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In this Letter, low-pass finite impulse response filters are proposed to flexibly reduce the envelope bandwidth (BW) requirements for envelope tracking (ET) supply modulators. A second-order power ET (PET) approach is used as a comparative study, and a `design zone' for shaping function coefficients is identified for the first time. Proposed 20 and 10 MHz low-pass filters (LPFs) are used to reduce the effective envelope BW in response to a 20 MHz long-term evolution signal applied to a 10 W power amplifier (PA). Experimental results verify that in comparison with PET, the proposed LPF method has the benefit of allowing arbitrary BW reduction and is compatible with the use of customised shaping functions. It is also shown that significant memory effects can be introduced when the effective envelope BW is reduced to less than the RF BW. To investigate the linearisability of the ET PA, the linearity before and after digital pre-distortion is also measured.

Envelope tracking (ET) is a promising power amplifier (PA) architecture for current and future communications systems, which uses dynamic modulation of the supply voltage to provide high efficiency and potentially very wide bandwidth over a large dynamic range of output power. However, the dynamic nature of the supply voltage can lead to a problematic variation in transistor gain, particularly in GaN HEMTs. This paper describes and analyzes this behavior and the detrimental effect it can have on ET PAs. Contributing factors and origins of gain variation are described in detail along with how, for the first time, meaningful comparisons can be made between different devices. Using these guidelines, gain variation is shown to be a widespread issue effecting most GaN HEMTs presented in literature. To allow an analysis of the intrinsic device behavior, an extended transistor model is developed that takes the effect of gate and source field plates into account. This model is refined using measurement data and used to demonstrate the fact that the parasitic gate–drain capacitance ( $C_{\textrm {GD}}$ ) is the main contributor to the small-signal gain variation—a significant part of the overall gain variation. Based on this knowledge, possible strategies to reduce gain variation at the transistor technology level are proposed, allowing the optimization of GaN HEMTs specifically for ET PAs. One identified strategy involves reducing the length of the gate field plate and is shown to be a viable approach to reduce the gain variation in GaN HEMTs, albeit at an increased RF/dc dispersion.

info:eu-repo/grantAgreement/MINECO//TEC2014-60283-C3-3-R/ES/NUEVOS MODELOS NO LINEALES COMPORTAMENTALES DE TRANSISTORES Y TECNOLOGIAS DE DISEÑO PARA AMPLIFICADORES DE POTENCIA DE BANDA DUAL PARA DRONES MULTICOPTERO LIGEROS To meet state-of-the-art specs in nonlinear circuit design for high frequency transceivers, we need computational efficient and reliable active device nonlinear models. Frequencydomain behavioral formulations, like X-parameters, have proven successful in this context. In this paper these formulations are exploited, through simplifications and transformations to the admittance/impedance domains, to algebraically guide the complex nonlinear design flow, in particular, in oscillator design. In addition, analytically the transistor behavioral model bandwidth can be extended, through the enhanced frequency scalability provided by the admittance domain. Simulations and measurements, HEMT and HBT devices will be used to validate the described approaches. Ministerio de Economía y Competitividad | Ref. TEC2014-60283-C3-3-R

This paper presents the robust characterization of nonlinear microwave devices driven by broadband modulated multi-sine stimuli by using a refined multi-tone waveform measurement system. The substantial contribution comes from the capability of the measurement system to measure and average the time domain waveforms of adjacently modulated signals on a similar time scale for different modulation frequencies. The enhanced system demonstrates the ability of presenting the constant baseband (IF) loads across wide modulation bandwidth which is extremely imperative for the precise evaluation of inherent nonlinearity of microwave devices. The measurement system is later applied for the experimental investigations of baseband impedance variation effects on 10W GaN HEMT through the optimization scheme based on the simultaneously engineering of significant baseband components(IF1 and IF2) and higher baseband components(IF3 and IF4).

The design and implementation of a class-J mode RF power amplifier is described. The experimental results indicate the class-J mode's potential in achieving high efficiency across extensive bandwidth, while maintaining predistortable levels of linearity. A commercially available 10 W GaN (gallium nitride) high electron mobility transistor device was used in this investigation, together with a combination of high power waveform measurements, active harmonic load-pull and theoretical analysis of the class-J mode. Targeting a working bandwidth of 1.5-2.5 GHz an initial power amplifier (PA) design was based on basic class-J theory and computer-aided design simulation. This realized a 50% bandwidth with measured drain efficiency of 60%-70%. A second PA design iteration has realized near-rated output power of 39 dBm and improved efficiency beyond the original 2.5 GHz target, hence extending efficient PA operation across a bandwidth of 1.4-2.6 GHz, centered at 2 GHz. This second iteration made extensive use of active harmonic load-pull and waveform measurements, and incorporated a novel design methodology for achieving predistortable linearity. The class-J amplifier has been found to be more realizable than conventional class-AB modes, with a better compromise between power and efficiency tradeoffs over a substantial RF bandwidth.

Baseband injection provides a useful approach for use in linearizing power amplifiers. The challenge is the determination of the required baseband signal. In [6] a generalized formulation quantifying the baseband voltage signal, injected at the output bias port, to linearize the device behavior was introduced. This envelope domain based solution requires the determination of only a small number of linearizing coefficients. More importantly these coefficients should be stimulus, hence bandwidth independent. This property has been experimentally investigated using a 10W Cree GaN HEMT device under a 3-tone modulated stimulus at 1.5dB of compression. It will be shown that the linearization coefficients were invariant when varying the modulation bandwidth from 2MHz to 20MHz.

This paper presents the design, implementation, and experimental evaluation of a high efficiency continuous Class BJ mode power amplifier, using a compact defected ground structure (DGS) to provide the necessary output matching and harmonic termination for broadband operation. Detailed consideration has been given to the DGS microstrip line impedance and corresponding equalization network, in order to achieve the required fundamental and harmonic matching. The method is demonstrated through design and realization of an amplifier using a commercially available 10W GaN HEMT transistor that achieves efficiencies above 70% over a 600MHz bandwidth with an output power of at least 40 dBm.

This paper compares two popular high power, high efficiency modes of operation, class F and inverse class F, and assesses the peak obtainable drain efficiencies when using Si LDMOS devices in a limited bandwidth design. Optimum class F and inverse class F conditions are presented using active harmonic load-pull measurements, and it was found that a higher drain efficiency was achieved in the class F configuration. This result is due to the limitations imposed by the soft voltage breakdown occurring due to the extended voltage swings inherent to inverse class F, as a consequence generating unwanted current content during the off cycle. This significantly reduces the peak measured efficiency using Si LDMOS devices when implementing an inverse class F design with a drain bias of 28 V. By reducing the drain bias to 18 V, to accommodate the voltage extension of inverse class F, it became possible to achieve peak measured efficiencies much closer to what theory predicted.

A new approach is presented that allows a Vector Network Analyzer to be operated as a Large Signal Network Analyzer without the need for a harmonic phase reference generator. The mode of operation exploits the architecture of a Rohde and Schwarz ZVA-67, which incorporates internal signal and local oscillator sources based on direct digital synthesis [1]. This unique capability leads to a Vector Network Analyzer based Large Signal Network Analyzer configuration that provides for time coherent receivers. This feature combined with a modified calibration procedure, allows the instrument, to provide error corrected RF current and voltage waveform measurements, requiring only the internal signal sources and an external phase (time) meter [2-5]. This approach simplifies the Large Signal Network Analyzer architecture and removes the complexities and bandwidth limitations introduced when employing a harmonic phase reference generator.

The introduction and formulation of the "Continuous" modes of transistor operation has provided an alternative design route for the realization of high efficiency power amplifiers (PA's) over a wide bandwidth (approaching an octave). The formulation of the "Continuous" modes provides a mathematical framework for extending the fundamental/harmonic impedance space over which the power and efficiency performance of the traditional PA modes, i.e. Class, B, Class F, etc, can be maintained. This mathematical framework has been experimentally validated using RF I-V waveform engineering measurements systems and the realization of demonstrator PA prototypes.

This paper demonstrates, for the first time, that the Class B to Class J concept can be successfully extended to X-band operation. X band power amplifiers with >30% bandwidth are required for a wide range of emerging applications where achieving optimal electrical efficiency is a key system driver. High efficiency is ordinarily achieved using circuit elements resonant at harmonic frequencies that are inherently narrowband in operation. The Class B to Class J continuum of modes, exploited here, offers Class B levels of efficiency over a continuum of impedance matching conditions, making wider bandwidth designs more feasible. This paper presents measurements of devices and MMIC designs which were undertaken to systematically use the continuum of modes for the first time at X-band. The 0.5W MMIC fabricated using a GaAs pHEMT process yields above 65% drain efficiency over >30% bandwidth.

A novel, highly efficient and broadband RF power amplifier (PA) operating in “continuous class-F” mode has been realized for first time. The introduction and experimental verification of this new PA mode demonstrates that it is possible to maintain expected output performance, both in terms of efficiency and power, over a very wide bandwidth. Using recently established continuous class-F theory, an output matching network was designed to terminate the first three harmonic impedances. This resulted in a PA delivering an average drain efficiency of 74% and average output power of 10.5W for an octave bandwidth between 0.55GHz and 1.1GHz. A commercially available 10W GaN HEMT transistor has been used for the PA design and realization.

This paper demonstrates how the linearity performance of a 10W GaN HEMT can be dramatically improved by actively engineering the baseband impedance environment around the device. An important refinement to existing active load-pull measurement capability is proposed that allows the precise and independent control of all significant baseband and RF components that result from the amplification of a complex 9-carrier multi-sine modulation. The synthesis of constant, modulation frequency independent negative baseband impedances, resulting in specific baseband voltage waveforms has delivered a 24dB improvement in ACPR compared to the classical baseband short case, even when the device is operating with RF components terminated into a non-optimal 50Ω RF environment. This linearization concept is further investigated through the broadband emulation of a class-J impedance environment around a single device. Using this enhanced system and a two-tone modulated excitation, optimum baseband loads are identified that result in a 18.5dB and 24dB improvement in IM 3 and IM 5 inter-modulation products respectively, again relative to the case of a traditional IF short circuit. The significance of this last observation is that unlike the 50Ω case, the optimum class-J IM 3 and IM 5 baseband impedances disperse, becoming reactive and moving away from the real axis.

A new methodology is presented for designing baluns exhibiting multi-decade bandwidths at microwave frequencies. Simulations show that resistors terminating the outer transmission line suppress the half-wavelength resonance and greatly extend the bandwidth. Using linear measurements at microwave frequencies, ferrite beads have been shown to behave as resistors with a small reactance, suitable for terminating the outer transmission line. This paper shows that ferrite beads can perform the dual role of improving magnetic coupling at low frequency and suppressing resonance at higher frequencies. The design methodology was applied to produce a balun that operates between 30MHz and 6GHz, displays less than 1dB of power loss up to 4.4GHz, and delivers an impedance transformation of 2∶1.

Characterisation of modern wireless power transistors and amplifies requires wideband-modulated stimuli for realistic performance evaluation. Normally power transistor performance is evaluated using passive load-pull techniques. However, these have some physical realization constraints that influence the amplitude and the phase of the presented impedance at the DUT plane as across the modulated bandwidth. The problem becomes more apparent as modulation bandwidth is increased beyond few MHz. It has been shown recently that a digitally controlled active envelop load-pull (ELP) system can completely address this problem [1], thus allowing for a more systematic investigation of the parameters that may affect the performance of power transistors under wideband-modulated stimuli. In this paper, non ideal multi-tone impedance conditions are purposely introduced for demonstrating their effects on power amplifier performance in order to illustrate the DUT sensitivity to measurement system imperfections at higher modulation bandwidths in excess of few MHz.

This paper presents, for the first time, a new active load-pull approach capable of supporting wideband multi-tone modulation based on the active envelope load-pull architecture. The new solution utilizes a digital control unit, implemented on a reconfigurable FPGA platform with high-speed signal processing capability, in the feedback loop to track the time varying amplitude and phase envelope of modulated RF communication signals. This novel approach enables a loop delay compensation scheme to be successfully implemented allowing for synchronization of the transmitted and the reflected signals over wide (>5 MHz) bandwidths. Initial, proof of concept, measurements on this active load-pull system have demonstrated that it can provide constant, frequency independent control of load impedances for signal excitations that are relevant for communication systems.

An active load-pull based investigation was conducted into the achievable performance of a 10 W gallium nitride (GaN) HEMT whilst operating in both narrow and wide bandwidth applications. It was found that a minimum efficiency of 40% was obtained over a broad bandwidth whilst delivering a constant output power of 10 W with the device biased in a class-AB mode. Conversely, in a narrow-band, high efficiency inverse class-F mode very high performance in terms of output power (12 W) and efficiency (81% PAE) has been measured. This has demonstrated the significant benefits in terms of the versatility of GaN devices for current and future PA applications. (1 page)

This paper presents broadband control electronics for active closed-loop envelope load pull system. This control electronics when integrated in the existing envelope load pull system provides a constant load with varying input power over large bandwidth. These two characteristics of the broadband control module make it a potential candidate for a novel tool in the power amplifier characterization techniques. The developed control electronics finds direct application in the development of broadband envelope load pull system to be used for the characterization of CDMA and W-CDMA power amplifiers (PA).

The paper presents an active envelope load-pull system for the active control of load impedance over a bandwidth of approximately 100 MHz, hence extending the employment of active load pull systems to modulated signals. As a result, the load-pull system can be also employed for linearity investigations of non-linear devices, which is crucial for telecommunication systems. Moreover, the presented load pull system offers the complete control of the impedances within the bandwidth and allows for novel applications such as accurate emulation of complete PA matching circuits over the entire operational bandwidth.

A nonquasi-static nonlinear table-based FET model, which has been implemented into a commercial Harmonic Balance Simulator (MDS), is described and demonstrated. It makes use of a simplified nonquasi-static charge formulation and also accounts for low frequency dispersion phenomena. It can be derived, fast and simply, from dc and small signal S-parameters and is general and technology and process independent. The model is accurate under dc, small and large signal operating conditions being able to perform excellent predictions up to high frequencies and over a wide bias range. Dynamic large signal model behaviour has been validated using pseudomorphic HEMT structures, by comparing measured and simulated input and output waveforms, dynamic load line and transfer characteristics. Excellent agreement between measurements and simulation results has been achieved in the frequency range from 2 to 50 GHz.

A great deal of effort has been devoted to simplifying the monolithic integration, and enhancing the high-speed modulation characteristics, of GaAs-based SQW lasers. MBE-grown, vertically-compact, high-speed GaAs/AlGaAs separate-confinement double heterostructure (SCDH) devices have been developed. >

We investigate p-dopant incorporation and the influence of p-doping on the gain and damping behaviour in GaAs-based strained multiple quantum well lasers. By adding 5x10high18-2x10high19 cmhigh-3 beryllium doping to the active region of In0.35Ga0.65As/GaAs strained multiple quantum well lasers (4 QW's), we are able to demonstrate both very efficient high-speed modulation (20 GHz at a DC bias current of 50 mA) and the first semiconductor lasers to achieve a direct modulation bandwidth of 30 GHz under DC bias (heat-sink temperature is equal 25 degree C).

A detailed comparison is presented between vertically-compact high-speed GaAs and In/sub 0.35/Ga/sub 0.65/As MQW diode laser structures suitable for integration with MODFET driver circuits. Both devices show superior performance, although the advantages of replacing the GaAs QW's with pseudomorphic InGaAs QW's are clearly demonstrated.

Summary form only given. The authors have developed GaAs/AlGaAs multiple-quantum-well (MQW) lasers with direct modulation bandwidths of 16 GHz. The devices utilize a compact vertical layer structure which also makes them particularly suitable for integration. The theoretical calculations leading to this optimized design as well as the DC and high-frequency characteristics of the lasers are discussed. A 3-dB electrical modulation bandwidth of 16 GHz was achieved at 90-mA pulsed bias in a 4- mu m*200- mu m device. The slope of the plot of the resonance frequency versus the square root of the optical power yielded values of dg/dn ranging between 4*10/sup -16/ and 7*10/sup -16/ cm/sup 2/ for short-cavity-length and narrow-mesa-width lasers. >