Bitstream Switched-Mode Transmitters for High-Efficiency Power Amplifiers
- Author(s): Song, Yonghoon
- Advisor(s): Wang, Yuanxun E
- et al.
Switched-mode RF power amplifiers (PAs) under bitstream modulations such as bandpass or envelope delta-sigma modulations have received increasing attentions over the past years. They have been proposed to overcome a trade-off between linearity and efficiency with an idea to discretize analog envelope from a complex modulation signal into discrete driving points for high efficiency operation. Such amplifiers offer enhanced efficiency under power back-off conditions with pulse train waveforms yet maintain linearity in amplification through digital coding and modulations. The quantization noise generated during a discretization process, however, is a challenge to be rejected properly and restore desired signal.
This work presents a switched-mode PA based on the pulsed load modulation (PLM) technique with multi-level envelope delta-sigma modulation (EDSM) to realize broadband efficient amplification of non-constant-envelope signals. PLM technique modulated load impedance to be optimized for dynamic power range and maintained optimum efficiency over 6 dB back-off. Multi-level EDSM introduced additional transition levels and reduced quantization noise power with improved signal to noise ratio.
In addition, active noise filtering scheme is proposed with a potential to replace an external high-Q passive bandpass filter in delta-sigma modulation. Multiple PA units were driven by identical signal with different phase. Challenged quantization noise power was suppressed by multi-path combining filtering.
The proposed transmitters were demonstrated at X-band using GaN devices. Both 2-level and 3-level EDSM techniques were tested and compared. Two channel active noise filtering is also presented with regard to their quantization noise levels and power efficiencies.