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Efficiency degradation effects of power combining transformers with partially disabled inputs are quantitatively analyzed. To improve efficiencies in lower-power modes of a multi-mode class-AB power amplifier (PA), a discrete resizing technique is introduced in combination with a parallel-combining transformer (PCT). The two-stage PA implemented in a 0.18-μm CMOS technology also includes varactor-based tunable matching circuits. The design method involves parallel-combining of two power stages, each of which are divided into three sub-cells to facilitate discrete resizing. The parallel-combining of concurrently resized power cells minimizes undesired power loss through the transformer and helps the PA to utilize the transformer efficiency maximally independent of the number of combining cells. When operating in the high-power mode, the PA exhibits a peak output power of 31 dBm with a PAE of 34.8%. Power back-offs are realized by discretely turning off parallel sub-amplifier cells concurrently, achieving output power levels of 26 dBm and 22.3 dBm with respective PAE of 22.5% and 15%. The EVM has been measured with IEEE 802.11g WLAN and 802.16e WiMAX modulated signals in three operation modes. In the high-power mode, the PA dissipates 590 mA from a 3.3 V supply.