GNUCAP is a general purpose circuit simulator. It performs nonlinear
dc and transient analyses, fourier analysis, and ac analysis
linearized at an operating point. It is fully interactive and
command driven. It can also be run in batch mode or as a server.
The output is produced as it simulates. Spice compatible models
for the MOSFET (level 1,2,3) and diode are included in this
Since it is fully interactive, it is possible to make changes and
re-simulate quickly. The interactive design makes it well suited
to the typical iterative design process used it optimizing a circuit
design. It is also well suited to undergraduate teaching where
Spice in batch mode can be quite intimidating. This version, while
still officially in beta test, should be stable enough for basic
undergraduate teaching and courses in MOS design, but not for
In batch mode it is mostly Spice compatible, so it is often possible
to use the same file for both GNUCAP and Spice.
The analog simulation is based on traditional nodal analysis with
iteration by Newtons method and LU decomposition. An event queue
and incremental matrix update speed up the solution for large
circuits (at some expense for small circuits).
It also has digital devices for mixed signal simulation. The
digital devices may be implemented as either analog subcircuits or
as true digital models. The simulator will automatically determine
which to use. Networks of digital devices are simulated as digital,
with no conversions to analog between gates. This results in
digital circuits being simulated faster than on a typical analog
simulator, even with behavioral models. The digital mode is
experimental and needs work. There will be substantial improvements
in future releases.