A low-noise amplifier system with a 3 dB bandwidth of 5 mHz to 150 kHz and an input impedance of the order of 1 Omega is described. The difficulty of bandwidth limitation due to input transformers was overcome by inserting the transformers into a negative feedback loop. The system is used for the investigation of flux flow noise in type II superconductors.
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Dirks et al. (1977) studied this question.