A low noise transimpedance amplifier for cryogenically cooled quartz tuning fork force sensors

We have designed and built a low noise, broadband transimpedance amplifier that allows for sensing both the amplitude and phase of an oscillating quartz tuning fork for a cryogenic atomic force microscope. The circuit uses a global feedback scheme, where the input stage, located next to the quartz tuning fork at low temperature, is followed by an operational amplifier stage operated at room temperature. At 4.2 K, with a 1 MΩ metal film feedback resistor, the amplifier yields an output noise floor of ${\text{2×10}}^{\text{−7}}\hspace{0.17em}\mathrm{V}/\sqrt{\mathrm{Hz}}$ and a bandwidth of 200 kHz. When it is used with a commercial 32 kHz quartz tuning fork, the calibrated sensitivity at 4.2 K is determined to be 30.8 μV/pm.