基于CMOS工艺的超宽带低噪声放大器设计
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摘要
近年来,无线通讯和CMOS工艺得到了很大发展,射频集成电路可以用CMOS工艺来实现。在各种无线通讯新技术中,超宽带技术(Ultra-Wide Band,UWB)以其高速率、低功耗的特点受到无线通讯学术界的广泛重视,是一种具有巨大发展潜力的新技术。低噪声放大器是无线接收机的第一级,其良好性能保证了射频前端的正常工作。研究基于CMOS工艺的超宽带低噪声放大器,具有重要理论与实践意义。
本文首先对超宽带低噪声放大器的研究现状和CMOS集成电路的特点作了论述。其次,分析了几种常见的输入匹配电路,比较了各电路的优缺点,研究设计了巴特沃斯滤波器作为输入匹配电路。共源共栅结构具有增益大,密勒效应小,结构简单等优点,本文决定将其作为电路主放大结构,然后用基于功耗限制的噪声优化理论推导出了输入MOS管的最优尺寸,并采用级间匹配结构来提高增益降低噪声。为满足宽带需要,电路设计采用了电容耦合负载电路。本文还分别设计了电压偏置电路和电流源电路,以促进低噪声放大器的性能最优化。
超宽带低噪声放大器的电源电压为1.5V,采用台积电0.18μmCMOS工艺。用ADS2008软件进行仿真,仿真结果显示：在3～5GHz带宽内,噪声系数低于2.3dB,增益大于12dB,最大增益大于14dB,增益波动小于2.4dB,一分贝压缩点约为-15dBm。输入输出匹配均优于-10dB,功耗为19mW。
With the rapid development of wireless communication technology and CMOS process in recent years, RFIC can be achieved by CMOS process.One of new wireless communication techniques is UWB, which people from wirless communication academe attach great importance to,because of its high data speed and low power consumption. So UWB can be considered as a new huge development potential technology. Low noise amplifier is the first stage in the wireless transceiver, its high performance can ensure front-end of RF working successfully.So the research of ultra-wideband CMOS LNA is great importance for theory and practice.
Firstly,this paper discussed something about ultra-wide band LNA study actuality and characteristic of CMOS RFIC.Secondly, we analyzed several familiar input impedance match circuits,and compared their advantages and disadvantages. After that,butterworth filter was designed as input match circuits.The cascode structure has many advantages,such as good gain, small Miller effect, simple structure etc.So the paper chose it as primary magnifying structure of circuits. Thirdly,we educed the optimal size of input transistor by noise improved theory under power limit. A stage matching scheme was used for heightening gain and reducing noise figure. Capacitance-coupling load circuit was used for fulfilling the demand of wide frequency. To optimize the performance of LNA,the paper designed voltage biased circuit and current-source circuit.
The voltage of ultra-wide band LNA is 1.5V,based on TSMC 0.18μm CMOS process.And this LNA is simulated by ADS2008.The simulation results show that,it works in 3-5GHz, the noise figure is under 2.3dB.The gain is better than 12dB,and the maximum gain is above 14dB.The gain fluctuating range is less than 2.4dB.1dB compression point is about-15dBm. The input and output impedance match are better than-10dB. The power consumption is 19mW.
本文首先对超宽带低噪声放大器的研究现状和CMOS集成电路的特点作了论述。其次,分析了几种常见的输入匹配电路,比较了各电路的优缺点,研究设计了巴特沃斯滤波器作为输入匹配电路。共源共栅结构具有增益大,密勒效应小,结构简单等优点,本文决定将其作为电路主放大结构,然后用基于功耗限制的噪声优化理论推导出了输入MOS管的最优尺寸,并采用级间匹配结构来提高增益降低噪声。为满足宽带需要,电路设计采用了电容耦合负载电路。本文还分别设计了电压偏置电路和电流源电路,以促进低噪声放大器的性能最优化。
超宽带低噪声放大器的电源电压为1.5V,采用台积电0.18μmCMOS工艺。用ADS2008软件进行仿真,仿真结果显示：在3～5GHz带宽内,噪声系数低于2.3dB,增益大于12dB,最大增益大于14dB,增益波动小于2.4dB,一分贝压缩点约为-15dBm。输入输出匹配均优于-10dB,功耗为19mW。
With the rapid development of wireless communication technology and CMOS process in recent years, RFIC can be achieved by CMOS process.One of new wireless communication techniques is UWB, which people from wirless communication academe attach great importance to,because of its high data speed and low power consumption. So UWB can be considered as a new huge development potential technology. Low noise amplifier is the first stage in the wireless transceiver, its high performance can ensure front-end of RF working successfully.So the research of ultra-wideband CMOS LNA is great importance for theory and practice.
Firstly,this paper discussed something about ultra-wide band LNA study actuality and characteristic of CMOS RFIC.Secondly, we analyzed several familiar input impedance match circuits,and compared their advantages and disadvantages. After that,butterworth filter was designed as input match circuits.The cascode structure has many advantages,such as good gain, small Miller effect, simple structure etc.So the paper chose it as primary magnifying structure of circuits. Thirdly,we educed the optimal size of input transistor by noise improved theory under power limit. A stage matching scheme was used for heightening gain and reducing noise figure. Capacitance-coupling load circuit was used for fulfilling the demand of wide frequency. To optimize the performance of LNA,the paper designed voltage biased circuit and current-source circuit.
The voltage of ultra-wide band LNA is 1.5V,based on TSMC 0.18μm CMOS process.And this LNA is simulated by ADS2008.The simulation results show that,it works in 3-5GHz, the noise figure is under 2.3dB.The gain is better than 12dB,and the maximum gain is above 14dB.The gain fluctuating range is less than 2.4dB.1dB compression point is about-15dBm. The input and output impedance match are better than-10dB. The power consumption is 19mW.
引文
[]Zhe-Yang Huang, Che-Cheng Huang, Chun-Chieh Chen and Chung-Chih Hung. A lV-2.39mW capacitor-coupling resonated low noise amplifier for 3-5GHz ultra-wideband system. IEEE International SOC Conference.2007:101-104.
[2]王晓红.超宽带CMOS低噪声放大器的研究。华东师范大学硕士论文.2008：9-21.
[3]李唐,刘亚峰.超宽带无线通信技术概述.天津通信技术.2003：35-38.
[4]Hui Zheng; Shuzuo Lou; Dongtian Lu; Cheng Shen; Tatfu Chan; Luong, H.C. A 3.1 GHz-8.0 GHz Single-Chip Transceiver for MB-OFDM UWB in 0.18-μm CMOS Process.Solid-State Circuits, IEEE Journal of.2009.44(2):414-426.
[5]Zhe-Yang Huang, Che-Cheng Huang, Yeh-Tai Hungand Meng-Ping Chen. A CMOS Current Reused Low-Noise Amplifier for Ultra-Wideband Wireless Receiver. ICMMT International Conference on Microwave and Millimeter Wave Technology.2008:1499-1502.
[6]池保勇,余志平,石秉学.CMOS射频集成电路分析与设计.清华大学出版社,2006：50-100.
[7]毕查德.拉扎维.射频微电子.余志平,周润德等译.清华大学出版社,2006：50-100.
[8]曹克.低电压低功耗CMOS射频低噪声放大器设计.清华大学博士论文.2005：10-30.
[9]Chun-Chieh Chen, Sheng-Hsiang Yen, Zhe-Yang Huangt, Meng-Ping Chen, and Yeh-Tai Hung. A Low-Power and Low-Noise Amplifier for 3-5GHz UWB Applications. International Symposium on Intelligent Signal Processing and Communications.2006:191-194.
[10]Zhe-Yang Huang, Che-Cheng Huang, Chun-Chieh Chen and Chung-Chih Hung. A 1V CMOS low-noise amplifier with inductive resonated for 3.1-10.6GHz UWB wireless receiver. IEEE International SOC Conference.2007:15-18.
[11]Zhe-Yang Huang, Che-Cheng Huang, Meng-Ping Chen and Yeh-Tai Hung. Design Considerations on Input Impedance Matching for Ultra-Wideband Low-Noise Amplifier. International Conference on Microwave and Millimeter Wave Technology,2008:245-248.
[12]Ro-Min weng, Ron-Chi kuo, Po-Cheng lin. An Ultra-Wideband LNA with Notch Filter. 17th International Conference on Radioelektronika.2007:1-4.
[13]Md. Mahbub Reja, Igor Filanovsky, and Kambiz Moez. A CMOS 2.0-11.2 GHz UWB LNA Using Active Inductor Circuit. IEEE International Symposium on Circuits and Systems.2008:2266-2269.
[14]Zhe-Yang Huang, and Che-Cheng Huang. A Switching-Band CMOS Low-Noise Amplifier for 6-11GHz MB-OFDM UWB Wireless Radio System.7th International Conference on ASIC.2007:411-414.
[15]Zhe-Yang Huang, Che-Cheng Huang. A CMOS Low Noise Amplifier with RLC-Impedance Feedback for 3-5GHz Ultra-Wideband Wireless System. ISIC International Symposium on Integrated Circuits.2007:600-603.
[16]杨光.超宽带射频前端的分析与设计.复旦大学硕士论文.2009：23-32.
[17]杨帆.3～5GHz宽带CMOS低噪声放大器研究与设计.电子科技大学硕士论文.2007：25-80.
[18]王贵,华明清,王志功,唐万春.0.18μmCMOS工艺3.1～5.2 GHz低噪声放大器.固体电子学研究与进展.2009,第29卷第1期：41-45.
[19]申华,吕昕,多新中,杨立吾.0.13μmCMOS超宽带低噪声放大器的设计.电子设计应用.2008：66-72.
[20]罗志勇,李巍,任俊彦.超宽带CMOS低噪声放大器的设计.微电子学.2006,第36卷第5期：688-692.
[21]华明清,王志功,李智群.0.18-gmCMOS 3.1-10.6GHz超宽带低噪声放大器设计.电路与系统学报.2007,第12卷第1期：44-47.
[22]罗志勇,李巍,李宁,杨光,任俊彦.CMOS全差分超宽带低噪声放大器.固体电子学研究与进展.2008,第28卷第4期：554-558.
[23]刘萌萌,张盛,王硕,张建良,周润德.具有抑制窄带干扰带阻特性的3～10 GHz超宽带LNA.清华大学学报(自然科学版).2009,第49卷第4期：574-577.
[24]桑泽华,李永明.一种应用于超宽带系统的宽度LNA的设计.微电子学.2006,第36卷第1期：114-117.
[25]石保勇.无线局域网收发机射频前端的CMOS实现.清华大学博士论文.2003：20-60.
[26]杨光.超宽带射频前端的分析与设计.复旦大学硕士论文.2009：15-55.
[27]Thomas H. Lee. CMOS射频集成电路设计.北京电子工业出版社,2004：50-100.
[28]李智群,王志功.射频集成电路与系统.科学出版社,2008：50-100.
[29]杜超.CMOS超宽带低噪声放大器和混频器设计.湖南大学硕士论文.2009：10-25.
[30]陈邦媛.射频通信电路.科学出版社,2002：50-100.
[31]Qiang Li; Yue Ping Zhang.A 1.5-V 2-9.6-GHz Inductorless Low-Noise Amplifier in 0.13-μm CMOS. Microwave Theory and Techniques, IEEE Transactions on.2007,55 (10) 2015-2023.
[32]邓建元.0.25μmCMOS工艺2.4GHz低噪声放大器及压控振荡器设计.湖南大学硕士论文.2005：10-50.
[33]Andrea Bevilacquaand Ali M. Niknejad. An Ultrawideband CMOS Low-Noise Amplifier for 3.1-10.6-GHz Wireless Receivers. IEEE Journal of Solid-State Circuits.2004, 12 (5):2259-2268.
[34]Chang, J.-F.; Lin, Y.-S. Low-power, high-gain and low-noise CMOS distributed amplifier for UWB systems. Electronics Letters.2009,45(12):634-636.
[35]Jeong, M.I.; Lee, J.N.; Lee, C.S. Design of UWB switched gain controlled LNA using 0.18 μm CMOS. Electronics Letters.2008,44(7):477-478.
[36]Reinhold Ludwing,Pavel Bretchko.射频电路设计-理论与应用.王子宇,张肇仪,徐承和等译.电子工业出版社,2004：145-155.
[37]Chetty Garuda, Xian Cui, Po-Chih Lin, Seok Joo Doo, Pengbei Zhang, and Mohammed Ismail. A 3-5 GHz Fully Differential CMOS LNA with Dual-gain mode for Wireless UWB Applications.48th Midwest Symposium on Circuits and Systems.2005:790-793.
[38]Ke-Hou Chen; Jian-Hao Lu; Bo-Jiun Chen; Shen-Iuan Liu. An Ultra-Wide-Band 0.4-10-GHz LNA in 0.18-μm CMOS. Circuits and Systems II:Express Briefs, IEEE Transactions on.2007,54(3):217-221.
[39]王磊.UWB射频接收系统相关模块研究.西安理工大学硕士论文.2006：12-30.
[40]毕查德.拉扎维.模拟CMOS集成电路设计.陈贵灿,程军,张瑞智等译.西安交通大学出版社,2003：50-100.
[41]Yu-Tso Lin; Hsiao-Chin Chen; Tao Wang; Yo-Sheng Lin; Shey-Shi Lu.3-10-GHz Ultra-Wideband Low-Noise Amplifier Utilizing Miller Effect and Inductive Shunt-Shunt Feedback Technique. Microwave Theory and Techniques, IEEE Transactions on.2007,55 (9):1832-1843.
[42]张杰.低功耗RFCMOS低噪声放大器的设计.安徽大学硕士论文.2005：15-35.
[43]Liao, C.-F.; Liu, S.-I. A Broadband Noise-Canceling CMOS LNA for 3.1-10.6-GHz UWB Receivers. Solid-State Circuits, IEEE Journal of.2007,42(2):329-339.
[44]Lee, J.-H.; Chen, C.-C.; Lin, Y.-S..0.18 urn 3.1-10.6 GHz CMOS UWB LNA with 11.4±0.4dB gain and 100.7±17.4ps group-delay.Electronics Letters.2007,43(24):1359-1360.
[45]Bevilacqua, A.; Sandner, C.; Gerosa, A.; Neviani, A. A fully integrated differential CMOS LNA for 3-5-GHz ultrawideband wireless receivers. Microwave and Wireless Components Letters, IEEE.2006,16(3):134-136.
[46]胡柳林.2.45GHz CMOS低噪声放大器设计与仿真.哈尔滨工业大学硕士论文.2006： 36-38.
[47]Chang, J.-F.; Lin, Y.-S.3-10 GHz low-power, low-noise CMOS distributed amplifier using splitting-load inductive peaking and noise-suppression techniques. Electronics Letters.2009,45(20):1033-1035.
[48]Chen-Ming Li; Ming-Tsung Li; Kuang-Chi He; Jenn-Hwan Tarng. A Low-Power Self-Forward-Body-Bias CMOS LNA for 3-6.5-GHz UWB Receivers.Microwave and Wireless Components Letters, IEEE.2010,20(2):100-102.
[49]Reiha, M.T.; Long, J.R. A 1.2 V Reactive-Feedback 3.1-10.6 GHz Low-Noise Amplifier in 0.13μm CMOS. Solid-State Circuits, IEEE Journal of.2007,42(5):1023-1033.
[50]Yang, H.-Y.; Lin, Y.-S.; Chen, C.-C..2.5 dB NF 3.1-10.6 GHz CMOS UWB LNA with small group-delay variation. Electronics Letters.2008,44(8):528-529.
[2]王晓红.超宽带CMOS低噪声放大器的研究。华东师范大学硕士论文.2008：9-21.
[3]李唐,刘亚峰.超宽带无线通信技术概述.天津通信技术.2003：35-38.
[4]Hui Zheng; Shuzuo Lou; Dongtian Lu; Cheng Shen; Tatfu Chan; Luong, H.C. A 3.1 GHz-8.0 GHz Single-Chip Transceiver for MB-OFDM UWB in 0.18-μm CMOS Process.Solid-State Circuits, IEEE Journal of.2009.44(2):414-426.
[5]Zhe-Yang Huang, Che-Cheng Huang, Yeh-Tai Hungand Meng-Ping Chen. A CMOS Current Reused Low-Noise Amplifier for Ultra-Wideband Wireless Receiver. ICMMT International Conference on Microwave and Millimeter Wave Technology.2008:1499-1502.
[6]池保勇,余志平,石秉学.CMOS射频集成电路分析与设计.清华大学出版社,2006：50-100.
[7]毕查德.拉扎维.射频微电子.余志平,周润德等译.清华大学出版社,2006：50-100.
[8]曹克.低电压低功耗CMOS射频低噪声放大器设计.清华大学博士论文.2005：10-30.
[9]Chun-Chieh Chen, Sheng-Hsiang Yen, Zhe-Yang Huangt, Meng-Ping Chen, and Yeh-Tai Hung. A Low-Power and Low-Noise Amplifier for 3-5GHz UWB Applications. International Symposium on Intelligent Signal Processing and Communications.2006:191-194.
[10]Zhe-Yang Huang, Che-Cheng Huang, Chun-Chieh Chen and Chung-Chih Hung. A 1V CMOS low-noise amplifier with inductive resonated for 3.1-10.6GHz UWB wireless receiver. IEEE International SOC Conference.2007:15-18.
[11]Zhe-Yang Huang, Che-Cheng Huang, Meng-Ping Chen and Yeh-Tai Hung. Design Considerations on Input Impedance Matching for Ultra-Wideband Low-Noise Amplifier. International Conference on Microwave and Millimeter Wave Technology,2008:245-248.
[12]Ro-Min weng, Ron-Chi kuo, Po-Cheng lin. An Ultra-Wideband LNA with Notch Filter. 17th International Conference on Radioelektronika.2007:1-4.
[13]Md. Mahbub Reja, Igor Filanovsky, and Kambiz Moez. A CMOS 2.0-11.2 GHz UWB LNA Using Active Inductor Circuit. IEEE International Symposium on Circuits and Systems.2008:2266-2269.
[14]Zhe-Yang Huang, and Che-Cheng Huang. A Switching-Band CMOS Low-Noise Amplifier for 6-11GHz MB-OFDM UWB Wireless Radio System.7th International Conference on ASIC.2007:411-414.
[15]Zhe-Yang Huang, Che-Cheng Huang. A CMOS Low Noise Amplifier with RLC-Impedance Feedback for 3-5GHz Ultra-Wideband Wireless System. ISIC International Symposium on Integrated Circuits.2007:600-603.
[16]杨光.超宽带射频前端的分析与设计.复旦大学硕士论文.2009：23-32.
[17]杨帆.3～5GHz宽带CMOS低噪声放大器研究与设计.电子科技大学硕士论文.2007：25-80.
[18]王贵,华明清,王志功,唐万春.0.18μmCMOS工艺3.1～5.2 GHz低噪声放大器.固体电子学研究与进展.2009,第29卷第1期：41-45.
[19]申华,吕昕,多新中,杨立吾.0.13μmCMOS超宽带低噪声放大器的设计.电子设计应用.2008：66-72.
[20]罗志勇,李巍,任俊彦.超宽带CMOS低噪声放大器的设计.微电子学.2006,第36卷第5期：688-692.
[21]华明清,王志功,李智群.0.18-gmCMOS 3.1-10.6GHz超宽带低噪声放大器设计.电路与系统学报.2007,第12卷第1期：44-47.
[22]罗志勇,李巍,李宁,杨光,任俊彦.CMOS全差分超宽带低噪声放大器.固体电子学研究与进展.2008,第28卷第4期：554-558.
[23]刘萌萌,张盛,王硕,张建良,周润德.具有抑制窄带干扰带阻特性的3～10 GHz超宽带LNA.清华大学学报(自然科学版).2009,第49卷第4期：574-577.
[24]桑泽华,李永明.一种应用于超宽带系统的宽度LNA的设计.微电子学.2006,第36卷第1期：114-117.
[25]石保勇.无线局域网收发机射频前端的CMOS实现.清华大学博士论文.2003：20-60.
[26]杨光.超宽带射频前端的分析与设计.复旦大学硕士论文.2009：15-55.
[27]Thomas H. Lee. CMOS射频集成电路设计.北京电子工业出版社,2004：50-100.
[28]李智群,王志功.射频集成电路与系统.科学出版社,2008：50-100.
[29]杜超.CMOS超宽带低噪声放大器和混频器设计.湖南大学硕士论文.2009：10-25.
[30]陈邦媛.射频通信电路.科学出版社,2002：50-100.
[31]Qiang Li; Yue Ping Zhang.A 1.5-V 2-9.6-GHz Inductorless Low-Noise Amplifier in 0.13-μm CMOS. Microwave Theory and Techniques, IEEE Transactions on.2007,55 (10) 2015-2023.
[32]邓建元.0.25μmCMOS工艺2.4GHz低噪声放大器及压控振荡器设计.湖南大学硕士论文.2005：10-50.
[33]Andrea Bevilacquaand Ali M. Niknejad. An Ultrawideband CMOS Low-Noise Amplifier for 3.1-10.6-GHz Wireless Receivers. IEEE Journal of Solid-State Circuits.2004, 12 (5):2259-2268.
[34]Chang, J.-F.; Lin, Y.-S. Low-power, high-gain and low-noise CMOS distributed amplifier for UWB systems. Electronics Letters.2009,45(12):634-636.
[35]Jeong, M.I.; Lee, J.N.; Lee, C.S. Design of UWB switched gain controlled LNA using 0.18 μm CMOS. Electronics Letters.2008,44(7):477-478.
[36]Reinhold Ludwing,Pavel Bretchko.射频电路设计-理论与应用.王子宇,张肇仪,徐承和等译.电子工业出版社,2004：145-155.
[37]Chetty Garuda, Xian Cui, Po-Chih Lin, Seok Joo Doo, Pengbei Zhang, and Mohammed Ismail. A 3-5 GHz Fully Differential CMOS LNA with Dual-gain mode for Wireless UWB Applications.48th Midwest Symposium on Circuits and Systems.2005:790-793.
[38]Ke-Hou Chen; Jian-Hao Lu; Bo-Jiun Chen; Shen-Iuan Liu. An Ultra-Wide-Band 0.4-10-GHz LNA in 0.18-μm CMOS. Circuits and Systems II:Express Briefs, IEEE Transactions on.2007,54(3):217-221.
[39]王磊.UWB射频接收系统相关模块研究.西安理工大学硕士论文.2006：12-30.
[40]毕查德.拉扎维.模拟CMOS集成电路设计.陈贵灿,程军,张瑞智等译.西安交通大学出版社,2003：50-100.
[41]Yu-Tso Lin; Hsiao-Chin Chen; Tao Wang; Yo-Sheng Lin; Shey-Shi Lu.3-10-GHz Ultra-Wideband Low-Noise Amplifier Utilizing Miller Effect and Inductive Shunt-Shunt Feedback Technique. Microwave Theory and Techniques, IEEE Transactions on.2007,55 (9):1832-1843.
[42]张杰.低功耗RFCMOS低噪声放大器的设计.安徽大学硕士论文.2005：15-35.
[43]Liao, C.-F.; Liu, S.-I. A Broadband Noise-Canceling CMOS LNA for 3.1-10.6-GHz UWB Receivers. Solid-State Circuits, IEEE Journal of.2007,42(2):329-339.
[44]Lee, J.-H.; Chen, C.-C.; Lin, Y.-S..0.18 urn 3.1-10.6 GHz CMOS UWB LNA with 11.4±0.4dB gain and 100.7±17.4ps group-delay.Electronics Letters.2007,43(24):1359-1360.
[45]Bevilacqua, A.; Sandner, C.; Gerosa, A.; Neviani, A. A fully integrated differential CMOS LNA for 3-5-GHz ultrawideband wireless receivers. Microwave and Wireless Components Letters, IEEE.2006,16(3):134-136.
[46]胡柳林.2.45GHz CMOS低噪声放大器设计与仿真.哈尔滨工业大学硕士论文.2006： 36-38.
[47]Chang, J.-F.; Lin, Y.-S.3-10 GHz low-power, low-noise CMOS distributed amplifier using splitting-load inductive peaking and noise-suppression techniques. Electronics Letters.2009,45(20):1033-1035.
[48]Chen-Ming Li; Ming-Tsung Li; Kuang-Chi He; Jenn-Hwan Tarng. A Low-Power Self-Forward-Body-Bias CMOS LNA for 3-6.5-GHz UWB Receivers.Microwave and Wireless Components Letters, IEEE.2010,20(2):100-102.
[49]Reiha, M.T.; Long, J.R. A 1.2 V Reactive-Feedback 3.1-10.6 GHz Low-Noise Amplifier in 0.13μm CMOS. Solid-State Circuits, IEEE Journal of.2007,42(5):1023-1033.
[50]Yang, H.-Y.; Lin, Y.-S.; Chen, C.-C..2.5 dB NF 3.1-10.6 GHz CMOS UWB LNA with small group-delay variation. Electronics Letters.2008,44(8):528-529.