Before the emergence of ultra-wideband (UWB) radios, widely used wireless communications were based on sinusoidal carriers, and impulse technologies were employed only in specific applications (e.g. radar). In 2002, the Federal Communication Commission (FCC) allowed unlicensed operation between 3.1–10.6 GHz for UWB communication, using a wideband signal format with a low EIRP level (−41.3dBm/MHz). UWB communication systems then emerged as an alternative to narrowband systems and significant effort in this area has been invested at the regulatory, commercial, and research levels.

IEEE Transactions on Microwave Theory and Techniques and Antennas and Propagation Announce a Joint Special Issue on Ultra-Wideband (UWB) Technology

This paper presents a modified Doherty configuration with extended bandwidth. The narrow band feature of the conventional Doherty amplifier is discussed in the view of the broadband matching. To extend the bandwidth, the post-matching architecture is employed in the proposed design. Meanwhile, broadband low-order impedance inverters are adopted to replace the quarter-wavelength transmission lines. Low-pass filter topologies are used to realize both the post matching network and the impedance inverters. A modified Doherty Power amplifier was designed and fabricated based on commercial GaN HEMT devices to validate the broadband characteristics of this configuration. The 6-dB backoff efficiencies of 47%–57% are obtained from 1.7 to 2.6 GHz (41.9% fractional bandwidth) and the measured maximum output power ranges from 44.9 to 46.3 dBm in the designed band. In particular, more than 40% efficiencies are measured at 10-dB backoff throughout the operation band.

A Post-Matching Doherty Power Amplifier Employing Low-Order Impedance Inverters for Broadband Applications

A novel graphical power amplifier (PA) design tool, the “clipping contour,” is introduced and described. Using the now well-publicized continuous Class-B/J voltage waveform formulation as a starting point, a process is derived that allows contours to be constructed on a Smith chart that define the “zero-grazing” fundamental and harmonic impedance conditions. Theoretical equations are defined and solved whereby the contours can be drawn in real time in a computer-added design environment. A key and novel result from this theory is the definition of a 2-D harmonic design space that opens up rapidly as small concessions from optimum power and efficiency matching conditions are made. A design example is described and fabricated, which demonstrates the utility of using the second harmonic clipping contour during the PA design process. A 10-W GaN demonstrator gives measured continuous wave power > 8.5 W, efficiency > 60%, and better than -30-dB adjacent channel power ratio over a bandwidth of 1-2.9 GHz.

Continuous Mode Power Amplifier Design Using Harmonic Clipping Contours: Theory and Practice

This paper presents an in-depth, systematic study of the impact of input and output harmonics in the design of high-efficiency power amplifiers (PAs). The study evaluates the performance of harmonically tuned amplifiers, tackling concurrently both input and output harmonics. The proposed theory starts with deriving an altered input voltage waveform under the impact of input nonlinearity. Intrinsic drain voltage and drain current components are formulated as a function of the conduction angle $\alpha $ considering both source and load terminations. Output power and drain efficiency are then computed as a function of input nonlinearity, $\alpha $ , and output loading conditions. The derived formulations allow to investigate the design sensitivity to input nonlinearity and its impact on fundamental design space. The impact of source harmonics is evaluated using harmonic source pull under different output loading conditions. Thereafter, PA design and implementation has been carried out using NXP 1.95 mm die to confirm the distinctive behavior of class GF and GF−1 amplifiers with respect to the input harmonic terminations. For practical validation, four different design cases with different second harmonic source impedances are investigated. At 2.6 GHz, drain efficiencies ranging between 76% and 83% were exhibited depending on the source and load harmonic tuning for each design case. Measurement results confirm the theoretical findings reported in this paper.

High-Efficiency Input and Output Harmonically Engineered Power Amplifiers

This article presents the theory and design methodology of broadband RF-input continuous-mode load-modulated balanced power amplifier (CM-LMBA) by introducing the CM output-matching networks in the LMBA architecture. It is illustrated that the CM impedance condition can be achieved by properly adjusting the phase difference between the different PA branches in the proposed CM-LMBA during the entire load modulation process. An RF-input CM-LMBA with 1.45–2.45-GHz bandwidth using commercial GaN transistors is designed and implemented to validate the proposed architecture. The fabricated CM-LMBA attains a measured 11.2–13.4-dB gain and around 40-W saturated power. Power-added efficiency (PAE) of 46.4%–56.5% and 43.2%–50.3% is achieved at 6- and 8-dB output power back-offs throughout the designed band. When driven by a 100-MHz OFDM signal with an 8-dB peak-to-average power ratio (PAPR), the proposed CM-LMBA achieves better than −46-dBc adjacent channel leakage ratio (ACLR) and higher than 45% average PAE after digital predistortion at 1.8 and 2.1 GHz.

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Broadband RF-Input Continuous-Mode Load-Modulated Balanced Power Amplifier With Input Phase Adjustment

A new design methodology is presented for the planar implementation of a classical Marchand balun. A novel intuitive analysis shows that the Marchand configuration can be designed optimally to eliminate the phenomenon of “trace separation,” which is frequently observed in planar implementations. The new theory shows that this unbalancing effect is caused by the parasitic transmission line formed between the inner strip and ground, which is not considered in Marchand's original coaxial structures. Compact design equations are derived, based on which a new innovative structure is proposed and fabricated. This demonstrates the elimination of trace separation and achieves flat equal port split over a double octave bandwidth, performing up to 10 GHz, using an industry standard single-layer thin-film process having a continuous unpatterned ground plane. Popular planar variants of Marchand's original structures are also designed and fabricated to verify the new design equations. These structures are compared in terms of bandwidth, trace separation, and balanced port impedances.

Optimal Design of Broadband Microwave Baluns Using Single-Layer Planar Circuit Technology

A new design method for Doherty–Chireix combined operation is proposed. Only one of the two amplifiers operates at the low power region like a Doherty amplifier, while both amplifiers are operating as a Chireix outphasing amplifier at the high power region, taking advantage of the high efficiency region of each operating mode. Quasi-envelope efficiency of both amplifier efficiency curves is extracted in a systematic way using only analog circuit design methods. The output combiner is used as an input splitter for the radio frequency input Chireix outphasing without using any digital signal processing. Input signal phase control versus power for outphasing operation is obtained by using only a linear circuit, exploiting the input impedance variation of nonideal devices. A 52.4-dBm peak output power, 6.6-dB back-off Doherty–Chireix combined amplifier, is built operating at 2.17 GHz. A 50% average power added efficiency was measured using 6-dB peak-to-average power ratio, 3.84 MHz modulated test signals. Class-B like efficiency behavior is obtained at the low power region without compromising the peak back-off efficiency.

RF-Input Self-Outphasing Doherty–Chireix Combined Amplifier

An active load pull waveform measurement system has been utilized to investigate, by drain current waveform analysis, the effect of short circuits at the second and higher harmonics on the gate of a 750 μm Gallium Arsenide transistor. It is confirmed that the half wave rectified sinusoidal current waveform necessary for modes of operation such as class B, J and F is not optimally generated at X-Band frequencies for transistors controlled purely by bias. The disrupting effect of the nonlinear device gate capacitance must be accounted for to take full advantage of these high efficiency modes. The addition of a simple wide band filter circuit at the input of a MMIC test cell to “short” the gate higher harmonics is shown to force the drain current waveform towards the ideal case and hence yield a significant increase in drain efficiency of up to 10 percent.

Waveform Evidence of Gate Harmonic Short Circuit Benefits for High Efficiency X-Band Power Amplifiers

A new outphasing signal splitter for a radio frequency (RF) single input Chireix power amplifier without using digital signal processing is introduced. Input signal phase control versus power for outphasing operation is obtained by using only a linear circuit, exploiting the input impedance variation of non-ideal devices. A 52.6 dBm peak output power, 6 dB back-off Chireix power amplifier is built operating at 2.17 GHz. More than 50% drain efficiency is maintained over 5.7 dB output power back-off range.

Tim Canning

Papers

Continuous Mode Power Amplifier Design Using Harmonic Clipping Contours: Theory and Practice

Optimal Design of Broadband Microwave Baluns Using Single-Layer Planar Circuit Technology

RF-Input Self-Outphasing Doherty–Chireix Combined Amplifier

Waveform Evidence of Gate Harmonic Short Circuit Benefits for High Efficiency X-Band Power Amplifiers

Self-outphasing Chireix power amplifier using device input impedance variation