Radio Frequency and Microwave Power Amplifiers. Volume 2: Efficiency and Linearity Enhancement Techniques
Radio Frequency and Microwave Power Amplifiers are finding an increasingly broad range of applications, particularly in communications and broadcasting, but also in the industrial, medical, automotive, aviation, military, and sensing fields. Each application has its own design specifications, for example, high linearity in modern communication systems or high efficiency in broadcasting, and, depending on process technology, capability to operate efficiently at very high frequencies, such as 77 GHz and higher for automotive radars. Advances in design methodologies have practical applications in improving gain, power output, bandwidth, power efficiency, linearity, input and output impedance matching, and heat dissipation. This essential reference presented in two volumes aims to provide comprehensive, state-of-the-art coverage of RF and microwave power amplifier design with in-depth descriptions of current and potential future approaches. Volume 1 covers principles, device modeling and matching networks, while volume 2 focuses specifically on efficiency and linearity enhancement techniques. The volumes will be of particular interest to engineers and researchers engaged in RF and microwave amplifier design, and those who are interested in systems incorporating RF and microwave amplifiers.
Inspec keywords: integrated circuit design; energy conservation; MMIC power amplifiers; power combiners; CMOS integrated circuits; distributed amplifiers; active networks
Other keywords: high-efficiency power amplifier design; efficiency enhancement techniques; outphasing power amplifiers; combiner synthesis; multiband power amplifier linearization; Doherty power amplifiers; linearity enhancement techniques; distributed power amplifiers; envelope tracking techniques; multichannel power amplifier linearization; behavioral linearization; behavioral modeling; CMOS power amplifiers; active load-modulation-based power amplifiers; radio frequency power amplifiers; nonlinear embedding models; microwave power amplifiers
Subjects: Education and training; CMOS integrated circuits; Active filters and other active networks; Microwave integrated circuits; Waveguide and microwave transmission line components; Amplifiers; Semiconductor integrated circuit design, layout, modelling and testing
- Book DOI: 10.1049/PBCS071G
- Chapter DOI: 10.1049/PBCS071G
- ISBN: 9781839530388
- e-ISBN: 9781839530395
- Page count: 500
- Format: PDF
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Front Matter
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1 High-efficiency power amplifier design
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High efficiency of the power amplifier can be obtained by using Class-F and Class-E operation modes or their different approximations, depending on the technical requirements. In all cases, an efficiency improvement in practical implementation is achieved by providing the nonlinear operation conditions when an active device can operate in pinch-off and saturation regions during most of the period, resulting in the nonsinusoidal collector current and voltage waveforms, symmetrical for Class-F and asymmetrical for Class-E operation modes. In Class-F power amplifiers analyzed in frequency domain, the fundamental-frequency and harmonic load impedances are optimized by short-circuit termination and open-circuit peaking to control the voltage and current waveforms at the device output to obtain maximum efficiency. In Class-E power amplifiers analyzed in time domain, an efficiency improvement is achieved by realizing the on/off active device switching operation (saturation and pinch-off regions) with special current and voltage waveforms so that high voltage and high current do not concur at the same time.
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2 High-efficiency Doherty power amplifiers
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This chapter discusses the power amplifier technique for amplitude-modulated (AM) radio-frequency signals was invented by William H. Doherty in broadcasting in the mid-1930s as a more efficient alternative to both conventional amplitude-modulation techniques and Chireix outphasing [4]. The amplifier was configured in a grounded-cathode structure where two vacuum tubes were connected in parallel, one as a Class B carrier tube and the other as a Class C peaking tube, and the tubes were split and combined through þ90° and -90° phase shifting networks. This novel power amplifier architecture removed limitation of low efficiency inherent in a conventional power amplifier circuit, permitting efficiencies of 60% to 65% to be realized, while retaining the principal advantages associated with low-level modulations systems and linear power amplifiers.
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3 Envelope tracking techniques
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The growing demand for high data rate cellular systems has pushed the adoption of 4G/5G Long-Term Evolution (LTE) which uses high peak-to-average power (PAPR) modulation schemes. 5G will bring high data capacity and low latency using the sub-6 GHz and mm-wave spectrum. The increased adoption of powerful worldwide smart phones has been in part possible due to increase of CMOS technology in lower feature nodes as FinFET 7 nm/14 nm. This has made possible the adoption of “old” techniques such as envelope tracking (ET) into smart phones. The ET technique was mentioned back in 1952 by Leonard R. Kahn who introduced a related technique, envelope elimination and restoration (EER). Envelope tracking improves the linearity and efficiency RF for power amplifiers, typically the most power-hungry components in a wireless transmitter. With explosive band proliferation as well as the use of carrier aggregation (CA) and multiple input multiple output (MIMO) techniques, the research area of improving the cost, size, and the performance of the RF transmit solution is very active.
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4 Outphasing power amplifiers
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This chapter presents a comprehensive introduction to the outphasing architecture, with particular focus on nonisolating outphasing techniques. When efficiency at output power back-off is required, the outphasing technique presents a significant advantage over conventional single-ended PAs. This performance benefit comes, as is typical in efficiency enhancement architectures, at the expense of complexity, linearity, and bandwidth. Nonetheless, when digital predistortion and the associated complexity can be tolerated, outphasing is clearly beneficial for systems using high peak-to-average power ratio signals. A direct comparison to other efficiency enhancement techniques is made complicated by the large trade-off space in efficiency, linearity, complexity, and so on. It is of interest, however, to consider a comparison to the popular Doherty PA (DPA). The main difference between these two techniques lies in how the branch PAs are operated. In outphasing systems, the branch PAs are driven identically, with both branch PAs operating in saturation in the outphasing regime and in back off at low output power levels. In Doherty PAs, on the other hand, in the higherpower regime only the main PA is operated at full voltage swing, while in back-off the auxiliary PA is completely off. As a result, at high output powers the outphasing PA has theoretically higher achievable efficiency, and better utilization of the device periphery at low power levels.
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5 Combiner synthesis for active load-modulationbased power amplifiers
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This chapter discusses the fundamental operation of Doherty and outphasing Pas. Rather than using the original combiner topologies as a starting point, our focus will be on the desired transistor operation. Based on the active loadmodulation conditions that the Doherty and outphasing Pas dictate, we will therefore instead derive the combiners that present the desired behavior. The original solutions will thus appear as special cases when ideal operation is assumed. For any other case, new solutions will appear. We will show how this approach can give both new insights, extended design spaces and improved performance. In the first part of this chapter, we will establish a theoretical framework for Doherty and outphasing combiner synthesis from the conditions that guarantee efficient operation. This will thereafter be applied and practically demonstrated, first in a Doherty design context and then in an outphasing design context. The chapter will be concluded with a brief summary and suggestions for future work.
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6 Power amplifier design based on nonlinear embedding models with design examples
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The chapter begins by discussing the motivations for starting power amplifier (PA) design at the current-source reference planes. Next, the technique of nonlinear embedding which bypasses the need for fundamental and harmonic source and loadpull simulations is considered. Simulation and measurement results for Class-B are presented together with the concept of optimal harmonic injection. The Chapter goes on to analyze Class-F, and design examples are provided. Applications to Doherty and Chireix Pas are then presented, and the Chapter concludes by presenting the designs of broadband Pas with the continuous Classes B/J and F.
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7 CMOS power amplifiers
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Accurate MOSFET device modeling is an extremely important procedure to develop low-cost silicon integrated circuits using CMOS technology for higher speed and higher frequency integrated circuits and subsystems within shorter design time. The accuracy of MOSFET models to describe their nonlinear behavior is crucial to predict the entire circuit performance. The lumped-parameter equation-based equivalent circuit model provides a clearer physical description and more design flexibility. The cross section of the MOSFET and substrate coupling networks are shown in Figure 7.1(a). Here, the considerable ohmic loss caused by the semiconductive silicon substrate is characterized by the resistances Rb1, Rb2, and Rsub. The junction capacitance corresponding to source-drain region is represented by the capacitances Cjs and Cjd. The capacitance Cgb represents the gate-to-substrate coupling capacitance consisting of the gate oxide capacitance and the depletion layer capacitance.
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8 Behavioral modeling and linearization
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This chapter describes the linearization of power amplifiers (PAs) used in radio frequency (RF) transmitters. An overview of wireless communications is provided to establish the context of how the transmitter is used and the importance of the PA. Measures and models of the PA nonlinearity are reviewed, after which the core topic of PA linearization is presented. Selected techniques for PA linearization are discussed. The most popular is digital predistortion (DPD), where the correction signal is applied in the digital domain prior to the PA. Also covered are adaptive methods used to estimate the DPD coefficients that optimize the system. Feedforward compensation is presented as an analog technique for linearization, where the correction is applied in the RF domain after the PA. The chapter is concluded with a section on more detailed discussions of selected topics. Topics include the use of both odd and even polynomial terms in the modeling of PA nonlinearity and predistortion, the effect of PA saturation on the estimation of DPD coefficients, modifications used to make descent-based estimators more robust, batch processing for coefficient estimation, and the recursive least squares (LS) (RLS) estimator.
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9 Multiband/multichannel power amplifier linearization
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This chapter addresses the practical problem of signal distortions due to hardware imperfections such as power amplifier (PA) nonlinearity, modulator imperfection, cross-modulation and cross-over interference due to the multiple-frequency (multiband) and multiple-input/output transmission schemes proposed for 4G/5G communication. Basically this chapter targets two problems whose system level description have a certain similarity, so that they can be tackled in a similar digital predistortion system (DPD); however, digital level algorithms and implementation challenges will differ from each other.
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10 Distributed power amplifiers
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The potential of traveling-wave or distributed amplification for obtaining power gains over very wide frequency bands has been recognized yet in the mid-1930s when it was found that the gain-bandwidth performance is greatly affected by the capacitance and transconductance of the conventional vacuum tube [1]. However, the first theoretical analysis and its practical verification were obtained for very broadband vacuum-tube amplifiers more than a decade later [2,3]. The basic concept was based on the idea to combine the interelectrode capacitances of the amplifying vacuum tubes with series wire inductors to form two lumped-element artificial transmission lines coupled by the tube transconductances. As a result, the distributed amplifier overcomes the difficulty of a conventional amplifier, whose frequency limit is determined by the factor which is proportional to the ratio of the transconductance of the tube to the square root of the product of its input grid- cathode and output anode-cathode capacitances, by paralleling the tubes in a special way, in which the capacitances of the tubes can be separated, while the transconductances may be added almost without limit and not affect the input and output of the device. Since the grid-cathode and anode-cathode capacitances form part of low-pass filters which can be made to have a substantially uniform response up to filter cutoff frequencies, whose value can be conveniently set within a wide range by suitable choice of the values of the external inductor coils, it became possible to provide amplification over much wider bandwidths than was achievable with conventional amplifiers.
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Back Matter
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