Noise measurements on proximity effect bridges

Audio frequency noise density measurements have been performed on weakly superconducting proximity effect bridges using a cooled transformer and room temperature low noise preamplifier. The noise temperature of the measuring system is approximately 4°K for a 0.9Ω resistor. Noise density was measured as a function of bias current and temperature for the bridges. Excess noise above that expected from Johnson noise for a resistor equal to the dynamic resistance of the bridges, was observed in the region near the critical current of the device. At high currents, compared to the critical current, the noise density closely approaches that given by Johnson noise.