This paper presents a compact high efficiency broadband power amplifier (PA) for 5G mm-wave applications covering both 5G bands n257 and n258 (24.25 - 29.50 GHz). Thanks to its two-stacked power cell architecture and its common source driver, the amplifier exhibits a high maximum power added efficiency (PAE) of 43% at 26 GHz for a saturation power of 18.8 dBm. Using 5G modulated signals with PAPRs of 9.9 dB, the power amplifier meets the requirements of the 5G standard with a Pavg of 10 dBm and a PAEavg of 15% at 26 GHz. In the n257 and n258 5G New Radio (NR) FR2 bands (24.25-29.50GHz), PAEmax and PAE6dBPBO of the PA remain above 32% and 16%, respectively. The amplifier maintains its performance with a VSWR of 1.5:1 with a PAEmax and Psat drop of 5% and 0.8 dB, respectively. The PA is implemented in a 28nm FD-SOI CMOS process and only occupies 0.30mm2.