High Efficiency Broadband Envelope Tracking Power Amplifiers

In order to meet the increasing demand for higher data rates while maximizing spectral efficiency, modern wireless communication systems transmit complex non-constant envelope modulation signals with high peak-to-average ratio (PAPR). As a result, conventional power amplifiers must be operated in back-off, leading to a significant efficiency reduction. Various power amplifier architectures (i.e. Doherty, outphasing, and envelope tracking) have been demonstrated to achieve high efficiency for these high PAPR signals. Unlike the other architectures, the envelope tracking power amplifier (ETPA) makes an excellent candidate for multi-band multi-frequency use, while maintaining high efficiency for high PAPR and under average power back-off, as it is fundamentally immune to changes in the frequency of the carrier; it depends only on the envelope of the RF signal. This dissertation focuses on the design of envelope tracking power amplifiers for enabling broadband wireless communication systems. First, a test-bench for evaluating broadband ETPAs is described. A calibration routine, acting as a pre-equalizer, is used to achieve a flat linear response over the entire instantaneous bandwidth of the system, resulting in less than 1% EVM for a 40 MHz LTE signal. Performances of envelope tracking power amplifiers on this test-bench at various frequencies from UHF to millimeter-wave are evaluated, and record efficiencies are demonstrated. Next, this dissertation describes how in under practical usage, the power transmitted fluctuates as a function of the load demands over time. The long-term efficiency of the ETPA is evaluated using Monte-Carlo simulations based on a projected time-varying power profile. Compared to a Doherty PA with the same peak efficiency, the ETPA can provide more than 1.4x reduction in overall energy consumed. The ETPA thus provides significant opportunities for system energy savings under realistic operation. Thirdly, to accommodate the wide range of carrier frequencies required for numerous emerging wireless systems, a multi-octave RFPA based on a compact GaN stacked IC with RC feedback is designed, fabricated, and evaluated. Multi-octave ET operation was demonstrated from 500 to 1750 MHz with >25% efficiency. Compared to its constant drain voltage counterpart, >2x improvement in RFPA efficiency is observed in ET. Lastly, while the ETPA provides advantages such as broad carrier bandwidths and high efficiency under back-off operation, one of the main challenges in ETPA design lies in accommodating wide modulation bandwidths. Adaptive de-troughing, a digital signal processing approach for extending the modulation bandwidth capability of an existing dynamic power supply, is described and evaluated. Measurements demonstrated the ability to extend ET operation to 20 MHz LTE signals. In addition, ~5-6% modulator efficiency enhancement was measured when comparing "standard" to "adaptive" de-troughing supply waveforms.