What happens with bipolar Rbb as a function of collector current? - Page 2 - diyAudio
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Old 2nd June 2012, 12:56 PM   #11
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Quote:
Originally Posted by knutn View Post
Correct, to my knowledge it is modeling the (gold wire) connection from the chip to the pad.
The bond wire resistance is a fraction of an ohm. It is trivial compared to the ohmic resistance of the base silicon. Rbb varies from about 2R for an ultra-low Rbb (low noise) transistor like the 2SB737 to say 30R for a low noise transistor and G & M use 100R to 300R for nondescript transistors in examples.
I should have remembered this immediately. The fall in Rbb is modelled in SPICE. Rbb is set by parameter RB and the decline of Rbb to parameter RBM is controlled by the current parameter IRB.
Best wishes
David

Last edited by Dave Zan; 2nd June 2012 at 01:15 PM.
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Old 2nd June 2012, 01:25 PM   #12
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Yes the base resistance is due to the conductivity of the base region. I presume that at high current levels the current is unevenly distributed (SOA hot spots developing) and the active parts of the base become isolated from the bond wire by now inert base material, raising the Rbb
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Old 2nd June 2012, 02:26 PM   #13
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Originally Posted by davidsrsb View Post
Yes the base resistance is due to the conductivity of the base region. I presume that at high current levels the current is unevenly distributed (SOA hot spots developing) and the active parts of the base become isolated from the bond wire by now inert base material, raising the Rbb
The resistance decreases as current rises. Read the earlier posts.
It is a bit counter intuitive but the effect is discussed in G & M.
Basically the emitter current moves towards the edges of the emitter, this shortens the distance that the base current must flow.

Best wishes
David

Last edited by Dave Zan; 2nd June 2012 at 02:34 PM.
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Old 2nd June 2012, 02:45 PM   #14
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www.onsemi.com/pub_link/Collateral/BC549B-D.PDF figure 6 is a rare occasion of details on this topic. As noted by others, rbb' surely decreases with collector current in any case. I don't know though if the shown data is typical.

Samuel
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Old 2nd June 2012, 03:24 PM   #15
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I would expect that rbb' vary with hfe. hfe varies with collector current, usually decreasing as collector current increases.
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Old 2nd June 2012, 04:14 PM   #16
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Quote:
Originally Posted by Dave Zan View Post
The resistance decreases as current rises. Read the earlier posts.
It is a bit counter intuitive but the effect is discussed in G & M.
Basically the emitter current moves towards the edges of the emitter, this shortens the distance that the base current must flow.

Best wishes
David
Sorry, yes this does make sense, the current is being pushed to regions with thicker base and lower resistance per square (and for the same reasons lower hfe)
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Old 2nd June 2012, 07:09 PM   #17
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Quote:
Originally Posted by Dave Zan View Post
The resistance decreases as current rises. Basically the emitter current moves towards the edges of the emitter, this shortens the distance that the base current must flow.

Best wishes
David
Dave, Really well said, thanks
Seems to tie well with Bonsia's expectant correlation of beta decrease with increasing Ic, as both effects would be dependant on the current crowding.

Thanks All
-Antonio
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Old 3rd June 2012, 06:45 AM   #18
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Hi all,

& thanks for your replies :-) It has been a very interesting read this morning to read through your posts and considerations about rbb.

@David: In your first post you ask me where I had found the information on the internet that rbb' decreases with increasing collector current. I can post this information again (it was a website related to testing the simulation algorithms of one of the major simulation softwares, I think) but given what you (all) have written here I doubt it is correct. In my memory the drops in rbb' were much larger than what have been discussed here. But let me know & I'll post a link.

So - just to sum up in a general way what you have written here:

rbb' does drop as a function of collector current (for various reasons), yet the drops are not expected to be e.g. 10* for a 10* increase in collector current but probably/more likely to be on the magnitude of some percent per doubling of the current (also depending on the level of the collector current)....?

Thanks again for replying - this is very informative ;-)

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Old 3rd June 2012, 09:13 AM   #19
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Originally Posted by gentlevoice View Post

So - just to sum up in a general way what you have written here:

rbb' does drop as a function of collector current (for various reasons), yet the drops are not expected to be e.g. 10* for a 10* increase in collector current but probably/more likely to be on the magnitude of some percent per doubling of the current (also depending on the level of the collector current)....?

Jesper
Yes, that's an excellent summary. The exact variation depends on the details of the transistor internals but the data sheet referenced by Samuel is probably typical. As you say it depends on the collector current, in particular at high currents the drop is a bit more, but simultaneously the Hfe also drops so it is not usually an area where you would run the transistor anyway.

Best wishes
David

Last edited by Dave Zan; 3rd June 2012 at 09:16 AM.
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