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We present a wideband, 12-bit four-way Doherty Cartesian digital transmitter (DTX) featuring an innovative 50%-LO signed $I/Q$ interleaved up-conversion technique that enables close to perfect orthogonal $I/Q$ summation. The DTX incorporates a compact four-way lumped-element Doherty power combining network to enhance its average efficiency at deep power back-off (DPBO). It comprises a signed second-order hold (SOH) interpolation filter to suppress the sampling spectral replicas significantly. The proposed DTX is realized in a 40-nm bulk CMOS and delivers a peak output power of 27.54 dBm with drain and system efficiencies of 46.35% and 30.77%, respectively, at 5.3 GHz. At 12 dB DPBO, the realized DTX demonstrates a drain efficiency (DE) of 41.74%–39.27% in a 5.2–5.5 GHz band, respectively. Its intrinsic $I/Q$ image, LO leakage, and C-IMD3/ $H_{{\mathrm {3BB}}}$ for a 200 MHz tone spacing over a 4.8–6.2 GHz band are −64, −65, and −69 dBc, respectively, without calibration. Applying a simple memoryless $2\,\,\times \,\,1$ -D digital pre-distortion, its error vector magnitude and adjacent channel leakage ratio are lower than −31 dB and −39 dBc, respectively, for a six-carrier “40 MHz 256-QAM OFDM” signal with 18 dBm average output power and a 41% average DE. The signed SOH functionality is verified employing a four-carrier “80 MHz 512-QAM OFDM” signal with spectral purity of better than −35 dBc, while its baseband sampling frequency is 675 MHz. [ABSTRACT FROM AUTHOR]