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This article presents a quasi-load insensitive (QLI) Doherty power amplifier (DPA). The proposed theory makes the amplifier load insensitive in terms of output power, while its efficiency is slightly degraded for complex loads. The load insensitiveness is achieved by dynamically changing the supply voltages ( ${V_{DD}}$ ) and the input power splitting for both the carrier and peaking transistors. The optimal, load-dependent, ${V_{DD}}$ values are theoretically derived from back-off (BO) and full power conditions using load line theory. The optimal input excitation signals for the carrier and peaking devices are also derived for these variable ${V_{DD}}$ conditions. A 3.6-GHz QLI DPA was designed, and a complete system, composed of the DPA output stage, a two-channel medium power driver, an adaptive input driving stage, and a load sensing system, was implemented. The laboratory measurements have been performed for loads distributed inside a 2.0 maximum voltage standing wave ratio (VSWR) circle and show an output power variation between 43.8 and 42.6 dBm and a BO efficiency between 50% and 35%. Under modulated signal excitation, for the worst case loads, the peak output power capability of the DPA is improved from 41.7 to 43.1 dBm, and the average efficiency is increased from 32.6% to 43%.

This letter presents an automatic power amplifier (PA) design methodology that uses a multidimensional search algorithm to find the best compromise between fundamental and harmonic impedance terminations for a specified bandwidth. In conventional design methodologies, PA designers need to preselect the optimum impedances, which normally follow non-Foster trajectories, and are thus impossible to achieve with passive matching networks (MNs) for a wide frequency range. Instead, the proposed automatic design method directly synthesizes the MNs to achieve the desired output power, efficiency, and gain performance, without forcing any impedance profiles. For that, load-pull data are interpolated using artificial neural networks and an algorithm based on the simplified real frequency technique is used to obtain the MNs. Finally, the method is validated with a single-ended PA implementation.

This work presents a new artificial neural network (ANN) model formulation for RF high-power transistors which includes the S-parameters of the active device. This improves the small-signal extrapolation capability, and the OFF-state impedance approximation, making it suitable for Doherty power amplifier (DPA) design. This extrapolation capability plays a key role in the correct Doherty load modulation prediction, since, at low power levels, the peaking PA is subjected to active loads that cannot be synthetized with a passive load-pull system, forcing the model to extrapolate. Thus, the proposed model formulation is able to solve the issues that are normally observed when ANN-based models are used in complex PA architectures as the Doherty PA. To validate the proposed behavioral model, a 700-W asymmetrical LDMOS DPA, centered at 1.84 GHz, was simulated and measured.

This article presents an automatic system that is able to dynamically restore the output power capability of a power amplifier (PA) under variable loading scenarios. This is accomplished by dynamically adapting the supply voltage, ${V_{\mathrm{ DD}}}$ , of the PA according to the developed theoretical model. To dynamically vary ${V_{\mathrm{ DD}}}$ , a GaN dc–dc converter, based on the commercial EPC 9067 Demo Board, was used. The load is measured using an impedance meter, which is based on the slotted line principle and was specifically designed for low insertion loss. This impedance meter also generates a load-dependent pulsewidth modulated signal that controls the dc–dc converter switching, regulating ${V_{\mathrm{ DD}}}$ . The prototype was designed for 3.55 GHz and tested with CW and modulated signal excitations, being able to compensate the output power degradation under variable loading conditions inside a 2.1 voltage standing wave ratio circle. For a constant gain compression level, the average efficiency degradation over the tested range of impedances is less than 5%, and the average power improvement is 0.7 dB. The worst case measured output power of the PA is compensated from 37.0 to 39.9 dBm, which corresponds to a 2.9 dB improvement. Using a modulated signal excitation, the peak envelope power, which is compressed by almost 3 dB without ${V_{\mathrm{ DD}}}$ compensation, is restored, and the average output power is improved by 0.6 dB. The system can track the load and adjust ${V_{\mathrm{ DD}}}$ with a worst case step-up delay of 25 ms.

Introduction Statin therapy is associated with an increased risk of developing diabetes. Among bariatric patients, the influence of this therapy on various metabolic outcomes, such as diabetes status and its remission, is largely unknown. Methods This was a retrospective study of 1710 patients who underwent bariatric surgery at our hospital between January/2010 and June/2017. We compared patients with and without statin therapy at baseline, 12 and 24 months after surgery regarding statin use and its impact on several clinical and analytical parameters. Multiple linear regression was performed, adjusting differences for age, sex, surgery type, antidiabetic drugs at baseline, hypertension at baseline, LDL cholesterol ˃ 130 mg/dL, weight variation one year after surgery, and age of obesity onset. Results The overall prevalence of statin use was 20.2% before, 13.6% 12 months after surgery, and 15.0% 24 months after surgery. There was a larger reduction in fasting glucose and HbA1c at 12 and 24 months after surgery among statin-treated patients, with the opposite trend for weight reduction and BMI. Statin-treated patients with diabetes had lower diabetes remission rates (45.3 vs 68.5%) 12 months after surgery, with the highest reduction in HbA1c (1.3±1.3 vs −1.1±1.2%; p=0.042), fasting glucose (−40.8±48.8 vs −30.9±41.6 mg/dL; p=0.028), and insulin (−21.7±28.2 vs −13.4±14.2 mIU/L; p=0.039). The proportion of new-onset cases of diabetes was equal between statin-treated vs non-treated individuals at 12 months (1.9%) and 24 months (1.0%) after surgery. Conclusion Bariatric surgery seems to lead to diabetes remission more frequently in patients not treated with statins. A larger reduction was observed in fasting glucose and HbA1c among statin-treated patients. Statin did not contribute to an increased proportion of new-onset diabetes after surgery.

This article presents a theoretical explanation of the efficiency degradation in wideband radio frequency power amplifiers (RFPAs) under concurrent dual-band operation that is still visible after eliminating all known causes related to the output matching network (OMN). First, the origin of this degradation, identified by the reduction of the PA’s efficiency and linearity, is traced to the input baseband impedance terminations. Then, a theoretical model that describes the phenomenon and qualitatively predicts the efficiency variation under concurrent dual-band operation is presented using a simple model based on two-tone excitation. Finally, the proposed explanation is confirmed by comparing the efficiency performance of two 30-W GaN RFPAs of distinct instantaneous bandwidth determined by different input video-bandwidth terminations, under two-tone and concurrent dual-band operation. This article shows that, in addition to the adequate OMN design, an optimal input baseband impedance profile is also an important design constraint needed to keep the efficiency and linearity performance of RFPAs, for scenarios that require a wide instantaneous bandwidth.

This paper presents optimal carrier and peaking drain voltages to preserve the output power capability and back-off level of Doherty power amplifiers (PAs) operating for non-optimal loads. These voltages are theoretically obtained from back-off and full power conditions. A performance comparison between simulations of a conventional Doherty and the proposed load insensitive version is presented, showing that the introduced theory can be very advantageous for operation under load varying scenarios. A 3.6 GHz load insensitive Doherty PA was designed, implemented and measured for loads distributed inside a 2.0 voltage standing wave ratio circle. These measurements validated the introduced theory, showing an almost constant maximum output power (42.9 – 43.2 dBm), and a back-off level varying between 5.0 dB and 6.2 dB. The 6-dB output power back-off efficiency varies between 42 % and 53 %, in which the highest degradation corresponds to loads with high imaginary parts.

This work is an overview of the current understanding of memory effects arising from AlGaN/GaN HEMT based RF power amplifiers. It reviews the most widely accepted physical sources of those memory effects and their impact on the amplifier's linearizability, to then discuss corresponding circuit design approaches to minimize the phenomena. This is illustrated by results of a power amplifier whose short-term and long-term nonlinear memory is compensated by an analog assisted digital pre-distortion scheme.

Marta Borges-Canha,1,2 João Sérgio Neves,1,2 Fernando Mendonça,1 Maria Manuel Silva,1 Cláudia Costa,3 Pedro M Cabral,4 Vanessa Guerreiro,1 Rita Lourenço,5 Patrícia Meira,5 Daniela Salazar,1 Maria João Ferreira,1 Jorge Pedro,1 Ana Rita Leite,2 Madalena von-Hafe,2 Catarina Vale,2 Sara Viana,1 Ana Sande,1 Sandra Belo,1 Eva Lau,1 Paula Freitas,1,6 Davide Carvalho1,6 1Serviço de Endocrinologia, Diabetes e Metabolismo do Centro Hospitalar Universitário de São João, Porto, Portugal; 2Departamento de Cirurgia e Fisiologia, Faculdade de Medicina da Universidade do Porto, Porto, Portugal; 3Serviço de Endocrinologia do Instituto Português de Oncologia do Porto, Porto, Portugal; 4Serviço de Patologia Clínica do Centro Hospitalar Universitário Cova da Beira, Covilhã, Portugal; 5Faculdade de Ciências da Nutrição e Alimentação da Universidade do Porto, Porto, Portugal; 6Investigação e Inovação Em Saúde (I3s), Faculdade de Medicina da Universidade do Porto, Porto, PortugalCorrespondence: Marta Borges-CanhaServiço de Endocrinologia, Diabetes e Metabolismo do Centro Hospitalar Universitário de São João, Alameda Prof. Hernâni Monteiro, Porto, 4200-319, PortugalTel +351918935390Email marta.canha@gmail.comPurpose: We aimed to evaluate the association between vitamin D status and hepatic function parameters and scores: Fatty Liver Index (FLI, predictor of hepatic steatosis) and BARD (BMI, AST/ALT ratio and DM, predictor of hepatic fibrosis) in patients with morbid obesity.Patients and Methods: Cross-sectional study including patients with morbid obesity followed in our centre between January 2010 and July 2018. Patients with missing vitamin D levels or hepatic profile parameters were excluded. We divided the population according to two cut-offs of vitamin D levels (12ng/mL and 20ng/mL).Results: The included population (n=1124) had an average age of 43.3± 10.7 years and 84.3% were female. Seventy-point eight percent of the population had vitamin D levels lower than 20ng/mL and 34.8% lower than 12ng/dL. Patients with lower vitamin D levels (< 12ng/mL) had higher BMI, hip and waist circumferences and higher prevalence of hypertension. Higher FLI scores [OR= 0.77 (0.07), p< 0.01] and ALP levels [β= − 0.03 (− 0.06, − 0.01), p< 0.01] associated to lower vitamin D levels.Conclusion: Vitamin D deficiency is associated with a higher risk of hepatic steatosis in individuals with morbid obesity. Correction of vitamin D deficiency may have a beneficial role in the management of NAFLD in patients with morbid obesity.Keywords: vitamin D, fatty liver, NAFLD, obesity

Power Amplifiers (PAs) are usually designed for fixed load terminations, which implies that their performance can be severely degraded for varying load operation. This paper presents a load dependent supply voltage ( $V_{\mathrm{DD}}$ ) adaptation that can be used to correct PAs' output power in these non-optimal scenarios. The calculated $V_{\mathrm{DD}}$ values are verified in simulation and laboratory measurements for a 3.55 GHz PA based on a Cree CGH40010F Gallium Nitride (GaN) device. To dynamically vary the $V_{\mathrm{DD}}$ , a GaN buck converter, based on the commercial EPC 9067 Demo Board, was used. In addition, to preserve the compression level, the excitation amplitude is accordingly adjusted. The measured output power of the PA was corrected from 37.0 dBm to 39.9 dBm - a 2.9 dB improvement for the worst case loads inside a 2.1 Voltage Standing Wave Ratio (VSWR) circle.

Introduction: Non-alcoholic fatty liver disease (NAFLD) is strongly associated with obesity, and the prevalence of both diseases is increasing notably. The lack of effective treatment options for NAFLD is leading to a great consideration towards the identification of new approaches. Aim: We aimed to evaluate the change one year after bariatric surgery of parameters of hepatic function and in the hepatic scores, Fatty Liver Index (FLI, predictor of hepatic steatosis), and BARD, BMI, AST/ALT ratio and DM, (predictor of hepatic fibrosis). Methods: Observational retrospective cohort study in morbidly obese patients that underwent bariatric surgery between January 2010 and July 2018. We excluded patients missing hepatic function parameters before or one year after the surgical procedure. We used two linear regression models: 1) unadjusted; 2) adjusted for surgery type (gastric sleeve, gastric band and gastric bypass), sex, age, body mass index, diabetes and dyslipidaemia. Results: The included population (n=1955) had an average age of 43.1±10 years and 85.8% were female. We observed a relevant decrease in transaminases (pre-operative AST and ALT, 24.8±12.4 and 29.5±19.5U/L, vs 22.4 ± 11.1 and 22.2±14.7 post-operatively, respectively, p Conclusion: Bariatric surgery is associated with a reduction of the hepatic enzymes and an improvement of FLI and BARD. Bariatric surgery may represent an effective therapeutic approach to NAFLD.

To evaluate the variation in parameters of hepatic function and in the scores Fatty Liver Index (FLI; predictor of hepatic steatosis) and BARD (BMI, AST/ALT ratio and DM, predictor of hepatic fibrosis), 1 year after bariatric surgery. This is a observational retrospective cohort study in patients with morbid obesity that underwent bariatric surgery in our centre. We used two linear regression models: (1) unadjusted and (2) adjusted for surgery type, sex, age, body mass index, diabetes, and dyslipidaemia. The included population (n = 1955) had an average age of 43.1 ± 10 years and 85.8% were female. Diabetes was present in 32.4% of the patients, 45.1% had dyslipidaemia, and 62.2% had hypertension. Twelve percent were submitted to gastric band, 29.6% to sleeve gastrectomy, and 58.4% to gastric bypass. We observed a relevant decrease in transaminases and gamma-glutamyltransferase, and an increase in alkaline phosphatase and total bilirubin. Both FLI and BARD markedly decrease 1 year after surgery (p

This paper addresses the mechanisms of wideband power amplifiers’ efficiency degradation in comparison with their narrowband operation. It starts by identifying some possible causes for this performance degradation, showing that the baseband terminations are dominant. A detailed explanation for the efficiency degradation is then presented, considering the scenarios where the most important baseband components fall before or after the resonance imposed by the bias and matching networks. A theoretical model that is able to describe this performance degradation is also presented. Finally, the proposed theory is validated through two implemented power amplifiers with the same fundamental and harmonic terminations, but with different baseband networks, showing that an optimized bias network design can improve the efficiency performance of a PA.

Background: Obesity is a multifactorial disease that is strongly associated to other metabolic disorders, such as insulin resistance and type 2 diabetes. Bariatric surgery is nowadays considered the most effective treatment of morbid obesity. The role of insulin resistance (IR) in weight loss after bariatric surgery is highly unknown. Aim: To evaluate the association between Insulin Resistance (IR) and percentage of excess weight loss (EWL%) one, two, three and four years after bariatric surgery in patients with morbid obesity. Methods: Retrospective longitudinal study in patients with morbid obesity followed in our centre between January 2010 and July 2018 were included. Patients were excluded if they had diabetes. We evaluated baseline Homeostatic Model Assessment of IR (HOMA-IR), Homeostatic Model Assessment of β-cell function (HOMA-beta), Quantitative Insulin Sensitivity Check Index (QUICKI) and Matsuda and DeFronzo index, and performed a linear regression concerning each year’s EWL%. Results: After applying the exclusion criteria, 1723 patients were included in this analysis. The logarithm of HOMA-beta was negatively associated with EWL% at second-, third- and fourth-years post-surgery (β=-1.04 [-1.82 to -0.26], p Conclusion: IR at baseline seems to be associated to long term weight loss, explicitly after the first year post bariatric surgery.

Introduction: Bariatric surgery (BS) is an effective therapeutic approach for obese patients. It is associated with important gastrointestinal anatomic changes, predisposing these subjects to altered nutrient absorption that impact phosphocalcium metabolism. This study aims to clarify the prevalence of secondary hyperparathyroidism (SHPT) and its predictors in patients submitted to BS. Methods: Retrospective unicentric study of 1431 obese patients who underwent metabolic surgery between January/2010 and June/2017 and who were followed for, at least, a year. In this group, 185 subjects were submitted to laparoscopic adjustable gastric banding (LAGB), 830 underwent Roux-en-Y gastric bypass (RYGB) and 416 sleeve gastrectomy (SG). Data comprising 4 years of follow-up were available for 333 patients. We compared the clinical and analytical characteristics of patients with and without secondary hyperparathyroidism (considering SHPT a PTH˃69pg/mL), taking also into account the type of surgery. A multiple logistic regression was performed to study the predictors of SHPT after BS. Results: The overall prevalence of SHPT before surgery was 24.9%, 11.2% one year after surgery and 21.3% four years after surgery. At 12 months after surgery, LAGB had the highest prevalence of patients with SHPT (19.4%, N=36), RYGB had 12.8% (N=274) and SG 5.3% (N=131). At 48 months after surgery, RYGB had the highest prevalence of SHPT (27.0%, N=222), LAGB had 13.2% (N=53) and SG 6.9% (N=58). Multi-variate logistic analysis showed that increased body mass index and age, decreased levels of vitamin D and RYGB were independent predictors of SHPT one year after surgery. The only independent predictor of SHPT four years after surgery was RYGB. Conclusion: The prevalence of SHPT is considerably higher before and four years after BS than 1 year after surgery. This fact raises some questions about the efficacy of the implemented follow-up plans of vitamin D supplementation on the long term, mainly among patients submitted to RYGB.

Modern and future transmitter architectures are being driven toward multi-input multi-output (MIMO) transceivers, which make use of several radio frequency (RF) power amplifiers (PAs) to drive an antenna array. When moving to high integration scenarios, as is expected for fifth-generation communications, the isolation between the array and the PAs is often removed. Since the array elements are electromagnetically coupled, the waves fed to the antennas are also driving the output ports of the PAs. This effect creates an apparent variable load at the output of each PA, depending on the operation of the transmitter. In this system, the behavior of each PA cannot be fully described solely as a function of its input, as it will change according to the coupled signal. To predict and understand the behavior of these MIMO systems, simulation becomes a powerful tool. These simulators should be able to reproduce the main figures of merit (FoMs) of the transmitter, namely, output power, distortion, and efficiency. In this paper, we show how a recently proposed dc behavior model and an RF signal behavior model can be used in simulation setups to predict the MIMO transmitter FoMs of interest.

The long-term memory effects of gallium nitride (GaN) transistors have prevented its use in situations where the modulated envelope signal has a wide amplitude variation over time, such as in time division duplex systems. These long-term memory effects are generally attributed to electron trapping in GaN high electron-mobility transistors (HEMT), which have shown to be very difficult to compensate, especially in cellular base station transmitters known to be subjected to highly restrictive linearity specifications. On top of the electron trapping effects, we show that thermal effects can also induce long-term memory behaviors, which should also be accounted for when linearizing these devices. Because the conventional behavioral modeling approach has been incapable to compensate these long-term memory effects on GaN HEMT-based power amplifiers (PAs), we started by investigating the physical mechanisms responsible for these semiconductor impairments in GaN devices. This physics-based knowledge was then used to design new predistorter models that could effectively compensate those PAs subjected to GaN trapping and thermal effects. In this paper, we describe the new predistortion models for PA linearization, as well as the characterization methods used to determine their parameters. To validate the linearization effectiveness of the proposed model, several high power GaN-based PAs are tested with multicarrier GSM signals, and their linearization results are compared against other state-of-the-art models, evidencing a clear and significant improvement. In fact, to the authors’ knowledge, the proposed approach is the first one to reduce the PA distortion effects due to GaN long-term memory effects to such low levels, allowing a comfortable compliance with the imposed linearity masks.

This paper addresses the instantaneous bandwidth degradation of radiofrequency power amplifiers (RFPAs) still visible after eliminating all known causes related with the output matching network (OMN). First, the origin of this degradation, identified by the reduction of the PAs efficiency and linearity, is traced to the input baseband impedance terminations and a qualitative description of the phenomenon is presented. Then, an instantaneous bandwidth comparison is performed between two 30W GaN RFPAs, in which all impedance matching conditions are the same except for the input video-bandwidth impedance terminations. This work shows that, in addition to the adequate conception of the OMN, an optimally designed input impedance profile is also an important design consideration to retain the efficiency and linearity performance of RFPAs, for scenarios that require a wide instantaneous bandwidth.

This paper addresses the efficiency degradation observed in wideband power amplifiers. It starts by presenting a detailed explanation that relates this observed performance degradation with the terminations at baseband. Then, a comparison between two implemented power amplifiers with the same fundamental and harmonic terminations, but with different baseband networks is presented, showing that an optimized bias network design can improve the observed efficiency degradation.

Isolators are a fundamental component of many radio frequency (RF) communication systems. Typical RF isolators are obtained using permanent magnets that drive a static magnetic field through ferromagnetic materials inducing non-reciprocal propagation through the medium. Magnetless isolation has been proposed in a number of recent publications to allow integration of these components and also avoid the use of strong magnetic fields. In the magnetless approach, non-reciprocity is achieved through spatiotemporal modulation of circuit parameters. In this paper, we present the base theory and design approach for mag-netless isolators using grounded transistors. We present a magnet-less isolator design with 20 dB isolation and 3 dB insertion loss at a frequency of 2.1 GHz.

This paper presents a simple method to extract the time constants associated with trapping effects observed in GaN HEMT devices. Instead of using specific and, sometimes, very expensive measurement systems, we propose a method that relies on common equipment present in all RF laboratories. The method uses measured information of a GaN power amplifier (PA) gathered from a VNA (small-signal gain collected for different gate-to source voltages) and a VSA (dynamic gain profiles obtained from a set of two-tone input signals with different separation frequencies). The paper starts by presenting the theoretical background of the proposed methodology followed by an experimental validation with a GaN HEMT PA.

This paper describes a simple method to estimate the output power and efficiency load–pull contours of a class-B power amplifier (PA). Using successively improved–but still simple and intuitive—field-effect transistor nonlinear ${i}_{\rm DS}({v}_{\rm GS},{v}_{\rm DS})$ current models, this work starts by explaining the poor efficiency predictions provided by the Cripps load-line method, to then derive refined expressions to predict the output power load–pull contours and a novel analysis that produces useful estimates of the efficiency load–pull contours. These estimates are believed to constitute an invaluable tool for the analysis and design of single-ended class-B or Doherty PAs.

Power efficiency is one of the main concerns in modern radio frequency (RF) power amplifier (PA) design. However, efficiency-driven designs typically impose operation of these devices in a compressed mode [1]. When operating in this region, the amplifiers will behave nonlinearly and produce a distorted output signal [1]. This distortion produces new spectral components at the PAs' output that are outside the bandwidth of the input signal, so the distorted output signal has a wider bandwidth than the input signal. This bandwidth enlargement effect, called "spectral regrowth," is undesired in RF transmitters due to wireless systems' spectral mask regulations [2].

The radio frequency carrier amplitude-burst transmitter constitutes a promising architecture capable of efficiently transmitting signals of highly demanding complex modulation schemes. However, the tested practical implementations present results that stay way behind the theoretically advanced promises. This study presents the first thorough study of the power efficiency and linearity characteristics that can be actually achieved with this architecture. The analysis starts with a brief revision of the theoretical idealised behaviour of these switched-mode amplifier systems, followed by the study of the many sources of impairments that appear in their real implementation. In particular, a special attention is paid to the dynamic load modulation caused by the often ignored interaction between the narrow-band signal reconstruction filter and the switched-mode power amplifier. The performance of this architecture is clearly explained based on the presented theory, which is supported by simulations and corresponding measurement results of a fully working implementation. The drawn conclusions allowed the development of a set of design rules for future improvements.

Power Amplifiers can either be designed directly from load-pull data or using CAD software with embedded nonlinear models. Both approaches have advantages and disadvantages and their own range of applicability. Availability of reliable transistor models is usually the main factor to decide which approach to be followed. This paper presents a double pulse S-parameter measurement system that enables the extraction of current and charge conservative models of GaN HEMT devices. The obtained characteristics were used to predict the transistor load-pull curves which finally led to a RF power amplifier design. The very good agreement obtained between the output power and efficiency load-pull predictions, and their corresponding PA measured values, attest the quality of the extracted current and charge data, using the developed system.

Modern transmitter architectures rely on multi input multi output (MIMO) techniques, using several radio-frequency (RF) power amplifiers (PAs) to excite an antenna array. The array elements are often coupled, creating apparent variable loads at the output of each PA. In this system, the behavior of each PA cannot be fully described solely as a function of its input, as it will change according to the coupled signal. Moreover, the impact of the RF mutual coupling is felt not only on the RF output, but also on the direct current (DC) consumption, and thus efficiency, of the device. In this paper, we propose a novel PA modeling approach capable of predicting both the RF output and the absorbed DC current, dependent on the excitation and coupled signals. The proposed model is, therefore, suitable for system level simulations including efficiency and linearity predictions.

Limit-cycle or self-oscillating power amplifiers (SOPAs) are seen as an effective and simple option to achieve linear yet efficient power amplification. Unfortunately, they suffer from unfiltered switching noise, also known as ripple, whose amplitude increases for progressively lower self-oscillating frequency. Moreover, such self-oscillating frequency changes with the input signal and its dependency curve, as well as its maximum, are determined by the overall loop delay and by the open-loop filter. To remedy this SOPAs' pitfall, this paper presents a method to increase the self-oscillation frequency and reduce frequency modulation for a fixed propagation delay and loop filter. This is achieved using a relay that exhibits a novel kind of hysteresis. For that, the theory of its working principle will be discussed in detail, its transfer characteristic will be defined, and an implementation suitable for VLSI will be proposed. To validate the concept, simulations and an experimental prototype were carried out to compare the behavior of existing types of SOPA with the proposed one.

This paper provides a comprehensive analysis of the AM/AM and AM/PM nonlinear distortion generation mechanisms arising in the most common RF power amplifier (PA) technologies presently used in cellular infrastructures: the Si LDMOS and the GaN HEMT. Considering all the known nonlinear elements of both transistors' equivalent circuits, namely, the drain-source current and the gate-source, the gate-drain and the drain-source capacitances, semi-analytical expressions are derived for the PA's AM/AM and the AM/PM distortions. This model is shown to offer an accurate nonlinear distortion prediction across the whole range of operation classes (Class C, Class B, and Class AB) and signal ranges (from small-signal to power saturation), and a qualitative view of the physical distortion generation mechanisms, which is useful for nonlinear circuit design and performance optimization.

This work describes a new power amplifier, PA, behavioral model intended to predict both the PA amplitude and phase input-output signal relationship and the average or instantaneous dc power consumption and thus average or instantaneous signal-dependent efficiency. Hence, contrary to other previously published PA behavioral models, such a model is not only capable of describing the complex low-pass equivalent fundamental zone output, but also the real low-pass current consumption output. In addition, a special attention is paid to accurately account for the multiple time-scale dynamics shown by modern GaN HEMT based high power amplifiers.

This paper presents a physical explanation of the AM/PM distortion of a Si LDMOS based Doherty PA. Contrary to what is observed in single-ended mode operation, the Doherty AM/PM characteristic does not present the monotonic phase-lagging behavior for all input power levels. Instead, a phase plateau or even a phase-leading behavior is observed when the peaking PA enters into operation. It was found that the nonlinear Cds and the input Cgd,Miller capacitances, along with the Doherty load modulation are the main responsibles for this Si LDMOS Doherty PAs' behavior. This was validated using a 700W, 1.8 GHz asymmetric Doherty PA using 230W Si LDMOS transistors for the carrier and peaking PAs.

This paper gives a comprehensive analysis of the major nonlinear distortion generation mechanisms present in modern wireless polar transmitters. Based on a simplified description of the physical operation of voltage supply modulated switching mode power amplifiers (PAs), it derives an analytical model of both the AM-AM and AM-PM distortions usually encountered on polar transmitters excited by the standard two-tone excitation. The predictions of this model match extensive measurement and simulation results for a real PA circuit of a polar transmitter architecture.

In this paper, the design of a general feed-forward topology for modeling RF power amplifiers (PAs) is presented. It consists of producing the best nonlinear nonrecursive approximator of a feedback model which traduces the physical characteristics of general PAs. Following the procedure presented in this paper, the feed-forward structure, which has a polynomial form with memory (Volterra series), can be exactly determined up to the desired order of approximation. Evidently, increasing the order of the feed-forward topology results in a more complex structure, with more laborious model extraction procedures. This paper presents in detail the structures of the general nonrecursive model up to the fifth-order kernel. A systematic extraction procedure, based on standard RF laboratory continuous wave and two-tone tests, is proposed under the assumption that the PA linear output does not vary within the considered fundamental frequency band (flat behavior). Model validation is demonstrated by means of ADS simulation data and from real data measurements.

This paper presents an explanation for the higher efficiencies commonly obtained from GaN HEMT based Doherty power amplifiers (PAs), when subjected to wider load-pull ratios as compared with their Si LDMOS counterparts, the two main players in base-station PA technology. The analysis starts with the recently derived theory of efficiency dependence on load impedance and quiescent current, to then relate them to the self-biasing effects introduced by GaN HEMT charge trapping phenomena. For validation purposes, measured load-pull ratios of two 15W Si LDMOS and GaN HEMTs transistors are used.

This paper presents a new nonlinear equivalent circuit model extraction methodology of a GaN HEMT subject to trapping effects. Contrary to previous approaches that rely on computationally involved behavioral modeling techniques and/or nonlinear optimization processes, which lack physical insight, this technique uses the physics of the trapping mechanisms to come up with an extraction procedure that is simple, direct and systematic. Based on a recently advanced double-pulse I/V measurement technique for obtaining iso-dynamic device states, we first measure a set of pulsed dc I/V curves from which the trap dependent equivalent circuit model is then extracted. The proposed model extraction technique is validated through two-tone RF measurements, in which the GaN HEMT trapping effects are clearly exposed.