In the following diff amp, only one output is taken from collector two. When I calculate the differential mode gain using the second differential input (-Vid/2) I get a positive gain. When I calculate the differential mode gain using the first differential input (Vid/2) I get a negative gain. A spice simulation of this circuit shows that the output is positive. I would then expect to get a negative gain for (-vid/2) and a positve gain for (vid/2) Does someone know why that is not the case?