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Old 7th November 2011, 01:41 PM   #3961
ingrast is offline ingrast  Uruguay
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Quote:
Originally Posted by Bob Cordell View Post
Hi Stinius,

..... Since moderate voltages and moderate collector currents are encountered in the measurement, it is very easy for heating of the device to influence beta and result in an Early voltage that is too low.....
.... as the resulting junction dissipation can lead to erroneous results due to beta dependence on junction temperature.
This is very interesting and unexpected at least for me, but of course makes lot of sense.

I mean, in the real world devices will heat up locally (not the circuit as a whole) and experience temperature variations dependent on actual signal levels and mechanical installation details.

This is not usually modeled except for families of simulations or Monte Carlo analysis with temperature as variable, but then for the whole assembly which may yield results different from those where only local sections of the circuit are actually subject to temperature variations.

Ideally a dream simulator may allow for on the fly power disipation results be coupled with thermal models as designed or set as design info, from which junction temperature is determined and input to simulation.

Out of ignorance I presume this type of simulation (local individual component temperature dependence) is not usually analyzed. I this true?

Rodolfo
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Old 7th November 2011, 09:38 PM   #3962
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Originally Posted by Edmond Stuart View Post
Hi Bob,

Nice to see you back. As for Modpex models, I've decided not to use them anyhow.
Instead, I prefer (for example) Fairchild, Philips and Zetex models which seem to be far more reliable.
BTW, Modpex (=Motorola=ON) models are automatically (read: brainlessly) generated, hence ridiculous parameters like VAF=4.91894

Cheers,
E.
Hi Edmond,

Thanks.

Whenever I see a SPICE model with parameters that have more than two or three significant digits I assume that it has been rather brainlessly created.

BTW, I sometimes suspect that they try to create the SPICE models just from datasheet curves. Unfortunately, the datasheets usually do not have enough information over a wide enough range to extract a decent model from.

Cheers,
Bob
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Old 8th November 2011, 05:33 PM   #3963
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Hi Bob,

Sure, many digits makes me suspicious too, but in this case it was the Early voltage of less than 5 Volts that is outrageous. Have you ever seen a (high voltage) tranny behaving so badly?

Cheers,
E.
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Old 7th January 2012, 05:12 PM   #3964
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Here are Thermal Time Constants for Cubes of Silicon

0.1 micron => 116 picoseconds (emitter-base thickness of modern bipolars)

1 micron ===> 11.6 nanoseconds (modern biCMOS can be this small)

10 micron ==> 100*11.6nS = 1,160nS = 1.16 microsecond
(you can squeeze several active elements into this area, getting good tracking)

100 micron => 100*100*11.6nS = 116,000nS = 116 microseconds
(modern opamps, and many small signal discrete transistors)

1,000 micron => 100^3 * 11.6nS = 11.6 milliSeconds
(this is 40 mils on a side, about what first generation OpAmps needed)

10,000micron (1cm) => 100^4 * 11.6nS = 1.16 seconds
(this is active size of many system-on-chip products)

To compute these, use Specific Heat, Thermal Conductivty, and Mass of silicon.

The useful name "thermal timeconstant" is obscured by the
title "thermal diffusivity" of the chemical properties community.
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Old 15th April 2013, 11:46 PM   #3965
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Originally Posted by Edmond Stuart View Post
I hope that these figures provide enough evidence that HEC is nothing more (or less) than NFB in disguise.
Has anybody indicated thatn HEC isn't negative feedback?

HEC is, in fact, a combination of a negative feedback loop and a minor loop positive feedback loop.

The gain generated by the positive feedback loop is merely throttled down by the major negative feedback loop.
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Old 16th April 2013, 01:19 AM   #3966
ingrast is offline ingrast  Uruguay
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Do you really want to pull the lid off this pot, again ????

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Old 16th April 2013, 01:45 AM   #3967
fas42 is online now fas42  Australia
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Quote:
Originally Posted by Bob Cordell View Post
BTW, I sometimes suspect that they try to create the SPICE models just from datasheet curves. Unfortunately, the datasheets usually do not have enough information over a wide enough range to extract a decent model from.

Cheers,
Bob
Which implies that a conventional user of a datasheet doesn't have enough information to work with ...
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Old 17th April 2013, 02:44 AM   #3968
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Originally Posted by michaelkiwanuka View Post
Has anybody indicated thatn HEC isn't negative feedback?

HEC is, in fact, a combination of a negative feedback loop and a minor loop positive feedback loop.

The gain generated by the positive feedback loop is merely throttled down by the major negative feedback loop.
Hi Mike,

Yes, it is correct to view HEC as an architecture that ultimately employs negative feedback. The simple explanation of HEC where a couple of summers are arranged to show the "error correction" aspect of it can be readily re-drawn to show an explicit positive feedback loop within a negative feedback loop. The loop gain of that positive feedback loop is nominally +1, implying that it is an infinite gain element embedded in the forward path of the negative feedback loop. Vanderkooy and Lipshitzed showed this many years ago. Indeed, this way of looking at HEC underlines the need for feedback compensation of this loop.

The "error correction" way of looking at HEC underlines the interesting optimum trimming attribute of this arrangement.

This was a long thread, and it was pointed out at several points along the way that different ways of looking at HEC can be useful, but that one must never ignore the reality of the need for compensation and that there is no free lunch.

Cheers,
Bob
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Old 17th April 2013, 02:46 AM   #3969
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Old 17th April 2013, 12:35 PM   #3970
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Originally Posted by fas42 View Post
Which implies that a conventional user of a datasheet doesn't have enough information to work with ...
If a datasheet is all you have, then you can at least create a model that is more workable than some out there. The datasheet that results may not be as comprehensive as we'd like, but may still be better and more reliable than some of the commercially-supplied models. When creating a model, it is always important to sanity-check the performance of the model against the datasheet curves. Of course, some datasheets are far more comprehensive in the data they supply than others.

Cheers,
Bob
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