High Dynamic Range SQUID Readout for Frequency-Domain Multiplexers

A 16-SQUID array has been designed and fabricated, which shows 0.12 μΦ ₀ Hz^−1/2 flux noise at 4.2 K. The readout amplifier based on a cryogenic silicon-germanium bipolar transistor employs short-delay negative flux feedback and reaches 7 MHz bandwith for a 1 Φ _0p-p signal. The −1 dB compression is reached approximately at 4.2 Φ _0p-p amplitude when the signal frequency is 1 MHz. In the feedback mode the flux noise is anomalously increased to 0.35 μΦ ₀ Hz^−1/2.