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In this paper, single-ended and differential class-E load transformation networks (LTNs) for wideband operation are investigated. For this purpose, a differential third parallel-tuned tank LTN and a parallel-circuit load LTN without suppressing tanks are proposed to fulfill the class-E wideband condition. The differential parallel-circuit load (DPCL), which considers the finite RF chokes, has higher output resistance, and because of the differential structure, which ensures an open circuit at even harmonic frequencies, it is able to cover a wide frequency range. Consequently, the DPCL is well suited for highly integrated monolithic designs, as well as wideband application. Based on this analysis, a wideband class-E switching-mode power amplifier in CMOS 90 nm using the DPCL is designed. By deliberately combining the LTN with an on-chip balun, a compact size of 1.2 mm2 is achieved. The circuit performance dependency on bond-wire length variation is analyzed and discussed. Measured results show a peak output power of 28.7 dBm, power-added efficiency (PAE) of 48.0%, and drain efficiency of 55.0% at 2.3 GHz. From 1.7 to 2.7 GHz, PAE is higher than 42% and output power is above 25 dBm.