Power Amplifier designs can unexpectedly start oscillating. Oscillation is caused by feedback from the output to the input through unexpected paths and spurious frequencies (i.e. RF coupling). Stability analysis shows the frequencies and locations of feedback paths that made this amplifier unstable using EM visualization. Once these paths are identified, we can fix the amplifier oscillation by changing the layout and adding more ground vias as shown in the larger picture. See how this is done in this tutorial video.