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Old 25th January 2010, 06:56 AM   #21
akis is offline akis  United Kingdom
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Wanted to recap, this thread is/was about the model's inconsistency of hfe vs Vce compared to the real world. I measured 1% whereas the model reports 50% over the same range.
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Old 25th January 2010, 01:06 PM   #22
klewis is offline klewis  United States
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Wanted to recap, this thread is/was about the model's inconsistency of hfe vs Vce compared to the real world. I measured 1% whereas the model reports 50% over the same range.
Akis,

I was measuring a bjt that had a spice model created by Andy_C to see how a real world sample compare to a model that had been tweaked to better match the data sheet. From all that I've read on this forum, it seems that if you are using models in critical locations, you probably need to tweak the model to better match the data sheet.

Ken
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Old 25th January 2010, 01:15 PM   #23
AndrewT is offline AndrewT  Scotland
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so all measurements are best done with 2 multimeters and even then it is hard to keep constant temperatures.
we amateurs cannot maintain constant temperature test conditions with our cheap and quick jury rigged test procedures.
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Old 26th January 2010, 07:30 AM   #24
akis is offline akis  United Kingdom
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Quote:
Originally Posted by klewis View Post
Akis,

I was measuring a bjt that had a spice model created by Andy_C to see how a real world sample compare to a model that had been tweaked to better match the data sheet. From all that I've read on this forum, it seems that if you are using models in critical locations, you probably need to tweak the model to better match the data sheet.

Ken
And that work is courageous, especially since you are messing with 10A currents!

I do not think the datasheet is correct either. It could easily be related to manufacturing : different batches, different factories, different companies - whatever reason there are great differences between datasheet and measurements. And on top of that it also appears the SPICE models are different.

Secondly it is not about "critical locations"; if you have a single stage single transistor amplifier, well, it is critical by definition. I spend considerable amount of time in SPICE trying to fine tune circuits and select the values of 80 resistors and 20 capacitors. If those calculations are out then even the topology of my circuit could be wrong.

For example I am measuring THD in the virtual world and biasing the various stages accordingly. If the model thinks there is a 50% hFE shift from Vce=3V to Vce=23V then it will show much higher THD, which simply is not there, because measurements show a shift of 1% not 50%.

This shortfall of the SPICE model has become more apparent in post-amp circuits where you do expect a swing of 20V, and has gone unnoticed in pre-amp circuits where the voltage swings are one ot two volts.

Why have others not noticed it? May be they have. May be they are all using hefty amounts of negative feedback and a VAS stage with an active load (as is the common practice) so that the Vce/hFE variations are the least of your worries.

But try to create a circuit with no negative feedback and ask it to amplify 20 times and swing 20V and see what THD the model will say you are getting.
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Old 26th January 2010, 02:34 PM   #25
klewis is offline klewis  United States
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Akis,

The Spice Simulation Sticky in this forum also covers this topic. I've started refining my models using Andy_C's techniques along with the Analog Services spread sheet. It is possible to tune the models to get them pretty close on things such as hFe. It's time consuming... but, less time consuming than physical testing, de-soldering, re-soldering, testing, rinse, repeat...

Ken
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Old 27th January 2010, 06:04 AM   #26
akis is offline akis  United Kingdom
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Quote:
Originally Posted by klewis View Post
Akis,

The Spice Simulation Sticky in this forum also covers this topic. I've started refining my models using Andy_C's techniques along with the Analog Services spread sheet. It is possible to tune the models to get them pretty close on things such as hFe. It's time consuming... but, less time consuming than physical testing, de-soldering, re-soldering, testing, rinse, repeat...

Ken

All 50+ pages of it, a daunting task :-)
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Old 3rd February 2010, 05:47 AM   #27
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Dear gentlemen,
I need a reasonably accurate model of either 2SA1186 or 2SA1860. These are very good Sanken power transistors but Sanken does not provide the model AFAIK. Does anyone know where to get them? Or would anyone build one so that many people can benefit from these as the transistors have very good characteristics. I would build them if I know how but it would take some time before I get to that place.

Thanks
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Old 3rd February 2010, 06:36 AM   #28
cbdb is offline cbdb  Canada
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just my 2 cents, if the transistor data sheet (which is usually data taken from a large population to find "worse case average") and a spice simulation both show large variation in hfe with Vce than it probably exists and there may be something wrong with your testing. hfe varies exponentialy with temp so unless you keep it controlled I dont know how you can be sure of your data. hfe also varies with Ic and from individual transistor to transistor, so its best to design around it, and thats what most people do.
If Andy_C was still around he could set us all straight.
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Old 16th June 2011, 06:52 AM   #29
dce is offline dce  United States
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Originally Posted by cbdb View Post
just my 2 cents, if the transistor data sheet (which is usually data taken from a large population to find "worse case average") and a spice simulation both show large variation in hfe with Vce than it probably exists and there may be something wrong with your testing. hfe varies exponentialy with temp so unless you keep it controlled I dont know how you can be sure of your data. hfe also varies with Ic and from individual transistor to transistor, so its best to design around it, and thats what most people do.
If Andy_C was still around he could set us all straight.
where is Andy_C by the way
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Old 2nd February 2014, 10:35 PM   #30
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I am reviving this long quiet thread for a while.

I am looking for good models for the BC546C and BC556C.

All I could find for the BC546 was the B or no suffix type.

If at all possible the model should reflect the real thing, or this is pointless.
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