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This paper presents the design insights for the implementation of a fully monolithic radio frequency (RF) power amplifier (PA) using both envelope-tracking (ET) and transistor resizing techniques for long-term evolution (LTE) applications. At the low output power region, some of the power cells in the PA can be disabled to further save power consumption, thus enhancing the efficiency from a traditional ET-PA. Our ET-PA system is first realized with a two-chip solution, consisting of a high voltage envelope modulator fabricated in a 0.35 μm Bipolar-CMOS-DMOS (BCD) technology, and a differential cascode PA in a 0.35 μm SiGe BiCMOS technology. This two-chip solution of the ET-PA is to showcase the effective efficiency enhancement of using the transistor resizing method. In the second design, a CMOS envelope modulator is integrated with the cascode PA on the same die in the 0.35 μm SiGe BiCMOS technology. Some insights are demonstrated regarding the optimization of the envelope modulator specific to our cascode PA for LTE broadband signals, where the finite bandwidth and the switching frequency of the envelope modulator are considered for achieving the minimal error-vector magnitude (EVM) and spurious noise. The fully monolithic BiCMOS ET-PA reaches the maximum linear output power (Pout) of 24 dBm and 23.4 dBm with overall power-added-efficiency (PAE) of 41% and 38% for the LTE 16QAM 5 MHz and 10 MHz signals at 1.9 GHz, respectively, without needing predistortion. At the low power mode of our ET-PA, an additional PAE enhancement of 4% is obtained at Pout of 16-20 dBm by disabling some of the PA power cells. Our fully monolithic ET-PA satisfies the LTE 16QAM linearity specs with high efficiency.