This is an example calculating output resistance that drives the inductor.
Source:Linear Circuit Transfer Functions: An Introduction to Fast Analytical Techniques by Christophe P. Basso
What I am confused is why the input and output are grounded references so they are connected together here as indicated in the red circles?
For calculating output resistance seen by the inductor, should we only short the input 1 so the output resistance will be infinite?
Update:
Why not just calculate seen by the inductor as normal (short input voltage source and leave everything else remain the same as below? So the resistance seen by inductor is infinity.
PS: Google book link - page 4:
https://books.google.co.kr/books?id=WGBFjgEACAAJ&printsec=frontcover#v=onepage&q&f=false
Or this online version:
http://dl.4mohandes.com/book/ba/Linear_Circuit_Transfer_Functions.pdf
Answer
He's finding the Thevenin resistance across the inductor. It's a two port network and the circuit shown in the figure is equivalent circuit of it. It's equivalent ckt will have lower port of input and output grounded. See here for all type of equivalent circuits. Since capacitor is getting open circuited and i/p voltage source short circuited we are getting Thevenin resistance as R1 + R2.
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