This work presents a high-efficiency and linear Ka-band power amplifier (PA) designed in a 0.13 µm depletion-mode GaAs pHEMT process, targeting 5G phased-array systems. To minimize passive losses, the output matching network employs an all-transmission line architecture. Phase mismatches among output branches are compensated directly within the interstage and output matching networks via tailored distributed and capacitive components. Device-level reliability is proactively addressed by maintaining adequate voltage headroom under worst-case load mismatch, based on voltage standing wave ratio (VSWR) analysis. The amplifier achieves a peak small-signal gain of 15.8 dB at 27 GHz. Under continuous-wave excitation at 27 GHz, it delivers 32.9 dBm output power at the 1-dB compression point with 32.8\% power-added efficiency (PAE), reaching a peak saturated output of 33.2 dBm and 35.9\% PAE. When driven by a 64-QAM signal with a 250 MHz symbol rate, the PA maintains an average output power of 26.3 dBm and an average PAE of 12.2\%, with an rms EVM of 3.4\% and an SNR of 25.5 dB.