PSPICE and negative resistance

[rtarbell]

Has anyone been able to successfully simulate a negative resistance circuit using pspice (or any other circuit simulator)?

I have been trying to simulate the attached circuit, and Pspice tells me that the input impedance as seen by the source is +1k (positive 1000 ohms).

I believe that the circuit should have an input impedance of:

-R3 * (R1/R2) = -1k for this circuit.

I tried simulating the same circuit in TopSpice, and I get the same result.

Am I doing something wrong, or do simulators not understand negative resistances?

 

[Elvee]

Ltspice apparently also gives a positive resistance; however, a closer examination shows that the analysis itself is correct, but the convention for the current direction from the source is that of a receiver, not a generator.
If, instead of the current from the source, the current in the wire leading to it is taken into account, then the resistance is negative.
Here is the result in this case:

 

[rtarbell]

AHA!!

Much obliged to you Elvee - I see that you are absolutely correct! I need to keep track of my sign conventions - measuring the current through the source does not necessarily give the direction I was expecting.

Thanks to you Elvee!