User`s guide

6
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26 Analo
g
Behavioral Modelin
g
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 attributes 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 attribute of the PWR part is defined as follows:
EXP = 1.5
The last major component is an ABM expression component to
take an input voltage and convert it into a current. The relevant
ABM1/I part attribute looks like this:
EXP1 = 200E-6 * V(%IN)
A final step in the model is to add device parasitics. For
example, a resistor can be used to give a finite output