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So I'm currently reading CMOS by R. Jacober Baker and in the first chapter there is a quick introduction into Spice but I have a hard time understanding one of its examples. So this is the circuit: a busy cat

The calculated output impedance is infinity.

While I understand how the .TF statement calculates the output for a voltage as output I'm kinda irritated on how the output impedance for this kind of setup is calculated. In the book it says Vmeas is getting removed and the circuit is open, but I don't understand why Vmeas is getting removed for this calculation. And even if it's getting removed, the impedance looking into one of the pins of the removed Vmeas source would still be 3k because you can "reach" the other pin via the ground connection. (same thing would happen if I apply a voltage source to my output pins -> I'd get a current over gnd and so the output impedance defined by Ro=Vo/Io=3k)

I assume I have a wrong understanding of what the output pins or the desired output is in this case because my defined output is a current now instead of a voltage, but I'm not really sure how to handle this correctly.

So why is the output impedance in this case infinity?

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You need to specify what happens in place of Vmeas when you remove it. Do you substitute an open circuit, a short circuit, or something else?

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  • \$\begingroup\$ I don't remove it. I just start the simulation as shown in the picture. Output impedance equals infinity. The remove part was only the explanation for this in the book, which didn't really help me. \$\endgroup\$ – roym899 Dec 27 '15 at 21:55

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