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[Help-gnucap] Determining impedance between two nodes...

From: Hugo van der Merwe
Subject: [Help-gnucap] Determining impedance between two nodes...
Date: Mon, 7 Oct 2002 15:10:37 +0200
User-agent: Mutt/1.4i

Are "newbie" questions such as these welcome on this list? :

How can I determine the impedance seen at the input and output nodes of 
an amplifier circuit? I've tried adding e.g. Z(23) to find the impedance 
at node 23 (my output node). I added it to my ".print ac" line, as I am 
interested in AC small signal impedance.

This gave values at the output which looked reasonable, though I haven't 
verified them. I want to do the same for the input, but since I had this 
as my input:

Vin 3 0 DC 0 AC 10m

the result is of course practically 0. I tried disconnecting Vin to see 
what it returns then, but that doesn't work, since that changes the DC 
operating point as well.

So how can I get the small signal resistance of the circuit as seen by 
the source? Is it also easy to see the small signal resistance seen by 
e.g. the internal capacitances of a transistor? (E.g. the small signal 
resistance seen between the base/emitter nodes, and the 
collector/emitter nodes..)

Hugo van der Merwe

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