一款JFET低噪声前置放大器的设计

为了与传感器相匹配，得到放大器的最小噪声系数，本文从对结型场效应管的等效输入电压噪声eN及等效输入电流噪声iN的分析中，得到结型场效应管的最佳源电阻比双极型晶体管要高出2-3个数量级的结论，并设计制作了一款结型场效应管低噪声前置放大器实用电路。并对其幅频特性、输入阻抗和等效输入电压噪声进行了测量，结果表明其输入阻抗高达71MΩ，等效输入电压噪声约为0.87nV／√Hz，是一种适合于高内阻传感器的较为理想的低噪声前置放大器电路，也可以通过阻抗变换后用于磁力仪等需要低噪声放大的场所。

Design of JFET Low Noise Preamplifier

In order to match the sensors and obtain the smallest noise coefficients of the amplifier, this paper analyzes the equivalent input voltage noise and input current noise of the JFET, and draws a conclusion that the best source resistance of the JFET is 2-3 order of magnitude larger than BJT, and an applied circuit of low-noise preamplifier is designed. When testing on the amplitude - frequency characteristic, the input impedance and the equivalent input voltage noise, the result indicates that the circuit＇s input impedance is 71MΩ and the equivalent input voltage moise is 0.87nV/√Hz. Therefore, it is an ideal circuit of low noise preamplifier for sensors with high output impedance, and also it can be used for many situations such as magnetometer after impedance transformation.