I have severals questions :

1) The first one is what is the mathematical definition of input and output impedance :

According to me :

- Input impedance is equal to Zin = Vin/Iin
- Output impedance is equal to Zout = (Vout - Vin)/Iout, however I ve already seen that the output impedance is equal to Zout = Vout/Iout. If you can explain me how Zout can be equal to Vout/Iout, it will be very nice to you

2)Here is a circuit, it is a voltage regulator with an emmiter follower :

What is the output impedance of this circuit ?

Can you please detail your calcul ? I m not interested in the result, i m interested in how you proceed to compute the output impedance.

Thank you very much !

Have a nice day