User Guide

Chapter B Convergence and “time step too small errors”
388
Example: A first approximation to an opamp that has an
open loop gain of 100,000 is:
VOPAMP 3, 5 VALUE = {V(in+,in-)*1e5}
This has the undesirable property that there is no limit on
the output. A better expression is:
VOPAMP 3, 5 VALUE =
+ {LIMIT(V(in+,in-)*1e5,15v,-15v}
where the output is limited to +/- 15 volts.
Transient analysis
The transient analysis starts using a known solution - the
bias point. It then uses the most recent solution as the first
guess for each new time point. If necessary, the time step
is cut back to keep the new time point close enough that
the first guess allows the Newton-Raphson repeating
series to converge. The time step is also adjusted to keep
the integration of charges and fluxes accurate enough.
In theory the same considerations which were noted for
the bias point calculation apply to the transient analysis.
However, in practice they show up during the bias point
calculation first and, hence, are corrected before a
transient analysis is run.
The transient analysis can fail to complete if the time step
gets too small. This can have two different effects:
1 The Newton-Raphson iterations would not converge
even for the smallest time step size, or
2 Something in the circuit is moving faster than can be
accommodated by the minimum step size.
The message PSpice puts into the output file specifies
which condition occurred.
Pspug.book Page 388 Wednesday, November 11, 1998 1:14 PM