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Efficiency Enhancement of Doherty Amplifier Through Mitigation of the Knee Voltage Effect
Junghwan Moon,Jangheon Kim,Jungjoon Kim,Ildu Kim,Bumman Kim IEEE 2011 IEEE transactions on microwave theory and techniqu Vol.59 No.1
<P>This paper presents an approach to maximize the efficiency of a Doherty power amplifier (PA) with the knee voltage effect. Since the carrier PA with , which is the usual matching impedance for a carrier PA at a low power region, does not reach to the saturated operation at the 6-dB back-off power level, the maximum efficiency could not be achieved. However, the carrier amplifier can be driven into the saturation using the load impedance larger than and can deliver the maximum efficiency even under the knee voltage effect. The optimized design for the maximum efficiency at the back-off level is derived. The optimized amplifier is analyzed and simulated in terms of its load modulation behavior, efficiency, and output power, then compared with the conventional Doherty PA. The enhanced performance is demonstrated by the Doherty PA built using CREE's GaN HEMT CGH40045 devices at 2.655 GHz. For worldwide interoperability for microwave access applications with a 7.8-dB peak-to-average power ratio, the proposed PA delivers an efficiency of 49.3% at an output power of 42 dBm with an acceptable linearity of 23.1 dBc. The linearity is improved to 43 dBc by employing a digital feedback predistortion technique, satisfying the system linearity specification.</P>
Energy Transfer Between Ce^3+ and Tb^3+ Ions in (Gd_1-x-y Tb_xCe_y)₂(MoO₄)₃Crystals
KIM, Junghwan,KIM, Sung Chul,YU, Yun Sik,KIM, Hongki,MOON, Byung Kee,KIM, Sun Il 동의대학교 기초과학연구소 1994 基礎科學硏究論文集 Vol.4 No.1
The energy transfers Tb^3+(^5D₃)→Ce^3+(5d¹) and Ce^3+(5d¹)→Tb^3+(5^D₄) in (Gd_1-x-y Tb_xCe_y) crystals were studied using time-resolved spectroscopy. It was found that the energy transfer Tb^3+(^5D₃)→Ce^3+(5d¹) occurs via the dipole-dipole interaction. The critical distance of the energy transfer was obtained as 13.2Å at 30 K. At room temperature, the strong 5^D₃-5^D₄cross-relaxation of Tb^3+ ions takes place simultaneously with the energy transfer Tb^3+(^5D₃)→Ce^3+(5d¹). The critical distance of cross relaxation in these crystal was 11.6Å at room temperature. By measuring lifetimes and emission intensities, the efficiencies of the energy transfers from Tb^3+(5^D₃) to Ce^3+ and from Ge^3+ to Tb^3+(5^D₄) were calculated.
Junghwan, Kim,Sang-Mun, Jung,Kyu-Su, Kim,Sang-Hoon, You,Byung-Jo, Lee,Yong-Tae, Kim The Korean Electrochemical Society 2022 Journal of electrochemical science and technology Vol.13 No.4
With the emerging importance of catalysts for water electrolysis, developing efficient and inexpensive electrocatalysts for water electrolysis plays a vital role in renewable hydrogen energy technology. In this study, a 1nm thickness of TiC-supported Ru catalyst for hydrogen evolution reaction (HER) has been successfully fabricated using an electron (E)-beam evaporator and thermal decomposition of gaseous CH<sub>4</sub> in a furnace. The prepared Ru/TiC catalyst exhibited an outstanding performance for alkaline hydrogen evolution reaction with an overpotential of 55 mV at 10 mA cm<sup>-2</sup>. Furthermore, we demonstrated that the outstanding HER performance of Ru/TiC was attributed to the high surface area of the support and the metal-support interaction.
Kim, Eun-Shik,Lee, So-Hee,Kim, Joon-Bum,Kim, Chan-Soo,Yoon, Bong-Taek,Lee, Sung-Hoon,Lim, Wontaek,Kim, Hyojung,Choi, Junghwan,Han, Hyerim The Ecological Society of Korea 2015 Journal of Ecology and Environment Vol.38 No.1
In 2013, the epidemics of pine wilt disease caused by the pine wood nematodes (Bursaphelenchus xylophilus) resulted in damages to the forests of black pine (Pinus thunbergii Parl.) trees in Jeju Island, Korea. Among the affected trees, an old black pine tree at Cave Temple on Mt. Sanbangsan was included and died due to the prevalence of pine wilt disease. The tree was on Mt. Sanbangsan, which was designated as a National Scenic Place with the Number 77 and was believed to be more than 400 years old in age. By examining the disc of the tree stem obtained from the height of 2 m, we counted the tree rings from 4 different directions and cross-dated the readings by comparing the records of drought simulated from the BROOK Model. Our analysis indicates that the tree seems to have grown since late 1860s. Contrary to the belief of the general public, we can conclude that the age of the tree was estimated to be at maximum 150 years, which means that it was not the same old tree as was shown in the painting of the Tam-Ra-Sun-Ryeok-Do (an old painting book for the Inspection Tour of Jeju Island) published in 1702. Discussion was extended to the life history of the tree in growth and leaning and the measures to protect the tree species from the damages of the pine wilt disease caused by pine wood nematodes.
Kim, Junghwan,Kim, Heejoo,Kim, Geunjin,Back, Hyungcheol,Lee, Kwanghee American Chemical Society 2014 ACS APPLIED MATERIALS & INTERFACES Vol.6 No.2
<P>We report a new method for developing a low-temperature solution processed vanadium oxide (s-VO<SUB><I>x</I></SUB>) and poly(4-styrene sulfonic acid) (PSS) composite to act as an efficient hole-transport layer (HTL) in polymer solar cells (PSCs). By compositing the s-VO<SUB><I>x</I></SUB> and PSS (s-VO<SUB><I>x</I></SUB>:PSS), the work function values of the s-VO<SUB><I>x</I></SUB>:PSS changed from 5.0 to 5.3 eV. Therefore, the energy level barrier between the HTL and organic active layer decreased, facilitating charge injection/extraction at the interfaces. In addition, the s-VO<SUB><I>x</I></SUB>:PSS films were denser and had more pin-hole-free surfaces than pristine s-VO<SUB><I>x</I></SUB> films, resulting in enhanced PSC performance due to significantly decreased leakage currents and excellent device stability in ambient condition. Because our approach of combining soluble transition metal oxide (TMO) and polymeric acid shows dramatically better performance than pristine TMO, we expect that it can provide useful guidelines for the synthesis and application of TMOs for organic electronics in the future.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/aamick/2014/aamick.2014.6.issue-2/am404353y/production/images/medium/am-2013-04353y_0007.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/am404353y'>ACS Electronic Supporting Info</A></P>
Kim, Junghwan,Kim, Huiyong,Kim, Jayeop,Hwang, Sung June,Lee, Kwang Soon Elsevier 2018 Journal of industrial and engineering chemistry Vol.61 No.-
<P><B>Abstract</B></P> <P>When screening solvents for carbon capture, a large amount of property data for each solvent in the CO<SUB>2</SUB>-loaded state is required. In this paper, a novel experimental method is proposed that involves modifying a wetted wall column in order to simultaneously measure the absorption rate, viscosity, pH, and heat of reaction of carbon capture solvents while the CO<SUB>2</SUB> loading is increased continuously. The proposed method was applied to 30wt% aqueous monoethanolamine and 30wt% aqueous piperazine as solvents and found to produce measurement results that matched well with literature data.</P> <P><B>Highlights</B></P> <P> <UL> <LI> A new wetted wall column setup and experimental procedure were proposed to expedite screening tests for CO<SUB>2</SUB> capture solvents. </LI> <LI> With the new method, CO<SUB>2</SUB> absorption rate, heat of absorption, viscosity, and pH are simultaneously measured. </LI> <LI> The performance of the method was validated for aqueous monoethanolamine and piperazine solvents. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Gate-Bias Control Technique for Envelope Tracking Doherty Power Amplifier
Junghwan Moon(문정환),Jangheon Kim(김장헌),Ildu Kim(김일두),Jungjoon Kim(김정준),Bumman Kim(김범만) 한국전자파학회 2008 한국전자파학회논문지 Vol.19 No.8
본 논문에서는 선형성 증가를 위해 도허티 증폭기의 게이트 바이어스를 조정하는 방식을 제시하였다. 도허티 증폭기의 선형성 향상은 출력 결합 지점에서의 고조파 상쇄를 통해 이루어진다. 하지만 고조파의 상쇄는 그 크기와 위상이 출력 지점에서 같은 크기와 서로 다른 위상을 가지고 있어야 이루어질 수 있는데, 넓은 출력 전력범위에서 위와 같은 조건을 만족시키는 것은 쉽지 않다. 선형성 증가를 위해 도허티 증폭기의 캐리어 증폭기와 피킹 증폭기의 선형성 특성을 입력 전력과 각 증폭기의 게이트 바이어스를 조정함으로써 살펴보았다. 살펴본 특성을 기본으로 하여 고조파 상쇄 전력 범위를 증가시키기 위해, 각 전력 레벨에 맞는 게이트 바이어스를 증폭기에 인가하였다. 게이트 바이어스 제어를 통한 선형성 향상을 알아보기 위해, 2.345 ㎓에서 Eudyna사의 10-W PEP GaN HEMT EGN010MK 소자를 이용하여 도허티 전력 증폭기를 설계하였고, P1㏈로부터 10 ㏈ back-off 지점인 33 ㏈m에서 고효율과 고선형성을 위해 최적화 되었다. WCDMA 1-FA 신호에 대해 제안된 게이트 바이어스 컨트롤 된 도허티 증폭기는 2.8 ㏈의 선형성 개선을 확인할 수 있었으며, 26 %의 PAE를 확인할 수 있었다. 또한, 802.16-2004 신호에 대해 RCE가 2 ㏈ 증가됨을 확인할 수 있었다. The gate-biases of the Doherty power amplifier are controlled to improve the linearity performance. The linearity improvement mechanism of the Doherty amplifier is the harmonic cancellation of the carrier and peaking amplifier at the output power combining point. However, it is difficult to cancel the harmonic power for the broader power range because the condition for cancelling is varied by power. For the linearity improvement, we have explored the linearity characteristic of the Doherty amplifier according to the input power and gate biases of the carrier and peaking amplifier. To extend the region of harmonic power cancellation, we have injected the proper gate bias to the carrier and peaking amplifier according to the input power levels. To validate the linearity improvement, the Doherty amplifier is designed using Eudyna 10-W PEP GaN HEMT EGN010MKs at 2.345 ㎓ and optimized to achieve a high linearity and efficiency at an average output power of 33 ㏈m, backed off about 10 ㏈ from the P1㏈. In the experiments, the envelope tracking Doherty amplifier delivers a significantly improved adjacent channel leakage ratio performance of -37.4 ㏈c, which is an enhancement of about 2.8 ㏈, maintaining the high PAE of about 26 % for the WCDMA 1-FA signal at an average output power of 33 ㏈m. For the 802.16-2004 signal, the amplifier is also improved by about 2 ㏈, -35 ㏈.