User Guide

Chapter 6 Analog behavioral modeling
172
The part would be defined with the following
characteristics:
EXP1 = V(%IN2,%IN3)+
EXP2 = 0.12*V(%IN1,%IN3)
This works for the main operating region but does not
model the case in which the current stays 0 when
combined grid and anode voltages go negative. We can
accommodate that situation as follows by adding the
LIMIT part with the following characteristics:
HI = 1E3
LO = 0
This part instance, LIMIT1, converts all negative values of
v
g
+.12*v
a
to 0 and leaves all positive values (up to 1 kV)
alone. For a more realistic model, we could have used
TABLE to correctly model how the tube turns off at 0 or at
small negative grid voltages.
We also need to make sure that the current becomes zero
when the anode alone goes negative. To do this, we can
use a DIFF device, (immediately below the ABM3 device)
to monitor the difference between V(anode) and
V(cathode), and output the difference to the TABLE part.
The table translates all values at or below zero to zero, and
all values greater than or equal to 30 to one. All values
between 0 and 30 are linearly interpolated. The properties
for the TABLE part are as follows:
ROW1 = 00
ROW2 = 301
The TABLE part is a simple one, and ensures that only a
zero value is output to the multiplier for negative anode
voltages.
The output from the TABLE part and the LIMIT part are
combined at the MULT multiplier part. The output of the
MULT part is the product of the two input voltages. This
value is then raised to the 3/2 or 1.5 power using the PWR
part. The exponential property of the PWR part is defined
as follows:
EXP = 1.5
Pspug.book Page 172 Wednesday, November 11, 1998 1:14 PM