User Guide

Worst-case analysis
309
Worst-case analysis example
The schematic shown in Figure 84 is for an amplifier
circuit that is a biased BJT. This circuit is used to
demonstrate how a simple worst-case analysis works. It
also shows how non-monotonic dependence of the output
on a single parameter can adversely affect the worst-case
analysis.
Because an AC (small-signal) analysis is being performed,
setting the input to unity means that the output,
Vm([OUT]), is the magnitude of the gain of the amplifier.
The only variable declared in this circuit is the resistance
of Rb2. Because the value of Rb2 determines the bias on the
BJT, it also affects the amplifier’s gain.
Figure 85 is the circuit file used to run one of the
following:
•
a parametric analysis (.STEP, shown enabled in the
circuit file) that sets the value of resistor Rb2 by
stepping model parameter R through values spanning
the specified DEV tolerance range, or
•
a worst-case analysis (shown disabled in the circuit
file) that allows PSpice to determine the worst-case
value for parameter R based upon a sensitivity
analysis.
Figure 84 Simple biased BJT amplifier.
Pspug.book Page 309 Wednesday, November 11, 1998 1:14 PM