This article presents a novel method of extending the high-efficiency range of Doherty power amplifiers (DPAs) by utilizing multiple output power back-off (OBO) parameters. The OBO extension techniques of the DPAs, such as complex combining load (CCL), virtual stub (VS), and out-phased current combining (OCC) methods, were defined, and a general expression for these OBO extension techniques was derived. Using the general expression, three OBO extension methods were comparatively and comprehensively analyzed. Based on the analysis, the DPA with an extended OBO can be designed by simultaneously employing multiple back-off techniques with a high degree of freedom. To verify the general expression and its analysis, a DPA design for the frequency band of 3.45–3.75 GHz with GaN HEMTs was conducted using multiple OBO extension techniques of VS and OCC methods for a large OBO of 8.5 dB. Using a long-term evolution (LTE) signal with a signal bandwidth of 20 MHz and a peak-to-average power ratio (PAPR) of 8 dB, the drain efficiency (DE) of 38.9%–56.8% was achieved at the average output power levels of 34.4–36.8 dBm while satisfying ACLR of no larger than −45 dBc with digital predistortion (DPD) linearization. Under excitation of a 5G new radio (NR) signal with a signal bandwidth of 100 MHz and a PAPR of 8 dB, the DE of 38.5%–50.2% was achieved at the average output power levels of 34.6–36.8 dBm with an ACLR of under −24.6 dBc without DPD linearization.

This paper presents a generalized expression of the modified load networks (MLNs) for Doherty power amplifiers (DPAs) with an extended output power back-off (OBO) range. Based on the analysis using this new expression, a new circuit structure generally applicable to any combination of the multiple MLNs is proposed. The load network has a simple structure including impedance matching circuits and offset lines for the carrier and peaking amplifiers. A design procedure for the proposed load network is presented. A DPA using any optimized combination of the multiple MLNs can be easily designed using the proposed load network. For verification, a GaN-HEMT symmetrical DPA based on a combination of the virtual open-stub (VOS) and out-phased current combining (OCC) methods was designed and implemented for the frequency band of 3.4 - 3.7 GHz. The reversed uneven power splitting method is adopted in the design for higher power gain. Using the 5G New Radio (NR) signal with a peak-to average power ratio (PAPR) of 7.9 dB and a signal bandwidth of 100 MHz, the proposed DPA exhibited DE of 55.3 - 60.9% with a power gain of 11.9 - 12.7 dB at an average output power of 33.7 dBm. An adjacent channel leakage power ratio (ACLR) of −42.6 - −44.8 dBc was achieved at the average power level by using a digital predistortion (DPD) technique across the band.

This paper presents a look-up table (LUT)-based focal beamforming system that can effectively transmit RF power up to mid-range distances (≤ 3 m) including when the Rx is in the near-field zones. The Tx elements control and radiate signals for the Rx even at the near-field zone ensuring the received signals are in-phase. Since the proposed system uses a LUT for storing the phase sets of the signals for the Tx elements, it requires only very simple hardware and a very simple adaptive control algorithm compared to conventional retroreflective method. In order to track the moving Rx, a 2-D adaptive sequential searching algorithm is proposed. The system can find the optimum phase set by sequentially searching the phase sets for a pre-determined 2-D area. The LUT of the phase sets are generated using geometric analysis over the entire 2-D area where the Rx could be located. To verify the proposed method, a 5.2 GHz mid-range (≤ 3 m) MPT system composed of a $4\times 8$ Tx array and a $2\times 3$ Rx array was designed and implemented. Using the proposed 2-D adaptive sequential searching algorithm, the optimum phase set for focal beamforming can be quickly found for a given position of the Rx. In our experiments, the results showed an RF power level of 177.8 mW was received at the Rx with a distance of 1 m with a total radiated RF power of 16W. Since the measured received power levels for various Rx positions agree well with the simulation results, the proposed system was proved to be an excellent candidate for the practical application.

A simplified all-in-one load network of Doherty power amplifier (DPA) was proposed for extended back-off range and broadband operation in this study. Although the proposed load network was simplified to just have a postmatching network (PMN) and compact $L$ -section matching networks for both carriers and peaking amplifiers, it could fully incorporate previously known back-off range extending methods, such as asymmetric structure, virtual stub (VS), out-phased current combining (OCC) method, and complex combining load (CCL). Accurate closed-form expressions for the elements and impedances of the load network based on the back-off range extending methods were derived and used to design the structure of the load network. Among available configurations of synthesized load networks, a structure suited for broadband operation was derived. To verify the proposed load network and its closed-form expression, an asymmetric DPA based on GaN-HEMT with an extended back-off range of 8 dB was designed and implemented for a frequency band of 2.9–4.0 GHz, which includes the N78 5G frequency band. Lumped inductors for the initial configuration were transformed using transmission lines for a quasi-lumped structure which was optimized to have a frequency response similar to the original inductor. Implemented DPA exhibited a drain efficiency (DE) of 50%–56% under an output power back-off (OBO) of 8.0 dB from a saturated output power of about 43 dBm, while it showed a DE of 62%–76% over a frequency range of 2.9–4.0 GHz using a continuous-wave (CW) signal. An efficiency of as high as 50.6%–58.0% and an adjacent channel leakage power ratio (ACLR) of under $-$ 46.0 dBc after a digital predistortion (DPD) based on a memory polynomial were achieved at an average output power of 35 dBm using a 5G new radio (NR) signal with a signal bandwidth of 100 MHz and a peak-to-average power ratio (PAPR) of 7.8 dB.

This paper presents a high-efficiency wireless power transfer (WPT) system that can charge multiple receivers (Rx's) of various types. For system analysis, the transmitted power and received power levels are derived using the parameters of the resonators based on electro-magnetic (EM) resonance. Using the analysis results, the power customized resonator (PCR) not only for deriving sufficient output power from the transmitter (Tx) but also for appropriately distributing the received power to the various and multiple Rx's is proposed. In addition to the PCR, the Tx unit is designed to work as a load-dependent voltage source using a differential Class-E power amplifier (PA) through a wide range of load impedances. Therefore, the Tx can naturally adapt to the required Tx power levels with a high efficiency corresponding to various Rx configurations without additional tunable circuits or adaptive control schemes. The WPT system, including the load-dependent voltage source for the Tx, full-bridge rectifiers for the Rx's, and the proposed PCR, was designed for the 6.78 MHz frequency band. The proposed system is validated for use with multiple mobile devices by conducting experiments with nine charging cases using three types of Rx's. For all cases, high system efficiencies of 70.7-85.5% were maintained over received power levels of 8.6-45.7 W at a charging distance of 30 mm, and each of the three types of Rx's were experimentally verified to receive sufficient power.

The load networks of advanced Doherty power amplifiers (DPAs) have traditionally been designed according to the ABCD parameters. In this paper, design conditions as an impedance transformation condition and an effective electrical length were used to design the output matching networks (OMNs) for the carrier and peaking amplifiers of the virtual-stub DPAs (VS-DPAs). An optimization method for the effective electrical length was proposed that was specifically constructed for the load impedances of the carrier amplifier at the low power level to have broadband characteristics through the load-pull simulation. Using the optimized design conditions for broadband design, compact OMNs for the carrier and peaking amplifiers were designed using quasi-lumped components. Moreover, an asymmetric structure with an increased power capacity of the peaking amplifier for the VS-DPA was proposed to compensate for a relatively low peak fundamental current of the peaking amplifier due to its deep class-C operation as well as the extended output back-off (OBO) range of the VS-DPA. To verify the proposed load network, a broadband asymmetric VS-DPA for the 3.3 - 4.2 GHz band was designed and implemented using GaN HEMTs with power capacities of 6 and 10 W. Using a 5G new radio (5G NR) signal with a signal bandwidth of 100 MHz and a peak-to-average power ratio (PAPR) of 7.8 dB, power gain of 8.2 - 9.1 dB, DE of 46.0 - 57.0%, ACLR of better than -45 dBc after DPD linearization at an average power of 35.0 dBm were achieved at the broad frequency range of 3.3 - 4.2 GHz.