A frequency-domain stability analysis of non-linear circuits driven by a microwave Random-Modulated (RM) signal is presented. This stability analysis generalizes the already proposed method for CW generators to random modulated signals. The method is applied to perform the stability analysis of a designed and fabricated 20W/S-Band Quasi-MMIC GaN asymmetric Doherty Power Amplifier (DPA). The amplifier operates in the 3.2-3.6GHz bandwidth. It is driven by a generator modulated by a 16 QAM signal at 10MS/s. The simulated dynamic and stability results are all extracted from a frequency-domain simulation.