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Envelope tracking provides the potential for achieving high efficiency in power amplifiers for next generation wireless systems with high peak-to-average ratio signals such as LTE. Envelope modulators with low cost, high efficiency and wide bandwidth are critical enablers for the widespread application of ET. This presentation reviews the development of various Si ICs for ET applications in basestation PAs and in handset PAs. Requirements of voltage swing, bandwidth, and accuracy are first described. BCD technology-based Si ICs for envelope modulators achieving voltages as high as 50V are presented, for operation in basestations with LDMOS and GaN RF power transistors. CMOS-based Si envelope modulator ICs for operation in wireless handsets are also discussed. ET amplifiers that achieve overall efficiency as high as 45% in 20MHz LTE handset applications are presented.