Hello Aomidee, and thanks a lot for your interesting answer.
Well I have a question (probably stupid..) for the first schematic, I have just one remark, you said that Zd has no voltage on it, is it because the impedance of R1 and R4 are the same ?
for the second image, I'm OK.
for the differential, you're probably right but what is the method to find differential input impedance ?
Thanks a lot for your help
PS : the book may be interesting but I'm wonder if I would have explanation of analog circuits not in physics terms like i probably find in microelectronic books, I mean that I would prefer a reference book in analog... maybe the title is not the reflect of the content of the book ... ?