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In this paper, the effect of antenna coupling in an active array on the integrated power amplifiers (PAs) is studied. It is found that for multiple-input multiple-output (MIMO) applications some antenna elements inputs can experience negative impedance. When isolators between antennas and PAs are missing, which is preferable in highly integrated millimeter-wave (mm-Wave) massive MIMO transmitters due to the fact that isolators are bulky and integration-incompatible, this negative impedance can cause PA oscillating/damaged, or at least non-functional. This non-linear problem is studied theoretically using an example of a coupled antenna array driven by Doherty PAs. In addition, the experimental results validate the existence of the negative impedance and reveal its relationship with the array excitation and PA non-linearity.