User`s guide

Plottin
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Hysteresis Curves 11
-19
The QSTD model is defined as:
.MODEL QSTD NPN( is=1e-16 bf=50 br=0.1 rb=50 rc=10 tf=.12ns tr=5ns
+ cje=.4pF pe=.8 me=.4 cjc=.5pF pc=.8 mc=.333 ccs=1pF va=50)
Instead of using the DC sweep to look at the hysteresis, we use
the transient analysis, (Print Step = .01ms and Final Time =
2ms) sweeping VIN from -1.8 volts to -1.0 volts and back down
to -1.8 volts, very slowly. This has two advantages:
• it avoids convergence problems
• it covers both the upward and downward transitions in one
analysis
After the simulation, when we are in Probe, the X axis variable
is initially set to be Time. By selecting X Axis Settings from the
Plot menu and clicking on the Axis Variable button, we can set
the X axis variable to be V(1). Then we can use Add on the
Trace menu to display V(7), and change the X axis to a user
defined data range from -1.8V to -1.0V (X Axis Settings on the
Plot menu). This plots the output of the Schmitt trigger against
its input, which is what we want. The result looks similar to
Figure 11-6.
Fi
g
ure 11-5
Netlist for Schmitt Trigger Circuit
* Schematics Netlist
R_RIN 1 2 50
R_RC1 0 3 50
R_R1 3 5 185
R_R2 5 8 760
R_RC2 0 6 100
R_RE 4 8 260
R_RTH2 7 0 85
C_CLOAD 0 7 5PF
V_VEE 8 0 dc -5
V_VIN 1 0
+PWL 0 -8 1MS -1.0V 2MS -1.8V
R_RTH1 8 7 125
Q_Q1 3 2 4 QSTD
Q_Q2 6 5 4 QSTD
Q_Q3 0 6 7 QSTD
Q_Q4 0 6 7 QSTD