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Transistor Model Parameters in ADS (Circuit Simulator) - RF Cafe Forums

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Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement

Post subject: transistor model parameters in ADS (circuit simulator).
Unread postPosted: Sat May 29, 2004 12:22 pm

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?).


David Straight


Post subject: test
Unread postPosted: Sat May 29, 2004 2:45 pm

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.




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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