Universal active dc biasing system for a high-T/sub c/ SQUID based on a liquid-nitrogen-cooled preamplifier

A system for the SQUID active dc biasing was developed and tested. This system provides an opportunity to change the output resistance of the biasing source over a wide range. The white flux-noise of a bicrystal SQUID-gradiometer has been measured in direct readout scheme with three different values of the biasing source output resistance. A maximum level of white noise of 11 /spl mu//spl Phi//sub 0//Hz/sup 1/2/@100 kHz was found in case of maximum output resistance (current-biasing). A white noise of 7,9 /spl mu//spl Phi//sub 0/Hz/sup 1/2/ was measured for the minimum of the biasing source resistance (voltage-biasing). The best level of the flux noise of 6 /spl mu//spl Phi//sub 0//Hz/sup 1/2/ was found at a biasing source resistance close to SQUID normal resistance R/sub n/ (power-biasing). Significant reduction of the cut-off frequency (near 10 kHz instead of 100 kHz) was found for the tested bicrystal SQUID-gradiometers due to the active dc voltage-and power-biasing systems in comparison with the standard current-biasing circuit.