Output resistance BTJ CE with emitt

Hello all,

I'm a little bit confused about finding the output resistance when using the hybrid pi model en taking ro into account :

this is the ac equivalent :
**broken link removed**

I don't understand why the currents ( I1 and I2 dIb) are drawn in that direction?

And also a guy (uart,https://www.physicsforums.com/showthread.php?t=635192) says Vbe=0 ? won't there be a voltage drop when across Rpi?

Hopefully you guys can point me in the right direction.

Kind regards

SneaKSz said:

Hello all,
I don't understand why the currents ( I1 and I2 dIb) are drawn in that direction?

Click to expand...

The direction of ΔIb is a convention; into the input port.

The directions of I1 and I2 are arbitrary.

SneaKSz said:

And also a guy (uart,https://www.physicsforums.com/showthread.php?t=635192) says Vbe=0 ? won't there be a voltage drop when across Rpi?

Click to expand...

The circuit in that other thread doesn't have the resistor ro. Without ro there is no path for current from the output to the input. If there is no such path, and if the input is grounded (meaning that no input voltage is applied), then there will be no current in r∏ to cause a non-zero Vbe.

Your circuit has a resistor ro, so the expression for the output resistance will involve r∏; the output resistance expression for the circuit in the other thread (it's just Rc) will not involve r∏.

Got it, drawed it with Rbb//Rin, and there is indeed no current path! thankss!