A low noise preamplifier for AC bridges

This paper describes the design, schematics, and performance of a very low-noise J-EFT preamplifier. The amplifier has a gain flatness better than 1dB in the 500Hz to 1MHz frequency region. Noise measurements on this amplifier yield a voltage noise less 0.5nV/√Hz and current noise less 1fA/√Hz in the 100Hz to 100kHz frequency region. It could be used as the preamplifier of the null detector of AC bridges. With modest changes it could be extended to a wide variety of uses requiring low noise gain.