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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.

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.

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.

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.

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 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.

This work responds to the commonly formulated question in PA design of “Why do GaN HEMTs' nonlinear behavior seems so distinct from the one of their Si LDMOS, GaAs MESFETs or HEMTs counterparts?”. Starting from some recent results on the origin of AM/AM and AM/PM distortion in power amplifiers made for these two device types, we demonstrate that there is no fundamental reason why the devices should be different, except that the formers suffer from a noticeable low-frequency dispersion. Then, we dig into this particular aspect of GaN HEMT operation, to show that this charge trapping-related phenomenon is responsible for a severe self-biasing capable of inducing soft-compression of an otherwise almost flat AM/AM gain plot - when measured with static CW tests - which, in fact, corresponds to a severe gain expansion when the AM/AM is assessed with more realistic dynamic tests performed with real communication signals. And it is this class-C PA like AM/AM that is responsible for the recognized GaN HEMT nonlinearity.

The carrier power amplifier (PA) of a Doherty architecture is designed with a load that maximizes efficiency for a particular load-pull ratio. With the increase of the input drive, the peaking PA provides the necessary current to reduce the carrier PA load impedance. From this point on, the carrier PA operates at a constant output voltage, suffering a severe voltage gain compression at its terminals. Since the input Miller reflected C gd depends on this gain, the device's total input capacitance changes substantially with drive level, inducing a significant input phase shift. Hence, in this paper we address the impact of this capacitance variation on the AM/PM distortion characteristic of a GaN HEMT based Doherty PA.

The Doherty architecture is presented as an ideally linear efficiency enhancement technique. The theory behind this statement assumes that the output of the Carrier and Peaking Power Amplifiers (PAs) are ideal current sources, which is far from the truth in practical implementations. Both the Carrier and Peaking PAs are composed of nonlinear current sources suffering from both AM/AM and AM/PM distortion. Furthermore, the AM/AM and AM/PM characteristics of the Carrier and Peaking PAs, when operating in a Doherty arrangement, are very different from the ones expected from their single-ended operation, especially in GaN HEMT based Doherty PAs. This is exactly the topic addressed by this paper.

Despite the integrability and reconfigurability offered by fully digital wireless transmitters, their efficiency is still low, and no effective ways exist to diagnose and predict its degradation at higher back-off powers. Having found that it is the linear dependence of the switching losses with the output amplitude plus the inevitable constant losses that cause efficiency degradation at higher back-offs, an efficiency model was derived for carrier amplitude burst transmitter architectures. This model, not only allows the designer to predict the efficiency for a given modulation and output stage, but also allows one to identify the origin of the losses given an observed efficiency curve.

Power amplifier (PA) behavior and distortion analysis requires the characterization of the PA nonlinear static input-output relationship, usually evaluated through the input amplitude to output amplitude, AM/AM, response. However, the PA input amplitude to output phase shift, AM/PM, behavior has an equivalent impact on distortion generation, and so it must be also measured. This can be obtained by vector signal analyzer (VSA) measurements, an expensive equipment that is not usually available at most microwave PA design laboratories. In this manuscript we demonstrate that both the AM/AM and AM/PM PA characterizations can be effectively performed from the spectral magnitude of continuous-wave, CW, and two-tone tests, not requiring any direct measurement of signal phase. This is very important since it enables the full AM/AM and AM/PM PA characterization to be performed with a typical spectrum analyzer, a common equipment in all RF and microwave laboratories.

Signal-dependent phase variation, AM/PM, along with amplitude variation, AM/AM, are known to determine nonlinear distortion characteristics of current-mode PAs. However, these distortion effects have been treated separately, putting more weight on the amplitude distortion, while the AM/PM generation mechanisms are yet to be fully understood. Hence, the aim of this work is to present a large-signal physical model that can describe both the AM/AM and AM/PM PA nonlinear distortion characteristics and their internal relationship.

The potential high efficiency of RF carrier-burst transmitters has drawn the increasing attention of various researchers. This architecture uses an on-off mode of a constant-amplitude phase modulated carrier, according to the selected envelope-to-duty-cycle conversion (PWM or sigma-delta), requiring an implementation with two transistors, often in class-D configuration. This paper proposes a significant modification to this traditional architecture, where now the phase modulated carrier is always on - and thus allowing a single transistor implementation - but where the amplitude is impressed into the carrier phase according to a bi-phase code. A proof-of-concept prototype tested with a CDMA2000 signal achieved 59% of average efficiency and more than 34dBc of ACPR for an integrated output power of 24.7 dBm.

Carrier amplitude-burst transmitters can ideally deliver 100% efficiency and perfect linearity. The amplitude to time conversion performed by a PWM or sigma-delta modulator allows the usage of switched mode power amplifiers, at the expense of introducing a large amount of out of band noise, which must be removed by a narrow bandwidth band-pass reconstruction filter. However, practical implementations suffer from several problems whose cause is the careless design of the amplifier-filter connection, which prevent the achievement of those ideal performance figures. This paper discusses how the linearity of these transmitters is affected by the interaction between the switched-mode amplifier and the band-pass reconstruction filter providing design rules for this amplifier-filter interconnection. The presented results are obtained by both simulations and practical measurements.

The importance of the bandpass reconstruction filter, BPF, in the proper operation of carrier-burst transmitters is widely accepted. However, it is often analyzed as if it could be considered isolated from the other system's components. To fill this gap, this paper presents a detailed study of the carrier-burst transmitter BPF, addressing both its most significant characteristics and its influence in the system. During this analysis, it was found that both the characteristics of the BPF and of the amplifier-BPF interconnection are extremely important to the correct behavior of the system. The proposed theoretically driven design rules are verified by several simulations and supported by corresponding laboratory measurements.

Limit-cycle or self-oscillating power amplifiers (SOPAs) are seen as a compact option to achieve efficient yet linear amplification of either complete RF signals or only their base-band envelopes. Unfortunately, one of their undesired drawbacks is that their self-oscillating frequency, and thus available bandwidth, is reduced when a significant propagation delay exists in the feedback loop, typically from the power stage. To remedy this SOPAs' pitfall, this paper presents a method to increase the self-oscillation frequency 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 is discussed, and a suitable implementation is proposed. To validate the concept, simulations are carried out to compare the behavior of existing types of SOPA with the proposed one.

This paper proposes an innovative power amplifier, PA, low-pass equivalent behavioral model, BM, intended to enable the simultaneous prediction of linearity and efficiency in modern wireless transmitter chains. Contrary to the main PA behavioral modeling stream that has been exclusively focused in predicting the device's input-output power characteristics in the fundamental zone, this new model also provides the system designer with predictions of the PA absorbed dc power, this way enabling power efficiency calculations. For that, this new BM gives the fundamental output current or voltage across the load, as well as the base-band current flowing from the power supply to the PA dc bias terminal, I dc . Beyond the model format, a model extraction methodology and the corresponding laboratory test bench are shown. These were then validated comparing static and dynamic I dc -versus-V in modeled and experimental data.

In this paper, two vector nonlinear characterization procedures are presented, aimed at improving available GaN HEMT models for an accurate reproduction of the device behavior operating as a current source and in switched-mode RF power amplifiers (PAs). In the case of the more traditional linear amplifying classes, a technique for simultaneously extracting the higher order derivatives of the I ds (V gs ), I gs (V gs ) and C gs (V gs ) transistor nonlinearities, along a desired load line, is described. This procedure conveniently isolates and models their contributions to the device intermodulation distortion (IMD) behavior. In the case of highly efficient switched-mode PAs, employed under drain modulation condition, a modified procedure for isodynamically measuring the higher order derivatives of the V dd -to-AM and V dd -to-PM amplifier profiles is put into consideration, as a way to refine the triode region reproduction for ON-OFF operation. The test set-ups, based on the combination of vector signal analysis and vector signal generation capabilities, are described in detail, together with some analytic expressions for the parameter extraction. Measured results for a 15 W device from Cree are finally included.

In this paper, the impact of self heating on the linearity of a drain modulated GaN HEMT power amplifier (PA) is studied. After characterizing the frequency response of junction temperature rise to power dissipation, as well as the effect of this variable on the PA modulation profiles, the dynamic envelope trajectory, under a two-tone test signal, appears to be slightly dependent on frequency spacing. Self heating related effects are shown to be negligible in presence of other nonidealities, as the case of feedthrough. In this way, punching a vector hole in the two-tone I/Q diagram and applying memoryless digital predistortion (DPD), will allow identifying the influence of self heating effects, over adjacent channel distortion. Finally, this long-term memory effect is proved to be responsible for only a minor residual distortion in a linearized EDGE polar transmitter.

This paper reports the design and implementation of a RF GaN Oscillator for test in space. This circuit/system will be applied in a space test of the European Space Agency and it will be one of the first experiments in space using European GaN devices. The main objective will be to study the material radiation hardness. In this paper we will present the proposed circuit for test and its main design procedure.

This paper describes the design of a class E power amplifier (PA) destined to be used as the final stage of a polar transmitter under IEEE 802.16e Mobile WiMAX signal excitation. Based on a 60 W GaN HEMT device, the load condition for optimum efficiency is set close to the maximum value of the power generating function, as defined in [1], for an OFDMA modulating signal. The possible impact of such average-based design on the AM profile is also put into consideration.

In this paper, we discuss the available transmitter architectures in a Software Defined Radio (SDR) context and present a suitable polar architecture implementation under three different modulation formats (TETRA, EDGE and CDMA2000FW). Then, we show how its recognized nonlinear impairments can be overcome using appropriate digital amplitude and phase pre-distortion algorithms.

This paper presents a new power amplifier (PA) linearizer model whose topology has support on the physical recursive behavior of the general PA electronic circuit. The resulting model presents a topology that, in its essence, is non recursive, which makes it suited for an easy implementation on digital hardware devices (such as FPGAs and DSPs). Validation tests were performed on a 900 MHz PA excited with WCDMA-3GPP signals and comparison with other linearizer models demonstrated its excellence. Due to its support on the PA physical behavior, the proposed predistorter model has the potential to be an adequate linearizer for a wide range of PA device technologies and excitation signals.

This paper investigates the minimum supply modulator linear processing bandwidth and slew rate (SR) requirements necessary to meet the standard specs of total signal distortion and adjacent signal power ratio in modern polar wireless transmitters. System level simulations, performed under CDMA2000FW and CDMA2000RV excitations, showed that, depending on the filter behavior in the amplitude envelope spectrum main lobe, relaxed bandwidths between 2 and 5 times the original signal are sufficient to guarantee desired specs of 45dB of ACPR and −35dB of NMSE. On what the SR is concerned, we concluded that 1/(2.5) times the maximum AM(t) signal slope is enough to achieve the same desired specs.

This paper presents the linearization of a polar transmitter for EDGE applications. The methodology used, complements the modulation of drain bias, vDD(t) with a proper variation of the input drive level, vg(t). It is believed to improve the linearity, while maintaining the efficiency of such transmitters.

In this paper, the particular impact of switching device ON resistance variation with drain supply voltage, Ron(VDD), on polar transmitter distortion is considered. Pulsed I/V measurement results over a GaN HEMT are used to predict the deviation in the Vdd-to-AM modulation profile. System-level calculations, in the presence of other nonidealities, allow the evaluation of the relative contribution of this frequency dispersion related effect.

The aim of this paper is to promote the discussion and raise peoplepsilas awareness towards the creation of recommendations and regulations to improve the installation of different technologies in the same site. This research was accomplished with a theoretical approach and verified with real measurements of commercial Tower Mounted Amplifiers (TMApsilas).

This paper reports on a new design method to overcome the nonlinear distortion associated to polar transmitters. The proposed methodology complements the modulation of drain bias, v DD (t), of conventional envelope elimination and restoration power amplifiers with a proper variation of the input drive level, v g (t). Allowing v g (t) to vanish when v DD (t) goes to zero, it obviates the inherent nonlinear distortion caused by the amplifier carrier feed-through. In addition, and beyond guaranteeing linear operation, such a design methodology is also beneficial for power added efficiency as it prevents the dissipation of instantaneous input power for a desired null instantaneous output power.

This paper presents a procedure to design a non-recursive model, suited for simulation and able to model short-and long-term memory effects, that is the best nonlinear approximator, up to a certain polynomial order, of a feedback behavioral model whose topology was derived from the PA physical behavior. A simple and systematic model extraction procedure, based on single-and two-tone tests, is given for a model truncated to 3rd order, and its validation is demonstrated through ADS simulation comparison.

This paper explains the nonlinear dynamic memory effects observed on both AM-AM and AM-PM conversions of nonlinear microwave power amplifier, PA, circuits. The study is introduced via an analytic Volterra series analysis which is then illustrated with envelope simulations gathered from a GaN HEMT based microwave PA. This way, a theoretically supported explanation of the small- and large-signal dynamic asymptotic behavior, observed in the amplitude and phase conversions of microwave/RF PA's, could be achieved.

It is well known that AM/AM distortion introduced by power amplifiers, PAs, is responsible for the degradation of the transmitted signals. Understanding the origins and behavior of that deterioration is thus of paramount importance for PA performance improvement. This paper is devoted to analyze the impact of bias networks on the dynamic AM/AM contours of microwave PAs and, using that knowledge, to give an interpretation to the often observed hysteretic paths' shape.

This paper presents a new method for phase characterization of nonlinear mixing products based on statistical averages of signal windows captured by a synchronous two-channel sampler. Passing some of the signal processing operations, previously performed by complex laboratorial arrangements, to the digital domain, the present method simplifies the measurement bench and allows absolute phase measurements, i.e., not depending on any arbitrarily selected phase reference nonlinearity.

This paper presents a new nonlinear device model, for microwave power GaN HEMTs, amenable for integration into a commercial harmonic balance simulator. All the steps taken to extract it are explained, starting with the extrinsic elements' determination and ending with the intrinsic ones. This model was validated by comparing measured and simulated output power and intermodulation distortion data of a GaN HEMT. Very good agreement was obtained from small- to large-signal excitation regimes.