Transistor output impedance

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.
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?
 
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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
 

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