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

[Al Davis]

On Monday 07 October 2002 07:10 am, Hugo van der Merwe wrote:
> 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? 

.print ac z(v1)
  is the impedance seen by v1, not counting itself.



>     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..)

Not yet in AC, but in DC and transient you can:
.print tran rpi(q5)
(or whatever transistor you want)

You can also probe "Z" for the internal elements of the transistor 
model.

If you want to find the impedance between an arbitrary pair of nodes, 
put a component there that is equivalent to an open circuit, and 
probe "Z" for it.  An admittance, capacitor, or current source with 
value zero will work.