What happens with bipolar Rbb as a function of collector current? - diyAudio
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Old 1st June 2012, 03:56 PM   #1
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Default What happens with bipolar Rbb as a function of collector current?

Hi,

I have a quick question that I hope you can help with:

It is "What happens to a bipolar transistors Rbb (base spreading resistance) as a function of collector current? Does it increase or decrease and how much?

I'd appreciate your insights ;-)

Best regards,

Jesper

P.S.: Just learned on the internet that the Rbb drops with increasing collector current. But would one of you know how much?
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Last edited by gentlevoice; 1st June 2012 at 04:04 PM.
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Old 2nd June 2012, 12:32 AM   #2
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Quote:
Originally Posted by gentlevoice View Post
Hi,

I have a quick question that I hope you can help with:

It is "What happens to a bipolar transistors Rbb (base spreading resistance) as a function of collector current? Does it increase or decrease and how much?
I believe that Rbb is dominated by ohmic resistance and therefore should not be very sensitive to collector current at typical values.
It is an excellent question, where is your reference that it decreases with collector current?

Best wishes
David
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Old 2nd June 2012, 12:56 AM   #3
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Hi

I think it just used as the change in base current for a given change in base emitter voltage or B*1/gm

Hope this helps
-Antonio
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Old 2nd June 2012, 01:02 AM   #4
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Rbb` doesn't change much with current. It is 'built in'.
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Old 2nd June 2012, 01:22 AM   #5
cbdb is offline cbdb  Canada
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From Analysis and Design of Analog Integrated Circuits by Gray and Meyer. "The value of rb varies significantly with collector current because of current crowding." Have to run, will try to add more later.
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Old 2nd June 2012, 02:40 AM   #6
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Originally Posted by cbdb View Post
From Analysis and Design of Analog Integrated Circuits by Gray and Meyer. "The value of rb varies significantly with collector current because of current crowding." Have to run, will try to add more later.
I don't have a copy of G & M handy to check their use of the symbols but Rb is perhaps not the same as Rbb, which is what you asked about in the first post.

Best wishes
David

Edit. It may be that they do use Rb for base spread R.
Google books shows only a short excerpt.
Edit2 Amazon has more. Typically 50% variation for 100X alteration of collector current.
Not very sensitive.

Last edited by Dave Zan; 2nd June 2012 at 03:06 AM.
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Old 2nd June 2012, 02:54 AM   #7
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There is a significant difference between Rbb' and rb'e. They are entirely different quantities.
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Old 2nd June 2012, 04:14 AM   #8
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Quote:
Originally Posted by magnoman View Post
Hi

I think it just used as the change in base current for a given change in base emitter voltage or B*1/gm

Hope this helps
-Antonio
Quote:
Originally Posted by sawreyrw View Post
There is a significant difference between Rbb' and rb'e. They are entirely different quantities.
Yes The OP was about Rbb. Antonio's comment is about rb'e, I think.
I just wasn't sure what G & M meant by Rb. Apparently it is Rbb.
If so then Rbb is not very sensitive but varies a bit more than I expected.
I like questions where I learn

Best wishes
David
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Old 2nd June 2012, 10:37 AM   #9
knutn is offline knutn  Norway
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Quote:
Originally Posted by john curl View Post
Rbb` doesn't change much with current. It is 'built in'.
Correct, to my knowledge it is modeling the (gold wire) connection from the chip to the pad.
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Old 2nd June 2012, 11:04 AM   #10
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Quote:
Originally Posted by Dave Zan View Post
Yes The OP was about Rbb. Antonio's comment is about rb'e, I think.
I just wasn't sure what G & M meant by Rb. Apparently it is Rbb.
If so then Rbb is not very sensitive but varies a bit more than I expected.
I like questions where I learn
Yep I was wrong, just thought of it as the standard diode equation even though I saw the words (base spreading resistance).

One more dog-ear to my G & M

Thanks
-Antonio
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