I understand how input and output resistance of small-signal model for bipolar transistor is defined and how draw one. What I don't understand quite yet is how actually input and output resistance of BJT are separated.
How do you know which resistances should be included for input and which for output resistance?
Because, if you ask me, models like this one could be identified as one whole resistor, whose value would be considered as input and output resistance simultaneously.
*For the record: ru - base-collector resistance, rpi - base-emitter resistance, ro - collector-emitter resistance, rb - base contact resistance, rc - collector contact resistance, rex - emitter contact resistance.