Electrical Engineering/bjt current


QUESTION: hello!
I am wondering is it true for ac current that
base current * beta factor = collector current?

in this picture B*ib not the entire collector current  right?
ic = Bib + vce/ro
so that relationship only holds in DC?

ANSWER: Maybe you need to see it in a circuit.  


It holds in dc and in small signal ac amplifiers but not in switching circuits where the current is sent from zero to saturation.

It is a matter of transistor operation for basic analysis point of view to define the Beta as the ratio of collector current to base current.  And, since the emitter current is nearly the same - at least for a high beta transistor - so it holds true for the emitter current as well.

Hope this helps.

[an error occurred while processing this directive]---------- FOLLOW-UP ----------

QUESTION: http://en.citizendium.org/images/thumb/8/82/Bipolar_hybrid-pi_as_two-port.PNG/35

Could you look at this picture ?

here ic = Bib+ Vce/ro right?

ANSWER: Ic = Ie - Ib


Ie = Ic + Ib

---------- FOLLOW-UP ----------

QUESTION: erm~ What I am trying to ask is  c -Bib isnt 0 it's vce/ro unless ro is infinite right?
therefore ic is only equal to B * ib when there is no ro? ( check the picture please)

You are making this too hard.

Ro is not a real fixed resistor; it is a function of the current and temperature and other things so it is not a useful term to use for making calculations usually. And since it changes with emitter current it provides distortion in an ac amplifier.  Therefore, in real circuits an emitter resistor is added in order to swamp out the effect of changing value of Ro.

The typical value of Ro in practical transistors is around 30 to 100 ohms - from my memory.

And, yes, if Ib is "0" then there is no Ro because no current is flowing which is a shut off condition.  

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