Low noise current preamplifier for qPlus sensor deflection signal detection in atomic force microscopy at room and low temperatures.

The resolution of frequency modulation atomic force microscopy is limited by instrumental noise. When using a qPlus sensor, the deflection detector noise is the dominant noise contribution. It can be reduced by improving the preamplifier used to amplify the sensor deflection signal. We present a simple single-stage differential preamplifier which outperforms previous designs known to us by at least a factor of two in the deflection noise density. We show specific versions of this preamplifier to use in ambient conditions, in ultra-high vacuum at room temperature, and at 4.2 K. Furthermore, we compare the thermal peak analysis and the frequency shift noise density method as a means to determine the deflection noise density. We note that this preamplifier can also be used for any current-generating sensors such as other piezoelectric sensors and photodiodes, but, in this paper, we restrict our analysis to qPlus sensors.