This paper presents highly-efficient RF polar transmitter (TX) systems that utilize the envelope-tacking (ET) technique with monolithic SiGe power amplifiers (PAs) for mobile WiMAX and 3GPP Long Term Evolution (LTE) applications. Monolithic single-ended cascode SiGe PA design capable of enhancing its power-added efficiency (PAE) is demonstrated. Four RF switches are adopted at the bases of the common-emitter transistors, which can be turned on/off in response to the desired output power. We found that self-biasing of the common-base device with ET can improve the output distortion of the cascode PA to output 18dBm and passing the stringent mobile WiMAX linearity specs with 30% PAE at 2.3GHz without predistortion. Furthermore, a differential cascode SiGe PA using the proposed ET-based polar TX system can improve the average output power to 21dBm with 33.6% PAE at 1.42 GHz, and it also passes the stringent LTE 16QAM linearity specs without needing predistortion.