低频低噪声测量放大器的设计

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 摘 要: 低频噪声是表征半导体器件质量和可靠性的一个重要敏感参数,为了能够测量电子器件低频噪声,使用分立器件SSM?2220组成偏置电路,由ADA4898?1构成前置放大器,采用噪声匹配变压器法设计一种测量低频低噪声的放大器。实验结果表明:在频率为80 kHz以下,放大器输入端共模抑制比高出集成运放OP?37 228 dB,其系统的噪声系数低于前置放大器ADA?40752 0.3 dB,满足低频低噪测量放大器的设计要求。 
  关键词: 分立元件; 前置放大器; 噪声匹配变压器; 低频噪声测量 
  中图分类号: TN722.3?34 文献标识码: A 文章编号: 1004?373X(2016)10?0116?04 
  Design of low frequency and low noise measuring amplifier 
  CHEN Xiaojuan1, FAN Xinxin2, WU Jie3 
  (1. College of Electronical and Information Engineering, Changchun University of Science and Technology, Changchun 130022, China; 
  2. School of Information Engineering, Northeast Dianli University, Jilin 132012, China; 
  3. School of Electrical & Information Engineering, Beihua University, Jilin 132013, China) 
  Abstract: The low?frequency noise is an important sensitive parameter to character the quality and reliability of semiconductor devices. To detect the low?frequency noise of electronic devices, an amplifier measuring the low?frequency and low noise was designed by means of the method of noise matching transformer, in which there are a biasing circuit composed of the discrete device SSM?2220 and a preamplifier constituted with ADA4898?1. The experimental results show that the common?mode rejection at input end of the amplifier is 228 dB higher than that of OP?37 as the frequency is below 80 kHz, and the system noise coefficient is 0.3 dB lower than that of preamplifier ADA?40752. This amplifier can meet the design requirements of low frequency and low noise detecting amplifier. 
  Keywords: discrete component; preamplifier; noise matching transformer; low frequency noise detection 
  0 引 言 
  低频噪声是表征半导体器件质量和可靠性的一个重要敏感参数[1?2],然而,常温下电子器件低频噪声极其微弱,其噪声电压往往集中于nV级别,这给噪声的测量带来了很大的困难。 
  GaAsFET(砷化镓场效应晶体管)具有超低噪声系数,被广泛用来设计前置低噪声放大器,但是GaAsFET适用于在高频、超高频以及微波无线电频放大器电路,无法满足对低频放大器的设计要求[3],因此迫切需要设计出能应用于低频超低噪声放大同时具有类似于GaAsFET超低噪声系数的放大器。 
  1 低噪声测量放大器设计的基本结构 
  低频噪声测量放大器设计的基本结构如图1所示,它包括噪声匹配变压器、前置级放大电路、带通滤波器和后级放大电路四部分。 
  式中:K为波尔兹曼常数1.38×10-23 J/K;T取296 K,Eni为单位频率下的等效输入噪声;RS取决于待测信号源的阻抗,本文中信号源内阻RS取100 Ω,1 kHz时,Eni为1.47×10-9 V/(Hz)1/2,计算出放大器的系统噪声系数为1.22 dB ,说明设计的放大器对本底噪声的抑制能力很强,具有GaAsFET 的低噪声系数特征,优于同类型市场上的前置放大器,比如ADI公司的麦克风前置放大器(ADA?40752)性能为1.5 dB超低噪声系数。 
  2.5 放大器的性能指标 
  根据上述分析的结果,可以确定要设计nV级低频噪声测量放大器的主要性能指标如下:差分放大器的增益为112.00 dB;共模抑制比为353.90 dB;放大器的输入阻抗为1.20×104 Ω,输出阻抗为 4.12×10-5 Ω;通频带为0.90 Hz~80.20 kHz;等效输入噪声为1.47×10-9 V/(Hz)1/2;系统噪声系数为1.22 dB。 
  3 结 语 
  在本文中,设计了nV级低频噪声测量放大器。使用了噪声匹配变压器,降低了放大电路的背景噪声。选用了由分立器件SSM?2220组成差放,消除电路共模信号,运用超低噪声运放ADA4898,组建二级放大电路。实验结果表明,该款噪声测量放大器的噪声系数和共模抑制比远大于市场上大部分低频低噪声放大器的数值 ,并且设计电路满足0.90 Hz~80.20 kHz范围内对低频噪声测量的要求。  参考文献 
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