This paper describes the approach used to implement a highly efficient Gallium Nitride (GaN) based Class-J Doherty Power Amplifier (DPA) delivering +45 dBm peak power at 3.5 GHz, supporting 5G mMIMO small-cell infrastructure transmitters. The DPA is implemented in a compact form factor with all circuitry, except the output combiner and input splitter, placed in a 7 mm x 7 mm quad-flat no-leads (QFN) plastic encapsulated package. A circuit topology to realize the class-J network inside the package is proposed. Operating with a 48 V supply, the DPA achieves power added efficiencies (PAE) of 54% at 8 dB output power back off (OBO), and 70% at saturated power. Excellent DPD linearized performance of better than 50 dB adjacent channel leakage ratio (ACLR) is achieved with multi-carrier long-term evolution (LTE) signals.