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TitlePreamplifier limitations on the accuracy of Johnson noise thermometers
Publication TypeJournal Article
Year of Publication2000
AuthorsWhite, D.R., and Zimmermann E.
Pagination11 - 23
Date Published2000
ISSN00261394 (ISSN)
KeywordsCapacitance, Error analysis, Feedback amplifiers, Johnson noise thermometers, Thermal noise, Thermometers
AbstractA detailed analysis is presented of noise and input impedance in noise thermometer preamplifiers. The analysis highlights the advantages of cascode amplifiers and the disadvantages of amplifiers employing global feedback. Feedback gives rise to two significant errors, one arising from an increase in the input-noise current, the other from the modification of the amplifier input impedance. With care, the errors arising from thermal-noise sources associated with the preamplifiers can be kept below 1 mK. However the largest error associated with the preamplifiers, typically 10 mK to 30 mK and previously unnoticed, arises from the interaction of the input capacitance and the transmission line inductance. This effect negates the common practice of matching the RC time constants in noise thermometers.

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