What are the mistakes in determining output impedance using h-parameter model?

[DamunaTaliffato]

I have this curcuit. I want to discover the output impedance. Using h-parameter model I get the second circuit. I know that Zout= V(open circuit)/I(short circuit) and ie = C ib, where C is an appropriate proportionality factor.

Vol = V1 Re/(Rb+Re) = (Rb ib + Re ie) Re/(Rb+Re)
Isc = ie = C ib
Then, if I consider Rb << Re I get:
Zout = Re + Rb/C
The problem is that the answer is just Rb/C. What are my mistakes?

 

[trurle]

View attachment 244129
I have this curcuit. I want to discover the output impedance. Using h-parameter model I get the second circuit. I know that Zout= V(open circuit)/I(short circuit) and ie = C ib, where C is an appropriate proportionality factor.
View attachment 244130
Vol = V1 Re/(Rb+Re) = (Rb ib + Re ie) Re/(Rb+Re)
Isc = ie = C ib
Then, if I consider Rb << Re I get:
Zout = Re + Rb/C
The problem is that the answer is just Rb/C. What are my mistakes?

Typically output impedance is defined as differential impedance, i.e. d(Vdrain)/d(Idrain), not as Vdrain/Idrain

 

[Baluncore]

I assume you want the output impedance between emitter and common, with the load Re.
See; https://en.wikipedia.org/wiki/Common_collector#Basic_circuit