be found in PSPICE will be discussed. A central problem when simulating analog circuits is the assessment of stability, and with respect to this problem the determination of the loop gain of a logarithmic amplifier will be discussed.

Originally it was intended to show the simulation of transmission line problems in connections of digital circuits on a printed circuit board, because these problems will occur in future at chip level when more transistors will be connected. Dealing with this kind of problem SPICE is used as a mixed mode simulator as it is not sufficient to describe a digital circuit only by its logical behavior when it co-operates with an analog element ‘transmission line’. As there are better ways to handle mixed- mode problems nowadays, e.g., by VHDL-AMS [10.21] the discussion of mixed mode problems with SPICE has been omitted.

It should be noted that some circuit problems can only be simulated rather clumsily by SPICE. This is the case when widely spread time constants are found in a circuit, which will result either in convergence problems or extremely long computer time. Examples of such circuits are SC filters or PLL circuits in carrier frequency systems. For these types of circuits the use of ADS [10.7] is of advantage.

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