This paper presents a GaN HEMT based harmonically tuned broadband power amplifier. The amplifier uses the CGH40120F GaN device from Cree Inc. and delivers 100 W output power across the bandwidth 1.55–2.25 GHz. The minimum power added efficiency (PAE) over the bandwidth is higher than 60 % with maximum values up to 70 %, respectively. Measurements using digital modulated signals were performed, too. At 38.5 dBm average output power more than 40 dBc ACLR has been achieved for an UMTS signal applying memory DPD. The PA covers the frequency range of modern wireless standards like DCS1800/LTE, PCS1900/LTE and WCDMA/LTE and is well suited for multi-band, mult-standard applications.