  To better understand the working principles of Superconducting Quantum
Interference Device (SQUID) direct readout schemes, which work in different
bias and amplifier modes with different internal feedback schemes, we present
the complete circuit analyses on SQUID small-signal model. SQUID bias and
amplifier circuits are analyzed using SQUID Thevenin equivalent circuit, and
the general equivalent circuit of SQUID with different internal feedback
schemes is derived and analyzed with a trans-impedance amplifier model.
Transfer characteristics and noise performances of different direct readout
schemes are analyzed and experimentally characterized. It is shown that
amplifier noise suppression is only dependent on SQUID flux-to-voltage transfer
coefficient and is irrelevant to the configuration of bias and amplifier; SQUID
with an internal feedback scheme improves the transfer coefficient with voltage
feedback and regulates the dynamic resistance with current feedback.
