噪声温度计用放大器设计与性能评估

Development of a High-performance Amplifier for Johnson Noise Thermometer

低噪声、低失真、高共模抑制比的放大器对于噪声法测量热力学温度至关重要。设计了由前置放大、仪表放大和缓冲放大3级组成的噪声温度计用高性能放大器。其中,采用具有超低噪声的结型场效应对管IF9030作为前置差分放大级的输入;研制了自动化测量对管转移特性的装置,用于在大批量对管中挑选匹配度更优的器件;采用互相关技术压低电路中不相关的噪声,从而确保能够准确测量到被放大器背景噪声淹没的非线性失真信号;重点评估了放大器在1 MHz带宽内的共模抑制比和放大器受双频激励时的非线性失真。测量结果表明,放大器的背景噪声为1.2 nV/Hz 1/2;100 kHz带宽内的共模抑制比优于90 dB,100 kHz至1 MHz带宽内的共模抑制比会随着频率的增加而降低,但不低于70 dB;放大器受频率为400 kHz、401 kHz,幅度有效值为8μV的双频信号激励时,产生的二阶交调失真信号比双频信号低79 dB,二阶谐波失真信号比双频信号低85 dB。

A amplifier with low-noise,low-distortion,and high common-mode rejection ratio(CMRR)is critical for the noise thermometer.A high-performance amplifier for noise thermometers composed of three stages of preamplification,instrumentation amplification and buffer amplification is designed.An ultra-low noise junction-field-effect-transistor(JFET)pair IF9030 is used as the input of the pre-differential amplifier stage.An instrument is developed to automatically measure the transfer characteristics of the JFET pair,so as to achieve a better matching of the characteristics.Cross-correlation technologies are performed to suppress the uncorrelated noise in the circuit,thus the nonlinear distortions under the background noise could be accurately measured.The results show that the background noise of the amplifier is about 1.2 nV/Hz 1/2.The CMRR within 100 kHz is higher than 90 dB,and in the 100 kHz to 1 MHz,the CMRR decreases to lower values with the frequency increasing.fortunately,it is still larger than 70 dB even at frequencies close to 1 MHz.When the amplifier is excited by a two-tone signal with a frequency of 400 kHz,401 kHz and an effective amplitude of 8μV,the second-order intermodulation distortion and the second-order harmonic distortion are 79 dB and 85 dB lower than the fundamental,respectively.