摘要
基于90 nm SOI CMOS工艺实现的应用于无线局域网的2.4 GHz集成的单刀三掷(SP3T)射频开关和低噪声放大器.射频开关采用了一种低功耗的等效负压偏置方法,该方法能够在不使用负电压的前提下使关断状态晶体管获得等效的负压偏置,从而提高射频开关的线性度.低噪声放大器采用了负反馈技术和导数叠加技术提高线性度,利用导数叠加技术减小低噪声放大器的三阶非线性,进一步提高了负反馈低噪声放大器的线性度.低噪声放大器与射频开关集成,并带有Bypass衰减通路.测试结果表明,射频开关的发射支路实现了0.95 dB的插入损耗和34 dBm的输入1 dB压缩点,蓝牙支路具有1.68 dB的插入损耗和30 dBm的输入1 dB压缩点.在2 V供电下,接收支路在高增益模式下具有15.8 dB的增益,1.7 dB的噪声系数和7.6 dBm的输入三阶交调点,功耗28.6 mW,在Bypass模式下具有7.2 dB的插入损耗和22 dBm的输入三阶交调点.
射频开关(Radio Frequency Switch, RF Switch)是无线通信系统中的重要控制模块,控制着射频通路间的切换,为了保证无线通信质量,它需要具备低的插入损耗以及高的线性度和隔离度.互补金属氧化物半导体(Complementary Metal Oxide Semiconductor,CMOS)工艺的发展推动着射频前端模块集成度的提高,射频开关的设计也逐渐由传统的GaAs pHEMT工艺转向具有更低成本、更高集成度的CMOS工
低噪声放大器(Low Noise Amplifier, LNA)是接收系统中的第一级放大模块,它的增益、噪声和线性度等性能对接收系统有着重要影响.CMOS工艺尺寸的缩减提高了LNA的增益和噪声性能,但恶化了LNA的线性
本文基于90 nm SOI CMOS工艺设计了一个应用于无线局域网(Wireless Local Area Networks, WLAN)的2.4 GHz集成的单刀三掷(Single Pole Three Throw, SP3T)射频开关和低噪声放大器.SP3T射频开关采用了低功耗的等效负压偏置方法,在避免使用负电压下使射频开关具备良好的线性度.LNA利用负反馈技术结合导数叠加技术实现了线性度的提高,导数叠加技术通过线性化输入晶体管的跨导能够减小LNA的三阶非线性,实现负反馈LNA线性度的进一步提高.
1 电路设计
1.1 电路整体结构

图1 电路整体结构
Fig.1 Block diagram of the circuit
1.2 SP3T射频开关

图2 SP3T射频开关原理图
Fig.2 The schematic of the SP3T RF switch

图3 等效负压偏置方法的晶体管偏置方式
Fig.3 Transistor biasing method of the equivalent negative voltage biasing method

图4 仿真的不同偏置方式下射频开关发射支路的IP1 dB
Fig.4 Simulated IP1 dB of the transmitting branch of the RF switch under different biasing methods
1.3 低噪声放大器

图5 低噪声放大器原理图
Fig.5 The schematic of the LNA
1.3.1 负反馈
(1) |
(2a) |
(2b) |
(2c) |
式中:g1为晶体管的跨导,g2和g3分别为跨导的二阶和三阶非线性系数.当忽略二阶交调时,源极退化电感Ls能够使LNA的输入三阶交调点(Input Third Order Intercept Point, IIP3)幅值提高

图6 源极退化电感LNA及其小信号非线性等效电路
Fig.6 Inductively source degenerated LNA and its small signal nonlinear equivalent circuit
反馈电容CFB提供了另一条反馈通路,能够进一步提高LNA的线性度.当放大器中存在两条反馈通路时,IIP3幅值可表示
(3) |
式中:a1、a2和a3分别为放大器的线性增益,二阶和三阶非线性系数,b1,dual和b3,dual分别为反馈系统的线性增益和三阶非线性系数,Tx=a1 fx(x=1,2)为反馈环路x的反馈环路增益,其中fx为反馈环路的反馈系统.由
为简化分析,利用
(4) |
(5) |
(6) |
(7) |

图7 简化的带反馈电容的源极退化电感共源级LNA及其小信号非线性等效电路
Fig.7 Simplified inductively source degenerated common source LNA with feedback capacitor and its small signal nonlinear equivalent circuit
反馈电容CFB为LNA的输出端和输入端提供了一条反馈通路,会直接影响LNA的传输特性,LNA的IIP3会受到源极退化电感Ls和反馈电容CFB等多种因素的影响.在设计过程中,晶体管的栅源电容Cgs和反馈电容CFB通常为fF级,因此,假设,忽略二阶交调的影响,源极退化电感Ls和反馈电容CFB能够使LNA的IIP3获得3/2倍的提升.与单独使用源极退化电感Ls相比,反馈电容CFB能够使LNA的IIP3获得进一步的提升.
负反馈在提高放大器线性度的同时会降低放大器的增益.利用
(8) |
射频开关具有较低的插入损耗和高的线性度,因此接收支路(ANT-RXout)的性能由LNA决定,在此展示接收支路的整体特性.

图8 接收支路IIP3和增益随Ls的变化
Fig.8 The IIP3 and gain of the receiving branch versus Ls

图9 接收支路IIP3和增益随CFB的变化
Fig.9 The IIP3 and gain of the receiving branch versus CFB
1.3.2 导数叠加
导数叠加技术的基本原理是利用辅助晶体管补偿主晶体管跨导的非线性.导数叠加技术通常将主晶体管与辅助晶体管并联,其中主晶体管偏置在饱和区而辅助晶体管偏置在弱反型区.由于主晶体管和辅助晶体管跨导的三阶非线性g3极性相反,当二者的g3幅值相近时,在一定偏置电压范围内便可使输入晶体管总的g3,t为
(9) |
导数叠加技术的优势在于能够直接减小晶体管的g3,并且由于辅助晶体管工作在弱反型区,它在提高LNA线性度的同时对LNA的增益、噪声等性能影响较小.

图10 仿真的接收支路IIP3随Vaux的变化
Fig.10 Simulated IIP3 of the receiving branch versus Vaux

图11 仿真的IIP3随CFB的变化
Fig.11 Simulated IIP3 of the receiving branch versus CFB
1.3.3 输入匹配
源极退化电感是LNA设计中常用的输入匹配结
(10) |
(11) |

图12 LNA简化的小信号等效电路
Fig.12 Simplified small signal equivalent circuit of the LNA
1.3.4 Bypass通路
Bypass通路为接收支路提供一条具有更高线性度的衰减通路,使接收支路能够通过工作模式的切换扩展输入信号的动态范围.在本设计中,Bypass通路主要由串联堆叠晶体管M1~M4以及R1、C6构成的对地RC串联网络构成.当晶体管导通时,晶体管可等效为一个小的导通电阻Ron,
(12) |
式中:Z0代表50 Ω阻抗.通过调整晶体管的尺寸以及R1、C6的值调整Bypass通路的插入损耗.

图13 Bypass通路简化电路
Fig.13 Simplified circuit of the Bypass path
晶体管M1~M4同样采用了等效负压偏置方法,以使其在关断状态时保持良好的关断状态.当接收支路工作在Bypass模式时,接收支路的输入功率通常较大,为避免输入信号泄露至放大通路,在放大通路中添加了开关晶体管SW1和SW2以增强放大通路的关断状态,这也有利于提高Bypass通路的线性度.

图14 SW1和SW2对Bypass通路IIP3的影响
Fig.14 The effect of SW1 and SW2 on the IIP3 of the Bypass path
2 结果与分析
设计采用90 nm SOI CMOS工艺实现,并将芯片焊接至印制电路板上进行测试.

图15 芯片和测试板照片
Fig.15 Photographs of the chip and the measurement board
芯片采用2 V供电电压.

图16 射频开关发射支路和蓝牙支路插入损耗测试结果
Fig.16 Measured insertion loss of the transmitting branch and the Bluetooth branch of the RF switch

图17 射频开关发射支路和蓝牙支路IP1 dB测试结果
Fig.17 Measured IP1 dB of the transmitting branch and the Bluetooth branch of the RF switch

图18 接收支路高增益模式S参数测试结果
Fig.18 Measured S parameters of the receiving branch in high gain mode

图19 接收支路Bypass模式S参数测试结果
Fig.19 Measured S parameters of the receiving branch in Bypass mode

图20 接收支路高增益模式噪声系数测试结果
Fig.20 Measured noise figure of the receiving branch in high gain mode

图21 接收支路高增益模式IIP3测试结果
Fig.21 Measured IIP3 of the receiving branch in high gain mode

图22 接收支路Bypass模式IIP3测试结果
Fig.22 Measured IIP3 of the receiving branch in Bypass mode
设计的接收支路性能与其他相近设计的性能对比如
文献 | 集成开关 | 频率/GHz | 增益/dB | 噪声系数/dB | IIP3/dBm | 工作电压/V | 功耗/mW | 面积/m | 工艺 |
---|---|---|---|---|---|---|---|---|---|
文献[ | 是 | 2.4 | 12.0 | 1.9 | 10 | 3.3 | 26.4 | — | 500 nm GaAs PHEMT |
文献[ | 是 | 2.4 | 11.7 | 3.4 | 2.1 | 3.3 | 35.3 | 1.81 | 180 nm CMOS |
文献[ | 是 | 2.4 | 13.5 | 2.8 | -1.5 | 3.3 | 16.8 | 1.54 | 180 nm SiGe BiCMOS |
本设计 | 是 | 2.4 | 15.8 | 1.7 | 7.6 | 2 | 28.6 | 0.51 | 90 nm SOI CMOS |
3 结 论
本文基于90 nm SOI CMOS工艺设计了一个 2.4 GHz集成的LNA和SP3T射频开关.射频开关采用了等效负压偏置技术以获得良好的线性度.LNA采用了负反馈和导数叠加技术,利用导数叠加技术实现了负反馈LNA线性度的进一步提高.经测试,射频开关发射支路和蓝牙支路分别具有34 dBm和 30 dBm的IP1 dB,实现了良好的线性度.接收支路在高增益模式下实现了15.8 dB的增益.1.7 dB的噪声系数和7.6 dBm的IIP3,验证了导数叠加技术对LNA线性度的提升作用.在Bypass模式下,接收支路实现了7.2 dB的插入损耗和22 dBm的IIP3.总体上,本设计的射频开关和LNA实现了较为优良的射频性能,能够应用于WLAN等无线通信系统,具有一定的研究和应用价值.
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