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

automagp68

Joined Nov 13, 2011
81
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?

MikeML

Joined Oct 2, 2009
5,444
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))

automagp68

Joined Nov 13, 2011
81
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

ian field

Joined Oct 27, 2012
6,539
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
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.

be80be

Joined Jul 5, 2008
2,049
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.

MikeML

Joined Oct 2, 2009
5,444
You are interested in what happens to the ratio of the currents as the β gets bigger. That is what Limits are used for

dl324

Joined Mar 30, 2015
11,938
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.

Alec_t

Joined Sep 17, 2013
11,758
Eh? If Ib=0 then Ie=0 and Ic=0. Both equations are satisfied.

automagp68

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

Normally Beta = infinity is a saturation excersise.

If Ie = 0 = IC

then we are talking about cut off

crutschow

Joined Mar 14, 2008
25,702
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.

dl324

Joined Mar 30, 2015
11,938
Normally Beta = infinity is a saturation excersise.
Beta decreases when Vce is low.