Single JFET front-end amplifier for low frequency noise measurements with cross correlation-based gain calibration

We propose an open loop voltage amplifier topology based on a single JFET front-end for the realization of very low noise voltage amplifiers to be used in the field of low frequency noise measurements. With respect to amplifiers based on differential input stages, a single transistor stage has, among others, the advantage of a lower background noise. Unfortunately, an open loop approach, while simplifying the realization, has the disadvantage that because of the dispersions in the characteristics of the active device, it cannot ensure that a well-defined gain be obtained by design. To address this issue, we propose to add two simple operational amplifier-based auxiliary amplifiers with known gain as part of the measurement chain and employ cross correlation for the calibration of the gain of the main amplifier. With proper data elaboration, gain calibration and actual measurements can be carried out at the same time. By using the approach we propose, we have been able to design a low noise amplifier relying on a simplified hardware and with background noise as low as 6 nV/√Hz at 200 mHz, 1.7 nV/√Hz at 1 Hz, 0.7 nV/√Hz at 10 Hz, and less than 0.6 nV/√Hz at frequencies above 100 Hz.