# BJT Differential Pair (Max Diff. Input?)

Discussion in 'Homework Help' started by jegues, Dec 14, 2011.

1. ### jegues Thread Starter Well-Known Member

Sep 13, 2010
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Hello all,

I was trying to do the following question.

See figure attached.

The part I'm confused about is how to figure out how large of a VB1 will cause one side of the pair to hog all the current.

How do we figure out if our VB1 is of large enough magnitude such that Q1 hogs all the current I?

Thanks again!

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2. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Dec 26, 2010
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Note that only a fairly modest value for VB1 will get a very large proportion of the current to flow in Q1, say 99% of it, but a much larger voltage would be required to reduce the current in Q2 to its absolute minimum. Which do you require?

4. ### jegues Thread Starter Well-Known Member

Sep 13, 2010
735
43
Enough to say that Q2 is essentially cut off.

In other words, it is not required that it is the absolute minimum.

Dec 26, 2010
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298
If you know the particular transistor used you may be able to get data on the variation of VBE with IC. Otherwise the Shockley diode equation will pretty much tell the story. http://en.wikipedia.org/wiki/Diode_modelling#Shockley_diode_model
Typically a small silicon transistor will have an ideality factor (n) pretty close to 1.

At room temperature, this boils down to a shift of collector current of ten fold for a change in base voltage of approximately 60mV.
At other temperatures the slope of the curve will be different. Here is a link to an article on the subject - it goes on into more detail than you may want, but it begins with a nice illustration of the logarithmic slope.

http://www.national.com/rap/Story/vbe.html

Last edited: Dec 16, 2011