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We have built a room temperature amplifier based on Si JFETs (junction field effect transistors) intended for ac-coupled SQUID (superconducting quantum interference device) readouts, such as in frequency-domain multiplexed transition-edge sensor systems. The amplifier operates at 5M Hzcentre frequency where it has the measured noise temperature of 27 K for a 60 � load, which includes the noise from the active termination and the room temperature transformer. When it was driven from a SQUID with an on-chip matching transformer, flux noise of 0.45 µ� 0 Hz −1/2 was obtained. Additionally, w ec onsider the potential o ft he recently introduced SiGe bipolar transistors for the same application. The dynamic range considerations for SQUID multiplexers, which require the use of an amplifier with a low noise temperature, are briefly discussed. (Some figures in this article are in colour only in the electronic version)
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Kiviranta et al. (2006) studied this question.
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