A LOW-NOISE CMOS PREAMPLIFIER OPERATING AT 4.2-K

A low-noise CMOS readout preamplifier operating at liquid helium tempe ratures is described. In conjunction with magnetic field sensors apply ing SQUIDS (Superconducting QUantum Interference Device) the preamplif ier can be used to measure biomagnetic fields of human brain and heart non-invasively. The input of the folded cascode amplifier can be atta ched directly to a low impedance SQUID output. This way the commonly u sed discrete LC tank resonator circuit for impedance matching can be o mitted. An equivalent noise voltage density of 0.3 nV/square-root Hz a t 500 kHz has been measured. Despite the occurrence of the kink effect and other abnormalities in MOS transistor characteristics at 4.2 K, d uring the tests no abnormal operation has been observed. Such a preamp lifier circuit is essential in simplifying the expensive shielding cur rently used in biomagnetic diagnosis systems.