An optimized Doherty power amplifier using an unequal quadrature input splitter

Abstract

frequency increases, the half-power beam-width reduces. In addition, the cross-polarization level is lower in the H-plane than the E-plane, and is about Ϫ22 dB for H cross-polar and Ϫ12 dB for E cross-polar at 4 GHz. CONCLUSIONA new spatial power combiner with EBG has been realized. From the measured results, the prototype can provide more than 90% impedance bandwidth and with good out-of-band suppression. A system gain of about 17 dB has been realized. ACKNOWLEDGMENTSThe authors would like to acknowledge the funding support received from both the Conicyt, Chile, Bicentenario Project ATC-11, and the A*Star Singapore-Poland Joint Collaboration Project. Both linearity and efficiency performances have been very important selection criteria for high-power RF amplifiers. There are many well-known techniques for the enhanced linearity and efficiency performance [1][2][3][4][5][6]. The Doherty power amplifier, employing a very simple load impedance modulation technique, has provided a powerful solution for an efficiency improvement without loss of linearity [4 -6].Kim et al. proposed an uneven power drive for the Doherty amplifier which drives a little more input power to the peaking amplifier to compensate for its lower gain due to a lower bias point [5]. However, they put an attenuator to an input of the carrier amplifier in order to drive relatively more input power to the peaking amplifier, which degrades the gain of the Doherty power amplifier. Hence, the power-added efficiency (PAE) improvement can be limited by the gain degradation.In this study, we adopted an unequal quadrature input splitter for better performance of the Doherty amplifier. An optimized uneven power drive using an unequal splitter allowed the proposed Doherty amplifier to have higher gain than the unevenly driven Doherty amplifier using an attenuator. The efficiency performance at a given linearity of the Doherty power amplifier was experimentally optimized for a down-link wideband code division multiple access (WCDMA) signal through the power splitters having varied unequal power split characteristics and the optimized bias conditions of the amplifier. The experimental results of the Doherty amplifiers using unequal splitters were compared with those of the conventional Doherty and class-AB amplifiers. Figure 1 shows a schematic diagram of the Doherty power amplifier using an unequal quadrature input splitter. In general, the peaking amplifier has a class-B or -C bias point to use its low gain at a low power level, while the carrier amplifier is biased for class-AB operation. A large gain expansion of the peaking amplifier can be used as an on/off transition for load impedance modulation. However, the fully expanded gain of the peaking amplifier, at a high power level, is still lower than that of the carrier amplifier which can be compensated by the unequal input power drive. CIRCUIT CONFIGURATIONThe unequal input power splitters were designed based on a modified branch-line hybrid coupler which also compensated for the phase offset due to a /4...

Highly efficient unsymmetrical doherty power amplifier with DPD for LTE application

“…Such unsymmetrical Doherty structures can be implemented by using two unequal devices [9], or can also by two equal‐sized devices with unequal saturation power due to different output matching networks [4]. The other solution is based on the use of equal‐sized devices but uneven power divider, which can deliver more input power to the peaking amplifier [5]. Then, the peaking amplifier can generate enough current to achieve proper load modulation.…”

The design of high‐performance Doherty power amplifier

“…[7] introduced a pair of off-set lines to enhance the isolation of the carrier amplifier. During the small signal stage, the peaking amplifier is shut off and presents very high impedance at the combination point, so the carrier amplifier works with 100X load, but it cannot assure the combine efficiency when the carrier amplifier is gradually turned on for the unequal signal from the two offset, load pull effect has taken place then.…”

“…Many advanced methods have been proposed to enhance the performance of the DPA, adaptive bias control [5], inverted DPA structure [6], and output offset lines [7] bring enhanced efficiency for the DPA, whereas defected ground structure [3] and derivative superposition technique [8] promise better linearity.…”

Linearization of a 3.7 GHz multi‐carrier GaN HEMT Doherty power amplifier using digital predistortion method

“…Both linearity and efficiency are important performance measures for RF power amplifiers, because the performance of the transmitter system is determined by that of the power amplifier [1][2][3][4][5][6]. Especially, for the upcoming 4th generation wireless systems, it is necessary to use a higher peak-to-average power ratio (PAR) and higher frequency operation of the digital-modulated signals.…”

“…As it has a high efficiency over a wide output power range, the Doherty power amplifier has been widely used for various wireless communication systems [1,2]. However, the Doherty amplifier needs to be linearized using additional circuits or systems, such as feedforward or digital predistortion methods, due to the stringent linearity requirements of the base transceiver or relay systems.…”