Analytical Approaches to Load Modulation Power Amplifier Design

In future mobile communication networks, there will be a shift toward higher carrier frequencies and highly integrated multiple antenna systems. The system performance will largely depend on the available radio frequency (RF) hardware. As such, RF power amplifiers (PAs) with improved performance, e.g. energy efficiency, are needed. Active load modulation (ALM) is one of the most common PA efficiency enhancement techniques. Unfortunately, different ALM techniques come at the cost of degrading other PA attributes. Through investigation of new ALM design techniques, the overall objective of this thesis is to improve upon different attributes and performance trade-offs in ALM PAs for future wireless systems.\ua0The working principle of ALM PAs is determined by both how the individual transistors are operated and how their outputs are combined. In the first part of the thesis, an analytical approach, where the output combiner is assumed to be an arbitrary black-box, is applied to the Doherty PA. The fundamental interaction between the main and auxiliary transistors is analyzed and generalized. New solutions with improved performance are identified, such as higher gain and an improved efficiency-linearity trade-off. This approach also introduces improved integration possibilities, which are demonstrated by a transmitter where the antenna acts as both the radiator and the Doherty combiner. Additionally, the analytical approach is applied to an isolated two-way power divider. This unlocks many new possibilities, such as improved integration and layout flexibility. \ua0In the second part, one embodiment of the emerging ALM architecture, the load modulated balanced amplifier (LMBA), is proposed: the RF-input Doherty-like LMBA. Design equations are derived and the fundamental operation is studied. This variant presents several advantages over other known architectures, such as higher gain and device periphery scaling of the different transistors.\ua0The third part proposes a new measurement-based ALM PA design procedure, which emulates the full behavior of the transistors in any ALM architecture using active load-pull measurements. This method can predict the intricate behavior in ALM PAs and it gives measurement-based insights into the internal operation of the circuit already at the design stage. This facilitates the design for optimal ALM PA performance. \ua0The thesis contributes with several promising techniques for reducing performance trade-offs and improving the overall performance of ALM PAs. Therefore, the results will contribute to the development of more energy efficient and high capacity wireless services in the future

Theory and Design of Efficient Active Load Modulation Power Amplifiers

The increasing demand for mobile data traffic has put new challenges and requirements for the development of the wireless communication infrastructure. The performance of the RF power amplifier (PA) is, in particular, of great importance, since it is the key building block for microwave transmitters in base stations and radio link equipment. The energy and bandwidth efficiency of the PA is vital for maximized channel capacity, reduced operational cost, and further integration. Among the efficiency enhancement techniques, active load modulation is one of the most widely used techniques. The overall objective of this thesis is to improve the average efficiency and bandwidth performance in active load modulation PAs for future wireless systems. In the first part of the thesis, an analytically based combiner synthesis approach for the three-stage Doherty PA (DPA) is proposed and presented. A compact output combiner network, together with the input phase delays, is derived directly from transistor load-pull data and the PA design requirements. The technique opens up new design space for three-stage DPAs with reconfigurable high-efficiency power back-off levels. The utility of the proposed technique is demonstrated by the implementation of a 30-W gallium nitride (GaN) three-stage DPA prototype at a center frequency of 2.14 GHz. Measurement results show that the prototype circuit can linearly reproduce 20-MHz long-term evolution signals with 8.5- and 11.5-dB peak-to-average power-ratio (PAPR), providing average efficiencies of 56.6% and 46.8% at an average output power level of 36.8 and 33.8 dBm, respectively. In the second part of the thesis, a novel PA architecture, the circulator load modulated amplifier (CLMA) is proposed and demonstrated. The CLMA is able to maintain high efficiency over large output power dynamic ranges. Moreover, the availability of wideband and low-loss circulators makes this architecture promising for wideband applications. Consequently, it has the potential to overcome many of the drawbacks of other architectures. The fundamental operational principle and theoretical performance of the CLMA are studied and presented. As a proof of concept, a demonstrator circuit based on GaN transistors is designed and characterized at 2.09 GHz. Measurement results show that the peak output power is 43.1 dBm and the drain efficiency is 73\% at 6-dB output power back-off level. In summary, the thesis presents two promising PA architectures for efficiency enhancement. The results of this thesis will therefore contribute to the development of energy efficient PAs for future mobile communication systems

Efficient and Wideband Load Modulated Power Amplifiers for Wireless Communication

The increasing demand for mobile data traffic has resulted in new challenges and requirements for the development of the wireless communication infrastructure. With the transition to higher frequencies and multi-antenna systems, radio frequency (RF) hardware performance, especially the power amplifier (PA), becomes increasingly important. Enhancing PA energy efficiency and bandwidth is vital for maximizing channel capacity, reducing operational costs, and facilitating integration.In the first part of the thesis, the bandwidth limitations of the standard two-way Doherty PA are discussed. A comprehensive analysis is provided, and the frequency responses of different Doherty combiner networks are presented. Furthermore, a Doherty combiner network is proposed, notable for its inherent broadband frequency and its capacity to account for the influence of output parasitics and packaged components from the active devices. The introduced Doherty combiner network is experimentally verified by a wideband gallium nitride (GaN) Doherty PA operating over 1.6-2.7 GHz.In the second part of the thesis, an analytically based combiner synthesis approach for the three-stage Doherty PA is proposed and presented. A compact output combiner network, together with the input phase delays, is derived directly from transistor load-pull data and the PA design requirements. The technique opens up new design space for three-stage Doherty PAs with reconfigurable high-efficiency power back-off levels. The utility of the proposed technique is demonstrated by the implementation of a 30-W GaN three-stage Doherty PA prototype at 2.14 GHz. Measurements show that a drain efficiency of 68% and 55% is exhibited at 6- and 10-dB back-off power, respectively.In the third part, a new PA architecture named the circulator load modulated amplifier (CLMA), is proposed. This architecture utilizes active load modulation for achieving enhanced back-off efficiency. Two active devices are incorporated in this innovative architecture, and a non-reciprocal circulator-based combiner is leveraged. Following this, the sequential CLMA (SCLMA) is introduced, characterized by its ability to enhance back-off efficiency without the necessity of load modulation. GaN demonstrator circuits for both CLMA and SCLMA architectures, whether with dual-input or RF single-input, are designed and fabricated, with excellent performance being measured.\ua0The thesis contributes novel design techniques and architectures to enhance PA efficiency and bandwidth. These findings pave the way for energy-efficient and adaptable RF transmitters in future wireless communication systems

Linear Operation of Switch-Mode Outphasing Power Amplifiers

Radio transceivers are playing an increasingly important role in modern society. The ”connected” lifestyle has been enabled by modern wireless communications. The demand that has been placed on current wireless and cellular infrastructure requires increased spectral efficiency however this has come at the cost of power efficiency. This work investigates methods of improving wireless transceiver efficiency by enabling more efficient power amplifier architectures, specifically examining the role of switch-mode power amplifiers in macro cell scenarios. Our research focuses on the mechanisms within outphasing power amplifiers which prevent linear amplification. From the analysis it was clear that high power non-linear effects are correctable with currently available techniques however non-linear effects around the zero crossing point are not. As a result signal processing techniques for suppressing and avoiding non-linear operation in low power regions are explored. A novel method of digital pre-distortion is presented, and conventional techniques for linearisation are adapted for the particular needs of the outphasing power amplifier. More unconventional signal processing techniques are presented to aid linearisation of the outphasing power amplifier, both zero crossing and bandwidth expansion reduction methods are designed to avoid operation in nonlinear regions of the amplifiers. In combination with digital pre-distortion the techniques will improve linearisation efforts on outphasing systems with dynamic range and bandwidth constraints respectively. Our collaboration with NXP provided access to a digital outphasing power amplifier, enabling empirical analysis of non-linear behaviour and comparative analysis of behavioural modelling and linearisation efforts. The collaboration resulted in a bench mark for linear wideband operation of a digital outphasing power amplifier. The complimentary linearisation techniques, bandwidth expansion reduction and zero crossing reduction have been evaluated in both simulated and practical outphasing test benches. Initial results are promising and indicate that the benefits they provide are not limited to the outphasing amplifier architecture alone. Overall this thesis presents innovative analysis of the distortion mechanisms of the outphasing power amplifier, highlighting the sensitivity of the system to environmental effects. Practical and novel linearisation techniques are presented, with a focus on enabling wide band operation for modern communications standards

Device level characterization of outphasing amplifiers

The outphasing technique proposed by Chireix in 1935 is one of the classical methods of addressing power amplifier (PA) efficiency degradation caused by operating in output back-off (OBO) conditions, where PA efficiency is typically low. Essentially, the envelope from the input signal is eliminated, and two CW signals are constructed; these have constant amplitude, while their relative phase offset holds the original information contained by amplitude modulation. Consequently, efficiency improvements are achieved by amplifying signals with constant amplitude using PAs operating in saturation, where efficiency typically peaks. The envelope is restored at the output by means of a vector summation of both signals, using a non-isolating combiner at the output stage The main focus of the work described in this thesis was placed on extending bandwidth of the inherently narrowband technique of outphasing and then adopting this method to modern telecommunication standards. Two prototype PAs were designed to investigate whether bandwidth improvements can be achieved by adopting a broadband balun as a combining structure in the outphasing PA. Two baluns were designed and fabricated to be used in the demonstrator circuits; one using a section of semirigid coaxial cable and the other, a planar balun realized on 10 mil thick Alumina substrate. A novel method of fabrication was proposed for the former structure, which achieved more than double octave bandwidth, from 1.25 GHz to 4.7 GHz with losses lower than 1dB, an amplitude imbalance (trace separation) below 0.75 dB and phase imbalance within ±5 degrees. The measured CW performance of the prototype circuits produced results comparable with the state-of-the-art solutions available in literature. Moreover, this work demonstrated that a balun with sufficient bandwidth allows load modulation to be prescribed at fundamental and second harmonic frequencies, opening the possibility of waveform engineering to implement continuous PA modes such as class J in outphasing PAs. The desired harmonic load termination was achieved without any specialized matching networks, and solely by means of load modulation provided through active device interaction. The thesis concludes with the formulation, analysis and description of the novel concept derived from Chireix outphasing. Several outdated assumptions still prevalent in outphasing analysis included in literature today are challenged and reformulated for modern semiconductor devices such as GaN HEMTs. Through this process, a new concept of Current Mode Outphasing (CMOP), is proposed and described in detail. One of the significant advantages of the proposed approach is it allows the elimination of the combiner structure, which typically dominates the size of the final outphasing circuit, due to the presence of λ/4 transmission lines. Consequently, the demonstrator MMIC circuit, containing DC bias, stability elements and pre-matched to 50 Ω on input and output, has been deployed on an area of 2.3 mm x 2.8 mm. The CMOP circuit was fabricated using 0.25 µm GaN technology and achieved a bandwidth of 1.6 GHz centered at 3.35 GHz, whereas the maximum CW output power remains within 43 dBm ± 0.5 dB. A total gain of more than 12 dB is reported from 2.95 to 3.95 GHz, while a maximum Power Added Efficiency was measured as 68.5% at 3.25 GHz and remains greater than 60% from 2.85 to 3.8 GHz, and above 50% for almost the entire frequency range. The output back-off (OBO) efficiency peaks at 3.25 GHz with 53.5% and 45.6% for 6 dB and 8 dB back-off, respectively, and remains above 30% and 23.7% for the entire frequency range. To the best of the authors’ knowledge, this is the largest fractional bandwidth achieved in an outphasing PA, that has been reported in literature

Digital Radio Encoding and Power Amplifier Design for Multimode and Multiband Wireless Communications

The evolution of wireless technology has necessitated the support of multiple communication standards by mobile devices. At present, multiple chipsets/radios operating at predefined sets of modulation schemes, frequency bands, bandwidths and output power levels are used to achieve this objective. This leads to higher component counts, increased cost and limits the capacity to cope with future communication standards. In order to tackle different wireless standards using a single chipset, digital circuits have been increasingly deployed in radios and demonstrated re-configurability in different modulation schemes (multimode) and frequency bands (multiband). Despite efforts and progress made in digitizing the entire radio, the power amplifier (PA) is still designed using an conventional approach and has become the bottleneck in digital transmitters, in terms of low average power efficiency, poor compatibility with modern CMOS technology and limited re-configurability. This research addresses these issues from two aspects. The first half of the thesis investigates signal encoding issues between the modulator and PA. We propose, analyze and evaluate a new hybrid amplitude/time signal encoding scheme that significantly improves the coding efficiency and dynamic range of a digitally modulated power amplifier (DMPA) without significantly increasing design complexity. The proposed hybrid amplitude/time encoding scheme combines both the amplitude domain and the time domain to optimally encode information. Experimental results show that hybrid amplitude/time encoding results in a 35% increase in the average coding efficiency with respect to conventional time encoding, and is only 6.7% lower than peak efficiency when applied to a Wireless Local Area Network (WLAN) signal with a peak to average power ratio equal to 9.9 dB. A new DMPA architecture, based on the proposed hybrid encoding, is also proposed. The second half of this thesis presents the design, analysis and implementation of a CMOS PA that is amenable to the proposed hybrid encoding scheme. A multi-way current mode class-D PA architecture has been proposed and realized in 130 nm CMOS technology. The designed PA has satisfied the objectives of wide bandwidth (1.5 GHz - 2.7 GHz at 1 dB output power), and high efficiency (PAE 63%) in addition to demonstrating linear responses using the proposed digital encoding. A complete digital transmitter combining the encoder and the multi-way PA was also investigated. The overall efficiency is 27% modulating 7.3 dB peak to average power ratio QAM signals

Multikilowatt transmitter study for space communications satellites, volume 2

Multikilowatt transmitter study for space communications satellites - amplifier design

Advanced Doherty power amplifier design for modern communication systems

Mobile communication technologies are becoming increasingly sophisticated and have experienced rapid evolution over the last few decades, and this is especially true for the base station transmitter. In response to the ever increasing demand in communication traffic and data throughput, largely driven by video based social media platforms, both spectral and power efficient device and systems are needed to fulfil the requirements. In terms of energy consumption, the power amplifier is an important component, and although developing efficient technologies for handset equipment is important, it is the base station element of the communications system that poses the greater challenge, having to deal with many channels simultaneously, resulting in the need to linearly and efficiently amplify highly dynamic phase and amplitude modulated signals possessing very large peak-to-average power ratios, at high power levels. This unique set of challenges has led to continuous research to improve the efficiency of amplifiers that can accommodate such signals, and the Doherty architecture has now become the architecture-of-choice. However, most of the previous research studies demonstrate Doherty performance enhancement through a ‘conventional’ design approach that uses one input source and a passive power splitter to deliver power to each half of the Doherty structure. They do not emphasize the additional efficiency and other performance improvements that are possible in Doherty amplifiers when using two different, independent and phase coherent input sources, attached to the input path of both main and auxiliary amplifiers. IV The novel research work presented in this thesis introduces an optimised design approach for Doherty amplifier architectures with individual input sources, as well as detailing a measurement architecture that is necessary to characterise such structures, using separate, phase-coherent input sources in a realistic measurement scenario. Finally, following extensive characterisation of a number of promising architectures, investigations around efficiency enhancement are focused around the adaption of gate bias applied to the auxiliary amplifier device, and identifying, for the first time, what is possible by generating different shaping functions that relate bias voltage to the magnitude of the input signal. One completely new area of research and novelty introduced in this work for example shows how choosing the right shaping function can give improved linearity and importantly linearisability by producing a flat gain over dynamic range. Note that linearisability is important, and is defined here as the term used to describe the ease with which the non-linearities of a device or power amplifier can be corrected. It is often assumed in power amplifier design that efficiency and power are the most important parameters, and that modern digital pre-distortion (DPD) techniques can easily correct any non-linearity that may result. Industry is now finding that this is not the case however, and the type and nature of the non-linearity in terms to AM-AM and AM-PM distortion is very important in determining of the degree of linearization possible