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In this paper, a four-level dynamic supply switching (DSS) modulator using two single inductor dual output DC-DC converters for improving the efficiency of CMOS power amplifiers (PAs) is presented. The proposed DSS modulator includes two single-inductor dual-output DC-DC converters and a supply switching circuit. Two single-inductor dual-output DC-DC converters generate four DC voltages. The supply switching circuit dynamically switches the four voltages according to the envelope of the modulated signal. The proposed four-level DSS PA operates in the 1.75 GHz band and is fabricated using the 0.18 μm CMOS process. For the LTE 16-QAM signal with a bandwidth of 5 MHz, the measured PAE of the DSS PA is 38.9 % at an average output power of 22 dBm, thereby satisfying the adjacent channel leakage power ratio (ACLR) of −30.0 dBc.