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 5 MHz centre frequency where it has the measured noise temperature of 27 K for a 60 Ω 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 μΦ0 Hz−1/2 was obtained. Additionally, we consider the potential of the 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.
- transition edge sensors
- superconducting microwave devices
- superconducting transistors
- bipolar transistors