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This is an email I received from Jaime Arbona, who describes a
convenient means of placing a power load line on a swept DC simulation for a BJT, Mosfet,
or Vacuum Tube. You've seen them before, the curved dotted line which appears on the
Ia/Va chart.....
I thought it was a nice tip, so I've placed it here for all to benefit from.
Have you ever needed the maximum power curve on a
valve (transistor) device obtained form pspice? What you usually to is to draw it by hand
using Ip=power/Vp. Well I got tired of that, so I devised a neat little constant power
device (which I call KPOW). You feed it a voltage, it produces a current so that power is
constant (and viceversa). I include the subcircuit below:
*---------------------------------------
* CONSTANT POWER SUBCIRCUIT
.SUBCKT KPOW 1 2
+PARAMS: POWER= 15 ; or any other default
G1 1 2 VALUE={POWER/(0.00001+V(1,2))}
.ENDS KPOW
*$
The .00001 term is to prevent infinite current when V(1,2)=0. Pspice has to be kept happy!
I then made a two terminal graphic device (just a box) using the Symbol Editor and the
following parameters:
TEMPLATE=X^@REFDES %1 %2 @model params: power=@power
PART=KPOW
REFDES=KP?
MODEL=KPOW
POWER=15W
When drawing the output characteristics of a valve, just place KPOW in parallel with the
(swept) plate source. Click KPOW and enter the max power dissipation of the valve.
Then using PROBE, display the current trhu KPOW, in addition to the plate current and
Presto! you then have a nice constant (max or whatever you like) power curve on your plate
characteristics.
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