Ok thanks, I want to know why we have to calculate rin and rout in small signal model?
Ok I change my question for the output of a bjt. Why we need to calculate ro in small signal models for a bjt? suppose I want to connect a small 8 ohms speaker to the output of a common emitter bjt, then I need impedance matching here, so can I tell that I have the impedance matching state when ro is equal to 8 ohms too?id not, then:
1: what's the usage of ro.
2: How to calculate the output resistance (impedance?) of the bjt so that I could relise if it is good for my speaker or not???