You need to ensure that your transistors are operating in saturation which means your dc operation point is correct. To ensure this, you need to have a DC feedback loop for your initial condition then you break the loop for your AC response. The very easy way of doing this is connecting your amplifier with a low pass filter in the feedback. So make a 1Farad!! cap to ground on your Vinm input then a 100Meg ohm resistor connecting your Vinm to your output. Then your input AC signal on Vinp. This is ok for low frequency amplifier design say below 100MHz GBW. But if your making fast stuff, you will need to use the STB analysis that Freebird is talking about. Connect your amplifier as the voltage follower (your DC feedback), with proper loading, then use the "iprobe" widget in the feedback path (it will "break the loop for you for AC", it is more complicated then this but if you want more info look up middlebrook, tian method). In the STB analysis, just select that "iprobe" unit and simulate. It will then say the DC gain and PM margins in your popup window. If you want to see the gain and phase plots go to your results brower and look it up!
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