how can I calculate input impedance?? (In learning AOE)

[JunHyeob LEE]

Hi, I start to study electronics. And I'm a beginner.

In Learning AOE, I have questions.

I know Rout is ((4k ll 4k) + 2k) ll 10k = 5.45k

but I don't know how to calculate Rin.

[Wimberleytech]

Rout = ((4k || 4k) + 10k) || 10k = 5.455k

Now for Rin

You must assume that the junction of the two 25k resistors is open circuit (no additional load)

Using the same methodology as was done for Rout

Rin = ((25k + 25k) || 10k)  + 10k = 18.33k

[JunHyeob LEE]

Thanks a lot! 

[bson]

WimberleyTech didn't explicitly state it, but input and output impedances (often called Z11 and Z22) are always measured open circuit.  Output impedance with the input open, input impedance with the output open.  Even with things like antennas the same model is used, although there is no "output" per se, the input is measured as if there is an open output.  Admittances (Y11, Y22) are measured with the opposite end shorted.

This means C has an input impedance.  Then B+C has an input impedance, but to measure B alone you'd need to disconnect C from it.  Same with A+B+C.  On the output side, A has an output impedance measured with the input open as drawn and B disconnected.  The A+B, with C disconnected, and finally A+B+C has a joint output impedance.

Hope this helps understand in/out impedances.

[JunHyeob LEE]

Thank you for your detail explanation.

[Wimberleytech]

WimberleyTech didn't explicitly state it, but input and output impedances (often called Z11 and Z22) are always measured open circuit.  Output impedance with the input open, input impedance with the output open.  Even with things like antennas the same model is used, although there is no "output" per se, the input is measured as if there is an open output.  Admittances (Y11, Y22) are measured with the opposite end shorted.

This means C has an input impedance.  Then B+C has an input impedance, but to measure B alone you'd need to disconnect C from it.  Same with A+B+C.  On the output side, A has an output impedance measured with the input open as drawn and B disconnected.  The A+B, with C disconnected, and finally A+B+C has a joint output impedance.

Hope this helps understand in/out impedances.

I do not disagree regarding two-port analysis.  However, for this problem, even with the limited insight into the context of the original question,  I still believe that you treat Vin as a short circuit for Rout analysis.