BJT impedance calculation

  • #1
636
2
A bit confused about direction of current flow in the BJT equivalent ckt.
I'm trying to calculate the resistance seen, looking into the emitter terminal.
In what direction does the current flow in the bjt output resistance ro?
 

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Answers and Replies

  • #2
625
138
Form + to -
And direction of a current doesn't really matter.
attachment.php?attachmentid=43007&stc=1&d=1327264695.png

And also remember that

Rin = Vin/Iin

Iin = Iro+IRe + Ie

Ib = Vin/(βre+RB)

Ie = Ib + Ic = (β+1)*Ib
 

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Last edited:
  • #3
636
2
Why doesn't the direction of current really matter?
 
  • #4
625
138
Because we are dealing here with a BJT model not a real device.
And this small-signal model is nothing more then mathematical abstraction.
Our small-signal model describe the behavior of a BJT in "special condition".
We need properly DC bias the BJT into the linear region and apply low-frequency very small AC signal. And in this condition we create a small-signal model.
As for Rin = Vin/Iin our AC signal source can only see a resistance nothing more.
Because our Vin source apply voltage and supplies Iin current.
So our signal source see the BJT as a black box. So the only thing that our source is able to see is the load resistance .
 
  • #5
vk6kro
Science Advisor
4,081
40
Why doesn't the direction of current really matter?

The input signal is AC, but for analysis purposes, we assume a direction for the input at one moment and then look at the directions of the currents in the circuit at that moment.

It is usual to take the input voltage and current as positive and then the other voltages and currents in the circuit depend on this.
 

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