In praise of: Cordell SPICE models of BJTs - diyAudio
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Old 11th September 2011, 11:55 PM   #1
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Join Date: May 2011
Location: Silicon Valley
Default In praise of: Cordell SPICE models of BJTs

I've been tinkering with LTspice simulations of high current drivers, which eventually led me to downloading and trying out Bob Cordell's free SPICE models at

I've been simulating the "sustained beta" complementary pair MJE1302 / MJE3281 using Cordell models, and thus far I'm quite pleased. The model results (from my LTspice simulations) are shown in color below. The measured results (from ON Semiconductor datasheets) are shown in black and white.

I'm pleased for a couple of reasons:
  • The modeled NPN and PNP are not perfectly complementary, which strikes me as a nice dose of "Murphy's Law WILL bite you in the ***" for the potentially unrealistic world of simulation. NPN is just plain faster at all collector currents. You don't get perfect cancellation of base currents between the push and the pull. Deal with it.
  • The shape of the modelled curves is about right; peak fT around 2 amps, falling off both above and below. It's not some ridiculous flatline nonsense with constant fT at all collector currents
So far, I'm a very satisfied non-paying customer; thank you, Bob Cordell!
Attached Images
File Type: png measured.png (147.5 KB, 174 views)
File Type: png simulated.png (31.0 KB, 167 views)

Last edited by Mark Johnson; 12th September 2011 at 12:00 AM. Reason: mention that models are free
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Old 12th September 2011, 12:35 AM   #2
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Location: Llanddewi Brefi, NJ
We should tempt Mr. Cordell to unleash some of the other LTSpice models he has for MOSFETs. As our French brothers say, (loosely) "Le livre vaut le voyage".
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Old 26th September 2011, 11:26 AM   #3
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How did you plot those fT curves? Must be a big bucks commercial software product- I haven't seen that in LTspice.
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Old 26th September 2011, 02:42 PM   #4
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It's from LTspice. I used .MEAS along with .STEP, then copied the .MEASUREd results from the "Error Log" window into a file that I fed to Microsoft Excel for plotting. Surely you recognize the Calibri font, and other visual signatures of the plots, which positively scream Here Is Yet One More Excel Chart.

Inspecting the horizontal axis and horizontal spacing of the data points on the chart (30 equally spaced points per decade of collector current), you can quickly deduce that I must have measured fT using the DECade option of the .STEP statement. And you'd be right.

It's a fun little homework problem to design and debug a simulation "circuit" (or call it a "test fixture" if you prefer), which enables you to .MEASURE the fT of a transistor at a given collector current and VCE bias condition. Once you've got that, then merely .STEP the collector current and Bob's your uncle.

Last edited by Mark Johnson; 26th September 2011 at 03:10 PM. Reason: repair a paragraphing error
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