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dstraight Post subject: transistor model parameters in ADS (circuit
simulator). Unread postPosted: Sat May 29, 2004 12:22 pm Offline
Captain Joined: Sun May 09, 2004 1:23 am Posts: 10 Location:
Morgan Hill, CA (Silicon Valley, Bay Area) Recently, I've been playing
around with importing BJT models into ADS and would be interested in
finding a model that would account for excessively large VCE causing
the collector current to run away. When using a transistor curve tracer
this phenomena can be seen as the collector current lines which are
mostly horizontal suddenly curving upward once VCE goes beyond what
it should. I understand that the physical reason for this is
as follows: The depletion region due to the collector-base junction
being reversed biased continues to grow larger as VCE increases and
eventually this region moves beyond the base region into the emitter
region and when this happens electrons in the emitter are easily swept
through into the collector. And, if you notice RF transistors
can't tolerate very much reverse collector-base voltage because of their
very thin base region whereas garden variety transistors can easily
tolerate 35V ~ 60V of reverse bias between their collector-base because
they have a much thicker base. Anyway, it would be really cool
if there was a specific model parameter that accounted for this or an
adaptation to my circuit that would model this (such as a zener diode
across the collector-emitter?). Thanks! David Straight
Top Profile dstraight Post subject: test
Unread postPosted: Sat May 29, 2004 2:45 pm Offline Captain
Joined: Sun May 09, 2004 1:23 am Posts: 10 Location: Morgan
Hill, CA (Silicon Valley, Bay Area) Okay, I hacked my way into something
that achieved what I want (although it's a bit lame). As I have described
in my previous post I wanted a transistor that showed uncontrolled collector
current once VCE became too big. The transistor models that I have at
this time do not do this (example: I can have VCE = 500 volts and other
than the Early effect the transistor model acts as if everything is
fine). My first attempt at building a better model wasn't through
the use of SPICE parameters, but rather, by connecting another transistor
across the collector-emitter of the first transistor. This extra transistor
used to cause runaway current when VCE becomes too high is driven by
the output of a Voltage Controlled Voltage Source (VCVS) that monitors
VCE of the first transistor. And, as you might expect at some point
the second transistor will turn on and the total current going into
this model's collector (which is really the collectors of two transistors)
will suddenly run quite high. Playing with the gain of the VCVS will
set the level of VCE needed to trigger the second transistor into conduction.
The output resistance of the VCVS will adjust how rapidly the 'run-away'
current will change with a changing VCE. In the schematic below
I set it such that when VCE is about 20 volts the collector current
will begin to increase rapidly I'm sure there's a better way
to do this. Image Image Top Profile
guestX Post subject: good tool kit Unread postPosted: Tue
Jun 01, 2004 3:05 am yes, with software you may fly to the Mars.
There is no software package able to fully simulate real-world circuit.
You can connect some cables for frequency shaping and use the tunneling
effect to generate ns pulses. we made radar with cheap transistors a
while ago. The software has no clue about tunelling.
Posted 11/12/2012
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