DC-Analysis模电的DC仿真.pdfVIP

Chapter 2 DC Analysis 2.1 Introduction Many of the analyses available for SPICE and Spectre compute op- erating points. For example, SPICE’s .op analysis computes and outputs information about the operating point such as the voltage, current, power, etc. at each component. The .dc analysis computes the operating point as a function of some independent variable. The linear small signals analyses, such as .ac and .noise, first compute the operating point and then linearize the circuit about that operat- ing point before computing the small-signal behavior of the circuit. Finally, the transient analysis computes an operating point for the initial state of the circuit. Clearly the ability to reliably compute an accurate operating point is very important. In general operating points are simply snap-shots of some solution trajectory. In DC analysis, the operating points are also assumed to be equilibrium points. Equilibrium points are constant-valued oper- ating points. In other words, equilibrium points are solutions that do not change with time. A circuit cannot reach an equilibrium point if the stimulus is still changing, so the first step of a DC analysis is to configure the independent sources so they are constant. In addi- tion, since all waveforms are constant-valued at equilibrium points, and and so capacitors act as open circuits and inductors act as short circuits. Therein lies the basic algorithm for computing an equilibrium point. 16 Chapter 2. DC Analysis 1. Configure all independent sources to be constant valued. 2. Replace all capacitors with open circuits. 3. Replace all inductors with short circuits. Solving the equations that describe the resulting