Hwiseob Lee

Broadband Doherty Power Amplifier Based on Asymmetric Load Matching Networks

This brief presents asymmetric load matching networks for broadband Doherty power amplifiers (DPAs). The output combiner consists of two λ/4 impedance transformers with optimized characteristic impedance values in order to provide a load modulation at a high output power level; the bandwidth limitation for efficiency at the back-off power level can be then mitigated. For a proper load modulation with the proposed output combiner, asymmetric load matching networks between the carrier and peaking amplifiers are proposed. The input and output matching networks were designed using balanced open stubs, which are more insensitive to frequency than unbalanced open stubs. For the load matching networks, two-section matching networks were adopted. The designed and implemented broadband DPA showed a fractional bandwidth of 23.5% (from 750 to 950 MHz). It also showed measured efficiency values of higher than 55.4% at the peak power level and 51.5% at the 6-dB back-off power level, respectively. For the operational frequency range, a peak output power value of higher than 48 dBm and a gain of 14-15.8 dB were achieved.

High-Efficiency Power Amplifier Using an Active Second-Harmonic Injection Technique Under Optimized Third-Harmonic Termination

This brief presents an active second-harmonic injection technique to improve the efficiency and bandwidth for high-efficiency power amplifiers (PAs). An optimum third-harmonic termination condition was examined for higher efficiency after the second-harmonic injection using a multiharmonic load-pull simulation. It was determined that the optimum third-harmonic termination is the same as that of the inverse class-F PA. Based on this result, a high-efficiency PA with an optimized third-harmonic termination for the second-harmonic injection was designed for a center frequency of 1 GHz as a main amplifier. The overall system requires an auxiliary second-harmonic amplifier and a diplexer between the main and auxiliary PAs. The PA with an optimized third-harmonic termination for the second-harmonic injection was implemented using a 10-W GaN high-electron-mobility transistor for both the main and auxiliary power stages. Compared with the PA without second-harmonic injection, the bandwidth with a power-added efficiency of more than 80% is extended from 60 (960-1020 MHz) to 180 MHz (880-1060 MHz) after the second-harmonic injection.

400 W broadband power amplifier using transmission-line transformers with 1:8 impedance transformation ratio

Abstract This paper presents a broadband high-power amplifier using transmission-line transformers with a high impedance transformation ratio. Two series-connected transformers with 1:8 and 1:1 impedance transformation ratios are used at the load network of a single-balanced high-power amplifier to match the low load impedance in broadband. The same transformer configuration is also used at the input network. A negative feedback is applied to improve the gain flatness and to increase the stability over the broadband. In order to validate the proposed circuit structure, a broadband high-power amplifier is designed and implemented to achieve an output power of more than 400 W in the frequency band of 30 to 500 MHz. The implemented power amplifier exhibits a flat gain response within 1.80 dB at an average of 19.1 dB and high power-added efficiencies of 28.0 to 56.9 at an output power of 400 W over the entire frequency band.

CMOS Power Amplifier Integrated Circuit With Dual-Mode Supply Modulator for Mobile Terminals

A CMOS power amplifier integrated circuit with an optimized dual-mode supply modulator is presented. The dual-mode supply modulator, based on a hybrid buck converter consisting of a wideband linear amplifier and a highly efficient switching amplifier, provides two operation modes: envelope tracking (ET) for high average output power and average power tracking (APT) for low output power. For the APT mode, the linear amplifier is switched off and the switching amplifier operates as a normal buck converter to supply DC voltage to the power amplifier according to the average output power. The optimum switch sizes of the switching amplifier were analyzed and applied for each operation mode for higher efficiency. An integrated circuit with a power amplifier and the dual-mode supply modulator was designed and fabricated using a 0.18-μm CMOS process for LTE applications at a frequency of 0.78 GHz. For the 16-QAM uplink LTE signal, the measured efficiency with an ET mode is as high as 45.4%, which is 7.0% higher than that from the stand-alone power amplifier at an average output power of 24 dBm. An efficiency of 14.1% was achieved with an APT mode at an average output power of 9 dBm. This is 3.2% higher than that with the ET mode.

Design of 1:4 Ultrawideband Hybrid Transmission-Line Balun

This article presents the 1:4 wideband balun based on transmission lines that was awarded the first prize in the Wideband Baluns Student Design Competition. The competition was held during the 2014 IEEE Microwave Theory and Techniques Society (MTT-S) International Microwave Symposium (IMS2014). It was initiated in 2011 and is sponsored by the MTT-17 Technical Coordinating Committee. The winner must implement and measure a wideband balun of his or her own design and achieve the highest possible operational frequency from at least 1 MHz (or below) while meeting the following conditions: ? female subminiature version A (SMA) connectors are used to terminate all ports ? a minimum impedance transformation ratio of two ? a maximum voltage standing wave ratio (VSWR) of 2:1 at all ports ? an insertion loss of less than 1 dB ? a common-mode rejection ratio (CMRR) of more than 25 dB ? imbalance of less than 1 dB and 2.5?.

Optimized Current of the Peaking Amplifier for Two-Stage Doherty Power Amplifier

This paper presents a method of improving efficiency for the two-stage Doherty power amplifier (DPA) using the optimized current of the peaking amplifier. The DPA has a two-stage structure for both the carrier and peaking amplifiers. The first stage of the peaking amplifier has an adjusted bias condition for a near Class-B operation, while the first stage of the carrier amplifier has a higher Class-AB operation. The gain expansion of the first stage due to its lower gate bias helps the second stage of the peaking amplifier to be biased for light Class-C operation and to have steeper turn-ON characteristics, which leads higher peak output power and higher back-off efficiency. The two-stage DPA was designed for the 2.655-GHz band. Using a downlink long-term evolution signal with a signal bandwidth of 10 MHz and a peak-to-average power ratio of 6.5 dB, the overall power gain of 25 dB and a peak output power of 54.2 dBm are experimentally obtained. Using an optimized shape of the peaking amplifier’s current, a drain efficiency (DE) of 53% and an adjacent channel leakage power ratio of −30 dBc were obtained at an average output power of 47.8 dBm. A DE of 56.8% and an adjacent channel leakage power ratio of −25 dBc were also obtained at an average output power of 49.5 dBm.

Doherty Power Amplifier Based on the Fundamental Current Ratio for Asymmetric cells

This paper presents a Doherty power amplifier (DPA) based on asymmetric cells using an even input power drive and an appropriate peak fundamental current ratio (FCR) between the peaking amplifier and the carrier amplifier. Using an appropriate output combiner and an even input power drive for the DPA, not only better load modulation but also enhanced power gain can be achieved. After providing an analysis for the fundamental currents, a design guide for the proposed DPA using the peak FCR is presented. For verification, conventional and proposed DPAs were designed using GaN-HEMTs for the 2.14-GHz band. Using a downlink long-term evolution signal having a peak-to-average power ratio of 6.5 dB, comparably better performances were achieved than the conventional DPAs, such as a high power-added efficiency of 55.7% and a power gain of as high as 16.6 dB, at an average output power level of 36.9 dBm.

Symmetric Three-Way Doherty Power Amplifier for High Efficiency and Linearity

This brief presents a three-way Doherty power amplifier (DPA) with a symmetric structure in terms of the output power capacities between the carrier and peaking amplifiers for high efficiency and linearity. Based on the analysis for the efficiency peak at the output power back-off, a symmetric structure was adopted to have higher overall efficiency for the modulated signal. Through the optimized bias condition for the two peaking amplifiers, the proposed three-way DPA can be linearized in the wide output power range. To validate the proposed scheme, a three-way DPA was designed and implemented using a 60 W GaN–HEMT for the carrier amplifier and two 30 W GaN–HEMTs for the peaking amplifiers. Using a 2.14 GHz long-term evolution downlink signal with a peak-to-average power ratio of 6.5 dB, the implemented symmetric three-way DPA exhibited a drain efficiency (DE) of 33.7% and an ACLR of −38 dBc at an average output power of 41.4 dBm. Compared to the condition with the same bias for the peaking amplifiers, the DE and average output power were improved by 8.0% and 2.4 dB for the given ACLR of −38 dBc, respectively.

Balanced\/Unbalanced Wideband: A Wideband, Bifilar Transmission-Line Balun

This article presents the design details of a wideband, bifilar transmission-line balun submitted for the student design competition for wideband baluns held during the 2015 IEEE Microwave Theory and Techniques Society (MTT-S) International Microwave Symposium (IMS 2015) in Phoenix, Arizona, and sponsored by the MTT-17 Technical Committee on HF-VHF-UHF Technology.

Doherty power amplifier using a compact load network for bandwidth extension

This paper presents a Doherty power amplifier (DPA) with a compact load network for an extended bandwidth compared to the conventional DPA. The compact load network takes less area since it has just one load matching network after the quarter-wave transmission line and no offset-lines. For the verification, both conventional and compact DPAs are designed and built using GaN HEMT for the band from 650 to 800 MHz. They were evaluated using the down-link 16-QAM FDD LTE signal that has a PAPR of 9.96 dB and a signal bandwidth of 5 MHz. At an average output power of 34 dBm, which is approximately 9 dB back-off from the 1 dB compression point, the proposed compact DPA exhibited a power-added efficiency (PAE) level of more than 40% and an adjacent channel leakage power ratio (ACLR) level of lower than -25 dBc over the frequency band of from 650 to 800 MHz, while the conventional DPA showed a PAE level of more than 26.6% and an ACLR level lower than -23 dBc. In addition, the proposed compact DPA has a reduced circuit size by 40.9% compared to the conventional one.