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Old 14th November 2007, 09:53 PM   #411
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Default Tweaking SPICE Models

I've been thinking a little more about my post above regarding the SPICE models for the 2SA1407 and 2SC3601 transistors.

It's a shame that the dc vbe-IC is so far off, and I was wondering if it is at all legitimate to tweak the models to get them a little closer to published data (on the naive assumtion that the other stuff isn't too far off).

What I'm thinking of is just tweaking IS and RE. Currently for the 2SA1407, IS=15.2n and RE=7.63 ohms. The low-current vbe spec sheet behavior suggests IS is on the low side, while the high current data suggests that RE is way high.

Is it legit to just tweak these two parameters to get closer?

BTW, it looks like the automated modeling that some manufacturers use comes up with some ridiculous, non-physically reasonable values in attempting to fit data. For example, RB in my MJE15030 model is 267 ohms. Unless I'm terribly mistaken, this is a ridiculous value for that transistor.

Cheers,
Bob
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Old 14th November 2007, 11:24 PM   #412
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Default Re: Tweaking SPICE Models

Quote:
Originally posted by Bob Cordell
I've been thinking a little more about my post above regarding the SPICE models for the 2SA1407 and 2SC3601 transistors.

It's a shame that the dc vbe-IC is so far off, and I was wondering if it is at all legitimate to tweak the models to get them a little closer to published data (on the naive assumtion that the other stuff isn't too far off).

What I'm thinking of is just tweaking IS and RE. Currently for the 2SA1407, IS=15.2n and RE=7.63 ohms. The low-current vbe spec sheet behavior suggests IS is on the low side, while the high current data suggests that RE is way high.

Is it legit to just tweak these two parameters to get closer?

BTW, it looks like the automated modeling that some manufacturers use comes up with some ridiculous, non-physically reasonable values in attempting to fit data. For example, RB in my MJE15030 model is 267 ohms. Unless I'm terribly mistaken, this is a ridiculous value for that transistor.

Cheers,
Bob

You're right Bob, brainless multi-parameter fitting can lead to silly answers. I have fought this in the past. rbb and RE have very weak sensitivity in some parameter fitting algorithms. Imagine trying to extract rbb where its effect is only manifest by 1/beta. You need good old fashion experiments designed to extract each parameter separately (sometimes).
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Old 14th November 2007, 11:46 PM   #413
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Bob,
are the (static) parameters
IS=9.85E-9 BF=487.8 NF=1.437 VAF=40 IKF=0.601 ISE=2.6E-7 NE=3.07 RB=0.178
for a MJE15030 reasonable ? Their fit is not so bad ...
Thanks
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Old 15th November 2007, 03:39 AM   #414
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Default Re: Tweaking SPICE Models

Quote:
Originally posted by Bob Cordell
I've been thinking a little more about my post above regarding the SPICE models for the 2SA1407 and 2SC3601 transistors.

It's a shame that the dc vbe-IC is so far off, and I was wondering if it is at all legitimate to tweak the models to get them a little closer to published data (on the naive assumtion that the other stuff isn't too far off).

What I'm thinking of is just tweaking IS and RE. Currently for the 2SA1407, IS=15.2n and RE=7.63 ohms. The low-current vbe spec sheet behavior suggests IS is on the low side, while the high current data suggests that RE is way high.

Is it legit to just tweak these two parameters to get closer?

BTW, it looks like the automated modeling that some manufacturers use comes up with some ridiculous, non-physically reasonable values in attempting to fit data. For example, RB in my MJE15030 model is 267 ohms. Unless I'm terribly mistaken, this is a ridiculous value for that transistor.

Cheers,
Bob

I imported the LTspice BJT library into a spread sheet that makes it a bit easier to look up and compare various model values.
I have about 10 of the older common small signal transistors, 2N2222, 2N2907, 2N5401, 2N5550, 2N4401, 2N4403 etc. mixed from Fairchild and Phillips and they all have Rb = 10 ohms. 2N3906/04 have Rb = 20 ohms.

The Onsemi models seem to have Rb ranging from below 1 ohm to several hundreds of ohms - probably the result of a poor curve fit and blindly trusting the results. I'd rather have a reasonable default value than an erroneous value resulting in a worthless model.

Nearly all of the models either do not spec Re, it defaults to zero as I recall, and those that do seem to have values below .1, even .01 ohms.

Rc seems to range from .01 ohm or less to 2 or 3 ohms max for the small sample that I'm looking at.

Seems Vce sat provides at least a sanity check for Re and Rc. It is less than .2 V @ 100 mA for the 2SC3601. Re is 5.63, Rc = 2.25 for the model provided on the forum here for that transistor. At 100 mA we have: .1 * (5.63+2.25) = .79 V which indicates that they're far too high. Rc alone is too high.

I too am trying to find a way to do a sanity check and determine a good enough fix for many of these models.

I started looking at Rb when I noticed that some med power transistors used as drivers tended to oscillate without base stoppers. Then checked the model to see that they often had less than 1 ohm Rb, those that were more stable had a value of 2 ohms or more and I think were all around better models which probably also contributed to better stability.

Would be nice if a semiconductor design specialist would comment on the Rb issue.

Pete B.
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Old 15th November 2007, 04:14 AM   #415
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Default Stumbled onto Toshiba RF models

Just found this, someone claimed Toshiba did not provide SPICE models. These are for RF devices only and package effects are included, one with a note of them being good to 3 GHz:
http://www.semicon.toshiba.co.jp/eng...ice/index.html
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Old 15th November 2007, 04:25 AM   #416
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This document provides "Key Parameters" for many devices and I used it to check the MJE171/181 parts:
http://eesof.tm.agilent.com/docs/ads...2/pdf/vcap.pdf

These too seemed to be off since the devices did not bias up correctly in the JE-990.

Pete B.
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Old 15th November 2007, 12:41 PM   #417
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Default Re: Re: Tweaking SPICE Models

Quote:
Originally posted by scott wurcer



You're right Bob, brainless multi-parameter fitting can lead to silly answers. I have fought this in the past. rbb and RE have very weak sensitivity in some parameter fitting algorithms. Imagine trying to extract rbb where its effect is only manifest by 1/beta. You need good old fashion experiments designed to extract each parameter separately (sometimes).

Thanks, Scott. It is indeed a shame that many manufacturers don't even seem to do a quick sanity check on their SPICE models before publishing them. Given all the work they do in collecting the data for the data sheets, it is especially a shame.

What is your opinion on just tweaking IS and RE to get existing models a little closer to reality?

What rule of thumb would you suggest for reasonable values of Rb for given types of devices?

Thanks,
Bob
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Old 15th November 2007, 12:43 PM   #418
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Quote:
Originally posted by teodorom
Bob,
are the (static) parameters
IS=9.85E-9 BF=487.8 NF=1.437 VAF=40 IKF=0.601 ISE=2.6E-7 NE=3.07 RB=0.178
for a MJE15030 reasonable ? Their fit is not so bad ...
Thanks

They don't look too bad to me, but I am definitely not the best person to ask.

Cheers,
Bob
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Old 15th November 2007, 12:53 PM   #419
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Bob,
I have just developed (with Mathematica) a procedure able to extract (without any explicit, manual, recursion) those static, forward, parameters. For the others I assume the "typical" value.
The great uncertainty is the value of VAF: many times there is no way to get it.
I've got reasonable results for MJL1302A, MJL3281A, MJL4302A, MJL4281A. With the MJE15030 I had some problems (BF too high, when compared to hFE, convergence problems).
More over I have problems with the displacement of the "beta droop" when varying VCE (I guess it is a problem of the SGP model).
Thanks
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Old 15th November 2007, 02:34 PM   #420
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Default Re: Re: Re: Tweaking SPICE Models

Quote:
Originally posted by Bob Cordell



Thanks, Scott. It is indeed a shame that many manufacturers don't even seem to do a quick sanity check on their SPICE models before publishing them. Given all the work they do in collecting the data for the data sheets, it is especially a shame.

What is your opinion on just tweaking IS and RE to get existing models a little closer to reality?

What rule of thumb would you suggest for reasonable values of Rb for given types of devices?

Thanks,
Bob
You're on the right track. You want to get the collector current vs. Vbe right first. rbb and RC and beta are second order, but keep in mind beta and Early voltage tend to be a constant product and a very low early voltage can complicate things a little (start at low Vcb for your first shot). I do rbb by measuring noise on a small signal transistor. Those 10-20 Ohm values are off for all but really low noise small signal transistors. Power devices OTOH have very low parasitic resistances by necessity.
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