# NCEES 121



## jdd18vm (Oct 19, 2007)

I dont understand why "since Beta is 100" Ie is approx equal 2mA (which is equal Ic)

John


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## benbo (Oct 19, 2007)

jdd18vm said:


> I dont understand why "since Beta is 100" Ie is approx equal 2mA (which is equal Ic)
> John


This is probably more information than you need. You can just asume Ic = Ie.

But, the reason =

a=alpha

b=beta

In a BJT Ic = a*Ie

a = b/(1 + b ) so, if b= 100 a=100/101 = .99

Ic = .99*Ie.

This wouldn't be the case if b were small, say 1. In that case a=1/2, Ic=.5*Ie


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## Flyer_PE (Oct 19, 2007)

Benbo will likely have a better explanation of this than I do but I'll give it a shot:

The beta value is the ratio of the base current to the collector current. The emitter current is the sum of the base current and collector current. With a high beta, the emitter current is very close in magnitude to the collector current. The totally scientific answer is that the current through the 1k-ohm resistor will be 1.01 times the value of the collector current.

Let me know if this helps.

Jim


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## Flyer_PE (Oct 19, 2007)

Gotta love answering the questions in parallel.

Also, he did do a better job than I did. 

Jim


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## jdd18vm (Oct 19, 2007)

Okay got that, and it makes some sense.

I got confused in EERM 43-18 eq 43.28 B=Ic/Ib. I used that I got Ib=100/2mA and that didnt make sense. Whats the distinction?

Thanks again, this is really helpful.

John


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## benbo (Oct 19, 2007)

jdd18vm said:


> Okay got that, and it makes some sense.
> I got confused in EERM 43-18 eq 43.28 B=Ic/Ib. I used that I got Ib=100/2mA and that didnt make sense. Whats the distinction?
> 
> Thanks again, this is really helpful.
> ...


No, thats also true = Ic = b*Ib

Beta is the relationship between the Ib and Ic, alpha is the relaiton between Ic and Ie, and you can derive beta from alpha and vice versa.


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