User Guide

Worst-case analysis
311
output is not monotonic with a variable parameter (see
Figure 87 and Figure 88)
For demonstration, the parametric analysis is run first,
generating the curve shown in Figure 87 and Figure 88.
This curve, derived using the YatX goal function shown in
Figure 86 illustrates the non-monotonic dependence of
gain on Rb2.
To do this yourself, place the goal function definition in a
PROBE.GF file in the circuit directory. Then start PSpice,
load all of the AC sweeps, set up the X axis for
performance analysis, and add the following trace:
YatX(Vm([OUT]),100k)
Next, the parametric analysis is commented out and the
worst-case analysis is enabled. Two runs are made using
the two versions of the Rbmod .MODEL statement shown
in the circuit file. The model parameter, R, is a multiplier
which is used to scale the nominal value of any resistor
referencing the Rbmod model (Rb2 in this case).
The first .MODEL statement leaves the nominal value of
Rb2 at 720 ohms. The sensitivity analysis increments R by
a small amount and checks its effect on Vm([OUT]). This
slight increase in R causes an increase in the base bias
voltage of the BJT, and increases the amplifier’s gain,
Vm([OUT]). The worst-case analysis correctly sets R to its
minimum value for the lowest possible Vm([OUT]) (see
Figure 87).
YatX(1, X_value)=y1{1|sfxv(X_value)!1;}
Fi
g
ure 86 YatX Goal Function.
Note T
h
e YatX goa
l
f
unction is use
d
on t
h
e
simulation results for the parametric sweep
(.STEP) defined in Figure 85. The resulting
curves are shown in Figure 87 and
Figure 88.
Pspug.book Page 311 Wednesday, November 11, 1998 1:14 PM