# need help with BJT Differential Pair

Discussion in 'Homework Help' started by pree2cool, Oct 5, 2006.

1. ### pree2cool Thread Starter New Member

Oct 5, 2006
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0
When you have a BJT differential pair, but the two BJTs are not matched, then, if one of the transistors has emitter junction area 1.5 times the other's, how does that affect the collector currents, if the differential input = 0 V. Also, what difference input is needed to equalise the collector currents? if alpha=1...

what are the concepts used here???

2. ### n9352527 AAC Fanatic!

Oct 14, 2005
1,198
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First you need to find out how the emitter junction area affects the hFE and to some extent the Vbe (hint: Vbe changes might be small enough to be ignored). In a differential pair, the hFE is usually matched to some degree and any different (unmatched) hFE will cause different amount of currents to flow on both legs due to different Ib.

If you do a little bit of analysis on the value of Ic for both legs corresponding to the both Ib values and Vbe you would see the effect of different areas.

3. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
Here's a couple of hints:
You can solve this using the diode equation, with the realization that saturation current is proportional to emitter area.
The offset voltage is around 10mV. I have calculated it and simulated it, and the results agree, but I'm not going to give you the solution or the exact value, since this is homework.

4. ### mark New Member

Oct 14, 2006
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hey guys, I've got another question about the BJT differential pair

when both emitters are connected to a current source, then Ic1=Ic2 and Id1=Id2 right?
but what happens when beta is small, how can the collector and emitter currents still be equal?

5. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
Ic1=Ic2 if both transistors are identical, at the same temperature, and have Vce1=Vce2.
Collector current is never equal to emitter current, because beta is never infinite.
What do you mean by "Id1=Id2"? Id generally refers to drain current of a FET. Your question was about BJTs.

6. ### mark New Member

Oct 14, 2006
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Right, sorry, I meant Ie1 = Ie2 and Ic1 = Ic2.
So if Ie and Ic aren't equal, where does the difference in current go?

7. ### n9352527 AAC Fanatic!

Oct 14, 2005
1,198
4
It's not actually go anywhere.

Ie = Ic + Ib

So, the current coming in at Ib terminal is the difference. If hFE is large enough, then Ib is far smaller than Ic and we can ignore Ib to simplify the analysis. If not, then we have to account for the difference.