| Johnson noise thermometers infer temperature from electrical measurement of the thermal noise generated by a resistor, it is one of methods of the few direct measurements of thermodynamic temperature. The important reason which restricts the development of Johnson noise thermometers is measurement time and system deviation, in order to reduce the measurement time and system deviation, we must reduce the amplifier’s background noise and nonlinear distortion. By analyzing the noise figure of the cascaded amplifier model, we can see that the preamplifier’s low-noise low-distortion performance is the most critical. Therefore, the research emphasis of this article is using unattached components to research and design a low-noise low-distortion preamplifier.By analyzing the electronic system internal inherent noise source and noise characteristics of field effect transistor, determine the selection basis of preamplifier’s main components. Analyzing the various low-noise circuit design methods in detail, including differential circuit, cascode circuit, low-noise bias circuit, tail current source and feedback circuit. Making a detailed discusses of amplifier distortion, the effects of distortion. The source of distortion of the differential circuit and the impact of distortion when the components are unmatched were analyzed emphatically. In order to reduce the distortion, designing a simple and easy to use field effect transistor parameter measuring instrument to select the high match transistor. By choosing the low noise components, reasonable PCB layout, taking the appropriate anti interference measures. Finally made a very low-noise low-distortion JFET cascode input preamplifier. Using the Josephson Arbitrary Waveform Synthesizer to produce the standard two-tone and three-tone waveforms to measure the background noise, nonlinear coefficient and so on accurately, the background noise is 1 Hz/n V, the nonlinear coefficient is 0.0075%. |
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