Advanced Design of High Efficiency Microwave Power Amplifiers for Wireless Applications

To accommodate the increasing demand for wireless communication, the cellular system underwent rapid development in the past decades. As the most power-consuming and expensive unit in RF frontend, the power amplifier (PA) is crucial to the overall performance of the system. However, to provide higher data rate and massive system capacity, advanced modulation schemes (e.g., OFMD and higher-order QAM etc.) have been widely employed, which renders the transmitting signal with large peak-to-average power ratio (PAPR). When traditional high efficiency PA (e.g., class-B/-F etc.) is adopted, the transmitter suffers from seriously degraded efficiency since the PA operates at large back-off power. For efficient amplification, Doherty PA (DPA) has been regarded as an ideal candidate due to its distinctive high efficiency profile over a wide dynamic range (i.e., from back-off to saturation power). However, conventional DPA suffers from limited output back-off (OBO) range, narrow bandwidth (BW) and complicated/bulky circuit designs. Therefore, advanced design methodologies offering extended dynamic range, higher power-efficiency, compact size, wideband and multiband operations are highly desirable for modern wireless applications.To begin with, a modified DPA structure is proposed providing extended BW and high power utilization factor (PUF). The impact of the excessive phase dispersion of the matching networks is analytically revealed based on frequency-dispersive DPA model. For dispersion compensation, an optimized adaptive combining load is introduced to enhance the achievable BW (~ 60%) and PUF (> 0.95) of DPA. Experimental prototype based on GaN HEMT devices has been designed, fabricated, and measured, which demonstrates good Doherty operation over an extended frequency range (1.4-2.55 GHz) with excellent efficiency, high PUF and simple circuit structure.Next, a new and generalized design methodology for dual-band DPA with extended dynamic range is presented. A novel output combiner is proposed for the provision of OBO-extended load modulation at two widely spaced frequency bands. For verification, a 9-dB-OBO DPA covering 1.4-/3.5-GHz operating bands is designed and fabricated. Excellent Doherty efficiency with back-off (9-dB) efficiency of over 55% and saturation efficiency of over 68% was achieved at both bands. Under 9-dB-PAPR modulated signal excitation, average efficiency of 52.1% and 51.3% is achieved at 1.4 and 3.5 GHz, respectively.Finally, for boosted power capability and dynamic range, a peak-current-ratio enhanced symmetrical DPA is proposed by using active harmonic control (AHC) technique. It is demonstrated that the inherent harmonic current injection from the two active devices (main and auxiliary) can be properly manipulated to enhance the auxiliary current/power capability. The enhanced peak current ratio results in improved load modulation and saturation power capability, approaching idealized class-B/-B DPA operation, which can deliver 25% more saturation power than conventional class-B/-C designs. The experimental prototype exhibits remarkable power-efficiency performance with state-of-the-art OBO range of 11 dB and uplifted PUF of 1.25.為了適應快速增長的無線通信需求，蜂窩移動通信技術在最近的幾十年中經歷了快速的發展。作為無線收發機系統中功耗最大最昂貴的器件，功率放大器（PA）的設計對於系統的整體性能和成本有著決定性的影響。然而為了提供更快的數據速度和更大的系統容量，現代無線系統往往采用了覆雜的調制方式（比如正交頻分多路覆用技術OFDM、高階正交幅度調制QAM），這導致了傳輸信號具有很高的峰均功率比（PAPR），當采用傳統的PA時，由於其工作在較大的回退功率狀態（OBO），系統的平均效率往往大大降低，進而引起能耗增加、系統發熱過高等問題。相比傳統PA，Doherty功率放大器（DPA）能夠在較大的功率範圍內維持高效率的工作，以此DPA被認為是一個理想的高效率PA方案。然而傳統DPA存在著動態範圍（高效率功率範圍）較小，工作帶寬過窄，實現電路覆雜等問題。因此，針對寬頻帶、雙頻帶工作、延展高效率區、提高功率利用度、小型化設計等技術點提出了多個創新性的設計方案。本文第一個工作介紹了一種改進型的高功率利用度的寬帶DPA設計。理論分析發現，傳統的寬帶設計中匹配網絡的相位色散特性會對PA的帶寬和性能有很大的影響。為了補償這一影響，我們提出了一種重新優化的自適應負載方案，在拓寬工作帶寬（~60%）的同時保持了較高的功率利用度（>0.95）。為了驗證理論，我們設計了一款基於GaN工藝晶體管的DPA，測試結果發現，在極寬的頻率範圍（1.4-2.55GHz）內，DPA均能保持高效率、高功率利用度的工作。在第二個工作中，我們創新性地提出了一種基於對稱架構的、寬動態範圍的雙頻DPA設計方案。我們首先提出了能夠拓寬對稱式DPA動態範圍的廣義網絡模型，並且利用該模型的多解特性，我們開發了一種支持任意雙頻工作的電路結構。為驗證理論，我們設計了一款工作在1.4 GHz和3.5 GHz的DPA，測試結果表明，該設計能夠在9-dB功率回退下保持高效率（> 55%）工作，同時飽和功率下的效率也超過了68%。在PAPR為9dB的調制信號測試下，在低頻和高頻段的平均效率分別達到52.1%和51.3%。在最後一個工作中，我們提出了一種功率增強型的DPA設計，針對丙類工作模式下副管輸出功率和電流不足的問題，我們創新性的提出有源諧波調制技術。通過利用兩個晶體管自身的諧波電流，將其合理操控，能夠有效地提升副管的電流和功率容量。在峰值電流比得到提升的情況下，DPA的負載調制、飽和輸出功率幾乎趨近於理想的"乙類/乙類"架構，相比較傳統的"乙類/丙類"架構，該增強型的設計能夠多輸出25%的飽和功率。實驗結果也成功驗證該設計能獲得高達1.25的功率利用度，並且其動態範圍也進一步拓寬至11dB的領先水平。LIU, Haoyu.Ph.D. Chinese University of Hong Kong 2021.Includes bibliographical references (leaves )Abstracts in English and Chinese.Title from PDF title page (viewed on ...