On a DC sweep, any plot of voltage vs. current has coordinates in ohms (or

reciprocal in mhos).

There are two kinds of resistance to note: static (DC) resistance, and

incremental (dynamic, differential) resistance. Static is when you take

the coordinates of a point on a curve, and divide. Incremental is the

slope of the curve at that point.

All linear resistor networks have static = incremental.

Note that if the resistance is measured with respect to different parts of

the circuit, it's more properly transresistance (e.g., an amplifier with

voltage output and current input has a gain of Vo / Iin, a resistance).

For general purpose, you probably want the AC impedance, not the DC

impedance. Use an AC analysis for this. You can find the impedance from

the current drawn from an AC voltage source applied to the port in

question (setting all other sources to 0V AC).

For example, here's a grounded emitter, class A, tuned amplifier:

http://t3sl4.dnsdynamic.net/Images/ResonantStage2.png
(Ignore T1.)

The S parameters measured are:

http://t3sl4.dnsdynamic.net/Images/ResonantStage1.png
Return loss (gamma) is a matching parameter. When:

gamma = -1: Load is open circuit (infinite resistance)

-1 < gamma < 0: Impedance is higher than source impedance (R4 = 100 ohm)

gamma = 0: Impedance equals source

0 < gamma < 1: Impedance is lesser than source

gamma = 1: Load is a short circuit (0 ohms)

gamma > 1: Load is negative!

Note that this circuit actually has a negative input impedance from about

380k to 500k. This type of circuit can oscillate very easily, and

precautions have to be taken to ensure the input isn't overly reactive.

I could've just as well plotted input impedance in the graphs, using the

same parameters, and the AC definition of input impedance.

Hmm, this graph is showing gamma as low as -4. Might've got the formula

wrong...

Tim