This article presents a wideband energy-efficient transmitter (TX) for 5G mm-wave phased-array systems. It features an advanced double-quadrature direct upconverter (DQ-DUC) to improve its in-band linearity and spectral purity. The proposed TX architecture incorporates an efficiency-enhanced balanced power amplifier (EEBPA) that mitigates VSWR fluctuations in phased-array systems while enhancing efficiency at power back-off (PBO). The EEBPA comprises two identical series-Doherty power amplifiers (PAs) combined through a quadrature hybrid coupler forming a balanced PA. The proposed DQ-DUC consists of a pair of I/Q modulators and the proposed EEBPA’s quadrature combiner to further suppress the I/Q image. To verify the proposed techniques, a 40-nm CMOS prototype is implemented. It delivers 20 dBm with 40%/31% drain efficiency at /6-dB PBO. The measured TX output reflection coefficient is better than −18 dB over a 22.5–30-GHz band. Its intrinsic LO feedthrough and image-rejection ratio for a 100-MHz tone spacing over a 24–30-GHz band are better than −45 dBc/50 dB, respectively, without calibration. The average error vector magnitude (EVM) is better than −27.1 dB without digital pre-distortion for an eight-carrier “100-MHz 64-QAM OFDM” signal with an 800-MHz aggregated bandwidth while generating an average output power of 8.4 dBm with 10.8% drain efficiency. Its maximum forward-power/EVM deviations are better than 0.3/1.65 dB, respectively, for a “100-MHz 64-QAM” signal under a voltage standing wave ratio of 3.