Stabilizing a SQUID-based amplifier in high magnetic fields*

dc SQUID-based circuits, when connected to sources exposed to strong magnetic fields, are often unstable in their fluxed-locked mode. We present a low noise SQUID-based current amplifier, which is unconditionally stable with a source exposed to magnetic fields up to 10 T.
Stability is achieved by inserting a four-pole low-pass filter between the source and the current
amplifier. With this circuit, we demonstrate Johnson noise measurements below 1 K for source
impedances in the kΩ range subjected to strong magnetic fields. We also discuss preliminary
high-resolution thermal transport data in a two-dimensional electron gas confined in GaAs/AlGaAs.