DC SQUID AMPLIFIER FOR NMR EXPERIMENTS ON SMALL SAMPLES

Low Noise preamplifiers are required for the detection of the magnetic resonance of small size samples or at low magnetic fields. According to calculations based on the ''e(n), i(n)'' model. SQUID based amplifi ers are more suitable to NMR experiments on small samples than semicon ductor amplifiers. We propose an amplification method by frequency tra nsposition using a DC SQUID. Such a system can be run over a broad ran ge of frequency as the noise impedance of the stages subsequent to the SQUID is continually optimised. Tile measurements are in good agreeme nt with our theoretical model based on a simplified voltage to flux SQ UID characteristic.