NCEES 121

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

John

jdd18vm said:

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

Click to expand...

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

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

Gotta love answering the questions in parallel.

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

Jim

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

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.

John

Click to expand...

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.