Post subject: transistor model parameters in ADS (circuit
Unread postPosted: Sat May 29, 2004 12:22 pm
Joined: Sun May 09, 2004 1:23 am
Location: Morgan Hill, CA (Silicon Valley, Bay Area)
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
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?).
Post subject: test
Unread postPosted: Sat May
29, 2004 2:45 pm
Joined: Sun May 09,
2004 1:23 am
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.
Post subject: good tool kit
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.