Analysis of a 5.5-V Class-D Stage Used in +30-dBm Outphasing RF PAs in 130- and 65-nm CMOS

This paper presents the design and analysis of a 5.5 V Class-D stage used in two fully integrated watt-level, +32.0 dBm and +29.7 dBm, outphasing RF Power Amplifiers (PA) in standard 130 nm and 65 nm CMOS technologies. The Class-D stage utilizes a cascode configuration, driven by an AC-coupled low-voltage driver, to allow a 5.5 V supply in the 1.2/2.5 V technologies without excessive device voltage stress. RMS electric fields (E) across the gate oxides and the optimal bias point, where the voltage stress is equally divided between the transistors, are computed. At the optimal bias point, the RMS E, the power dissipation of the parasitic drain capacitance of the common-source transistors, and the equivalent on-resistances are reduced by approximately 25 %, 50 %, and 25 %, compared to a conventional cascode (inverter) stage. To the authors’ best knowledge, the Class-D PAs presented are among the first fully integrated CMOS outphasing PAs reaching +30 dBm and demonstrate state-of-the-art output power and bandwidth.