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-   -   What happens with bipolar Rbb as a function of collector current? (http://www.diyaudio.com/forums/solid-state/213681-what-happens-bipolar-rbb-function-collector-current.html)

gentlevoice 1st June 2012 03:56 PM

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?

Dave Zan 2nd June 2012 12:32 AM

Quote:

Originally Posted by gentlevoice (Post 3044853)
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

magnoman 2nd June 2012 12:56 AM

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

john curl 2nd June 2012 01:02 AM

Rbb` doesn't change much with current. It is 'built in'.

cbdb 2nd June 2012 01:22 AM

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.

Dave Zan 2nd June 2012 02:40 AM

Quote:

Originally Posted by cbdb (Post 3045358)
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.

sawreyrw 2nd June 2012 02:54 AM

There is a significant difference between Rbb' and rb'e. They are entirely different quantities.

Dave Zan 2nd June 2012 04:14 AM

Quote:

Originally Posted by magnoman (Post 3045340)
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 (Post 3045416)
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

knutn 2nd June 2012 10:37 AM

Quote:

Originally Posted by john curl (Post 3045345)
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.

magnoman 2nd June 2012 11:04 AM

Quote:

Originally Posted by Dave Zan (Post 3045458)
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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