Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would
be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate
that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views.
It seems that the more formal social media like Facebook pretty much dominate this kind of venue anymore anyway, so if
you would like to post something on RF Cafe's
page, please do.
Below are all of the forum threads, including all
the responses to the original posts.
Post subject: transistor model parameters in ADS (circuit simulator).
Sat May 29, 2004 12:22 pm
Joined: Sun May 09, 2004 1:23 am
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.
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
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?).
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
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
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
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
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.