**Circuit
simulators Are Available In Several Types. **

Circuit
simulators can be broadly grouped into those that
simulate a circuit in an analog way, a digital
way, or a combination of analog and digital.

This
last category are often called mixed mode simulators
since analog simulation is used for part of the
circuit and digital simulation for the rest. The
most common analog simulator, SPICE (Simulation
Program with Integrated Circuit Emphasis), grew
out of a class project and subsequent doctoral
dissertation at Berkeley. It continues to be developed
there and elsewhere. It is available as freeware
(at least in demo versions) and as a commercial
product from several companies. In this class
Eldo, a commercial spice like simulator, will
be used via direct invocation and via the DAIC
user interface.

SPICE simulates a circuit by solving simultaneous
differential equations that describe voltages
or currents in a circuit network. The equations
come from mathematical models for components such
as resistors, capacitors, bi-polar transistors
(BJT), MOSFET transistors, transmission lines,
etc. While precision limited numbers are used
in these calculations, numerical resolution is
adequate to typically consider the nodal solutions
to be continuous (watch out though, I have experienced
circuits whose simulation showed behavior, caused
by numerical limitations, not observed in a real
circuit). For a SPICE simulation, the following
are needed: