A 38-41 GHz 27-dBm Direct-Conversion I/Q Transmitter in 0.12-μm GaN Technology with Enhanced Linearity for 4096-QAM Modulation

This paper presents a 38-41 GHz direct-conversion I/Q transmitter in 0.12-μm GaN technology that achieves high linearity and low error-vector-magnitude (EVM) for high-order 4096-QAM modulation. The proposed architecture integrates two high-swing active double-sideband (DSB) modulators, a local oscillation (LO) amplifier, and a two-stage RF power amplifier (PA). To minimize AM-AM and AM-PM distortions across wide power range, the DSB modulators employ reflection-type modulation with distortion-minimization biasing, together with a differential amplifier and PA featuring conduction-angle optimization. These techniques collectively suppress second-order and third-order distortions and improve mixer-core symmetry. The demonstrated transmitter delivers output powers of 27, 25, 22 dBm under QPSK, 64-QAM, 4096-QAM modulation schemes, respectively, corresponding to EVMs of 6%, 6%, and 1.4%. This work represents one of the first GaN zero-IF I/Q transmitters to simultaneously achieve high output power, wide bandwidth, and EVM performance suitable for emerging millimeter-wave high-order modulation transmitters.