A design methodology for a concurrent dual-band Doherty power amplifier (PA) with frequency-dependent backoff power ranges is presented in this paper. Based on a dual-band T-shaped network and a coupled line network, different dual-band components needed in Doherty PA topology, including a 3-dB branch-line coupler, an offset line, and a quarter-wavelength transformer, are developed. Two prototypes with balanced and imbalanced backoff power range modes are implemented to verify the feasibility. Continuous wave signal test results show that the proposed dual-band PA successfully achieves a power-added efficiency of 33% and 30% at the 6-dB backoff point from the saturated output power at 880 and 1960 MHz, respectively. To meet linearity requirements, the PA nonlinear behavior is characterized by using digital multitone signals, which categorize the distortions of a concurrent dual-band PA into intermodulation and cross-modulation. Finally, a 2-D digital predistortion technique is used to compensate for the nonlinearity of PA in dual bands. Two two-tone signals are applied to the dual bands for linearization, and the experimental results show that this technique achieves improvements of better than 19.1 and 24.6 dB for the intermodulation and cross-modulation in the dual bands, respectively.

Design and Linearization of Concurrent Dual-Band Doherty Power Amplifier With Frequency-Dependent Power Ranges

A comprehensive comparison of a wide collection of compact microstrip resonant cells (CMRCs) found in the extensive body of literature has been presented. The evaluation of different CMRC designs has led to the selection of the most promising CMRC geometry for the efficient miniaturisation of modern microwave components. To showcase the vital effectiveness of the approach, the initially selected CMRC has been notably improved and applied to an exemplary microwave component, that is, a branch-line coupler, to obtain its size reduction without degradation in its performance. Exemplary prototype circuits have been designed to mimic the frequency characteristics of a classic coupler, simultaneously presenting a substantial 75, 80 and 85% surface area reduction. The best solution obtained has been experimentally validated. The measured in-band results are in excellent accordance with the theoretical characteristics, whereas the broadband performance depicts an additional effect, that is, the strong suppression of unwanted signals in 2.6-7 GHz frequency band. The multi-step CMRC evaluation and improvement procedure proposed presents an efficient design methodology for novel compact circuit topologies developed in microstrip technology.

Development of a compact microstrip resonant cell aimed at efficient microwave component size reduction

A new dual-band 90° coupler using coupled transmission lines is proposed in this paper. As the coupler consists of only three quarter-wave coupled-line sections, it occupies less circuit area as compared to other existing dual-band branch-line couplers. A desired dual-band operation can be achieved by proper design of the coupling level for those coupled-line sections. Moreover, the coupler's configuration contains a degree of freedom so that its design is “customizable,” allowing itself to be conveniently realized on a variety of circuit and substrate technologies. As a result, it is suitable for use in many practical applications.

A Compact Dual-Band 90° Coupler With Coupled-Line Sections

With the rapid development of modern wireless communication systems, the desirable miniaturization, multifunctionality strong harmonic suppression, and enhanced bandwidth of the rat-race coupler has generated much interest and continues to be a focus of research. Whether the current rat-race coupler is sufficient to adapt to the future development of microwave systems has become a heated topic.

Microstrip Rat-Race Couplers

This letter presents a dual-band branch-line coupler with compact size and planar structure. The $\pi$ -shaped dual transmission lines structure is used to realize 90 $^{\circ}$ phase shift at two arbitrary frequencies. Explicit design equations are derived using the ABCD and admittance matrices. For verification, a prototype is designed, fabricated, and tested. The coupler has a compact size with 76% size reduction compared with the conventional circuit at the first band. The measured results are in good agreement with the simulated results.

Compact Dual-Band Branch-Line Coupler With Dual Transmission Lines

This study constructs stepped-impedance-stub lines for a dual-band branch-line coupler design with improved design flexibility. The proposed structure demonstrates dual-band performance and a compact size due to additional stepped-impedance stubs to branches. The developed synthesis method has two degrees of freedom which can be exploited to miniaturize circuit size and/or replace impractical impedances with more realizable ones. Observations also show the advantage of a wide-range realizable frequency ratio of dual bands. The current work fabricates three experimental dual-band branch-line couplers, including a two-section coupler, and achieves a size reduction up to 21.7%, compared with conventional structures. The measured results validate good dual-band performance at 2.4/5.8 GHz with enhanced bandwidths up to 21% and 12%, respectively. This research also successfully applies the proposed circuit to synthesize a dual-band rat-race coupler.

Compact Dual-Band Branch-Line and Rat-Race Couplers With Stepped-Impedance-Stub Lines

This paper presents a dual-band −90°/+90° transmission line for the miniaturization of dual-band rat-race couplers. This line was constructed using two diagonally end-shorted coupled-line sections tapped by open stubs at their center. The proposed structure has the advantages of −90°/+90° electrical lengths at two arbitrary frequencies and high equivalent characteristic impedances, enabling the creation of small couplers that have a large power-split ratio of up to 1:32. Using the proposed structure, a rat-race coupler operated at 2.4/5.1 GHz with a power-split ratio of 1:4 was fabricated for demonstration. This circuit occupies only 44% of the area of existing dual-band rat-race couplers.

Design of dual-band −90°/+90° transmission lines for miniaturized dual-band 1:4 rat-race couplers

This study presents a dual-band rat-race coupler featuring two controllable center frequencies with arbitrary power-split ratios. Novel dual-band stub-loaded 90° lines were used to replace the conventional single-band 90° lines according to the required branch impedances for specific power-split ratios. The proposed structure has two degrees of freedom, providing more design flexibility and obtaining short lengths and realizable impedances. This research successfully synthesized a 2.4/5.2 GHz rat-race coupler with a 1:2 power-split ratio at dual bands. The measured results were well-correlated with simulation results.

Yen-Hsiu Wei

Papers

Compact Dual-Band Branch-Line and Rat-Race Couplers With Stepped-Impedance-Stub Lines

Design of dual-band −90°/+90° transmission lines for miniaturized dual-band 1:4 rat-race couplers

Dual-band rat-race coupler with arbitrary power-split ratios