A low-noise CMOS preamplifier operating at 4.2 K

A low-noise CMOS readout preamplifier operating at liquid helium temperatures is described, In conjunction with magnetic field sensors applying SQUIDs (superconducting quantum interference devices) the preamplifier can be used to measure biomagnetic fields of human brain and heart noninvasively. The input of the folded cascode amplifier can be attached directly to a low impedance SQUID output. This way the commonly used discrete LC tank resonator circuit for impedance matching can be omitted. An equivalent noise voltage density of 0.3 nV//spl radic/Hz at 500 kHz has been measured. Despite the occurrence of the kink effect and other abnormalities in MOS transistor characteristics at 4.2 K, during the tests no abnormal operation has been observed. Such a preamplifier circuit is essential in simplifying the expensive shielding currently used in biomagnetic diagnosis systems.<
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