Flux Feedback Concepts and Parallel Feedback Circuit

Summary

This chapter highlights the direct readout scheme with feedback circuits in the “head stage” at cryogenic temperature. Generally, there are two typical kinds of feedback circuits, i.e. the parallel feedback circuit (PFC) and series feedback coil (circuit) (SFC). In a sense, the modified SQUID's “head stage” with PFC (or SFC) can be regarded as a new device with apparent parameters. The chapter discusses the apparent parameters of the SQUID with PFC, e.g. the change in the SQUID's transfer coefficient at the working point. It qualitatively analyzes the PFC in current bias mode, namely, the additional positive feedback scheme. The chapter also focuses on the flux noise contribution of the preamplifier and discusses the flux noise contribution from the Johnson voltage noise. Indeed, for a stable low‐noise SQUID readout technique, operating a SQUID with a PFC in range, or possibly in the transition range is recommended.