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CMOS Envelope Tracking (ET) amplifier and a RF SiGe BiCMOS power amplifier (PA) for mobile WiMAX applications is presented. Several versions of the ET amplifiers are designed and their power consumption and bandwidth considerations are discussed. Consequently, the entire polar TX system performances and design trade-offs are presented and compared against different ET amplifiers design. The IC occupies a total area of 1×1.4 mm2 and is being fabricated in a TSMC 0.35μm SiGe BiCMOS process. RF/Analog/Digital system co-simulation indicates that the overall ET-based polar TX system exhibits 30% Power-Added-Efficiency (PAE) and 4.6% Error Vector Magnitude (EVM) at 19dBm PA output power and passes the stringent output spectral mask for a 10 MHz mobile WiMAX 64QAM signal with -10dB Peak-to-Average-Power Ratio (PAR).