# BJT question because i can't remember what i learned 10 years ago

Discussion in 'General Electronics Chat' started by automagp68, Oct 25, 2015.

1. ### automagp68 Thread Starter Member

Nov 13, 2011
81
0
Im considering the situation where Beta = infinity

here is my issue

Everyone knows thats

Its assumed when beta = infinity ib = 0

Ie = (b+1)(ib)

Also

Ie = ib+ic

Hence most people say when Ib=0
Ic = ie

From the first equation we see that if ib = 0 then Ie = 0

from the second equation we see that if ib = 0 then Ic = ie

How is this possible.

I know beta = infinity is commonly referred to as saturation
But i can't recall why these two equations are contradictory like this.

They must agree and for the reasons stated above they do not
Someone help me remember why please?

2. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
Beta is never infinity....
Try:
1. beta = big number = Ic/Ib,
2. Rearrange: Ib = Ic/(big number)
3. Ie = Ib + Ic
4 Subst. 2 into 3: Ie=Ic(1+1/(big number))

3. ### automagp68 Thread Starter Member

Nov 13, 2011
81
0
I know beta is never really infinity. Its a theoretical problem commonly given to students

How is this handled when a problem specifically states beta = infinity

I know what your getting at above
aka ic = ie

4. ### ian field Distinguished Member

Oct 27, 2012
4,459
792
That might be similar to an op-amp problem/exercise.

AFAICR: the gain of an op-amp is considered infinite (for all intents and purposes) for some parts of the mathematical considerations.

5. ### be80be Senior Member

Jul 5, 2008
432
57
A lot of people say this because it doesn't matter for the part your trying to figure.
But some parts of the figure need the value you trying to get. Maybe that more unstandable. Maybe not.

6. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
You are interested in what happens to the ratio of the currents as the β gets bigger. That is what Limits are used for

7. ### dl324 Distinguished Member

Mar 30, 2015
3,384
653
For sufficiently large β it's okay to think of Ie as the same as Ic. Even when β is only 100, that assumption introduces less than 1% error.

8. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,979
1,138
Eh? If Ib=0 then Ie=0 and Ic=0. Both equations are satisfied.

9. ### automagp68 Thread Starter Member

Nov 13, 2011
81
0
And that is the problem Alec

Normally Beta = infinity is a saturation excersise.

If Ie = 0 = IC

then we are talking about cut off

10. ### crutschow Expert

Mar 14, 2008
13,509
3,385
It really makes no sense to talk about a Beta of infinity, since then you have no way to control the current in the transistor (the collector current would either be 0 or infinity since even an infinitesimal base current causes infinite collector current).
I think discussing the operation of a transistor with infinity Beta is as pointless as arguing about how may angels can sit on the head of a pin.

11. ### dl324 Distinguished Member

Mar 30, 2015
3,384
653
Beta decreases when Vce is low.