Analog Circuit Simulation:Purpose of Simulation

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Introduction

Analog circuit simulation usually means simulation analog circuits or very detailed simulation of digital circuits. The most widely known and used circuit simulation program is SPICE (simulation program with integrated circuit emphasis) of which it is estimated that there are over 100,000 copies in use. SPICE was first written at the University of California at Berkeley in 1975, and was based on the combined work of many reasearchers over a number of years. Research in the area of circuit simulation continues at many universities and industrial sites. Commercial versions of SPICE or related programs are available on a wide variety of computing platforms, from small personal computers to large mainframes. A list of some commercial simulator vendors can be found in the Appendix. The focus of this chapter is the simulators and the theory behind them. Examples are also given to illustrate their use.

Purpose of Simulation

Computer-aided simulation is a powerful aid during the design or analysis of VLSI circuits. Here, the main emphasis will be on analog circuits; however, the same simulation techniques may be applied to digital circuits, which are, after are, composed of analog circuits. The main limitation will be the size of these circuits because the techniques presented here provide a very detailed analysis of the circuit in question and, therefore, would be too costly in terms of computer resources to analyze a large digital system. However, some of the techniques used to analyze digital systems (like iterated timing analysis or relaxation methods) are closely related to the methods used in SPICE.

It is possible to simulate almost any type of circuit SPICE. The programs have built-in elements for resistors, capacitors, inductors, dependent and independent voltage and current sources, diodes, MOSFETs, JFETs, BJTs, transmission lines, and transformers. Commercial versions have libraries of standard components which have all necessary parameters prefitted to typical specifications. These libraries include items such as discrete transistors, op-amps, phase-locked loops, voltage regulators, logic integrated circuits, and saturating trans- former cores. Versions are also available which allow the inclusion of digital models (mixed mode simulation) or behavioral models which allow the easy modeling of mathematical equations and relations.

Computer-aided circuit simulation is now considered an essential step in the design of modern integrated circuits. Without simulation, the number of “trial runs” necessary to produce a working IC would greatly increase the cost of the IC and the critical time to market. Simulation provides other advantages, including:

• The ability to measure “inaccessible” voltages and currents which are buried inside a tiny chip or inside a single transistor.

• No loading problems are associated with placing a voltmeter or oscilloscope in the middle of the circuit, measuring difficult one-shot waveforms or probing a microscopic die.

• Mathematically ideal elements are available. Creating an ideal voltage or current source is trivial with a simulator, but impossible in the laboratory. In addition, all component values are exact and no parasitic elements exist.

• It is easy to change the values of components or the configuration of the circuit. Unfortunately, computer-aided simulation has it own set of problems, including:

• Real circuits are distributed systems, not the “lumped element models” which are assumed by simulators. Real circuits, therefore, have resistive, capacitive, and inductive parasitic elements present in addition to the intended components. In high-speed circuits, these parasitic elements can be the dominant performance-limiting elements in the circuit, and they must be painstakingly modeled.

• Suitable predefined numerical models have not yet been developed for certain types of devices or electrical phenomena. The software user may be required, therefore, to create his or her own models out of other models which are available in the simulator. (An example is the solid-state thyristor, which may be created from an npn and pnp bipolar transistor).

• The numerical methods used may place constraints on the form of the model equations used. In addition, convergence difficulties can arise, making the simulators difficult to use.

• There are small errors associated with the solution of the equations and other errors in fitting the non-linear models to the transistors which make up the circuit.

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