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M. Stubian, J. Bobek, M. Setvin, U. Diebold, M. Schmid:
"Fast low-noise transimpedance amplifier for scanning tunneling microscopy";
Poster: DPG-Frühjahrstagung (DPG Spring Meeting) of the Surface Science Division, SurfaceScience21, virtual; 04.03.2021.

Scanning tunneling microscopy is one of the most versatile techniques in surface physics. One of the factors limiting its performance is the bandwidth and noise of the preamplifier. Higher bandwidth enables faster scanning, and also implies low phase shifts, which reduces the susceptibility to feedback loop oscillations. STM preamplifiers are transimpedance amplifiers (TIAs), usually with a high feedback resistor. Increasing its resistance leads to lower current noise (Johnson noise of the resistor), but at the same time usually results in lower bandwidth. Using a multi-stage amplifier design, we could achieve an input noise of ≈ 5 fA/√Hz at room temperature and low frequencies, but nevertheless a large bandwidth of up to 200 kHz and large dynamic range (<0.1 pA to 50 nA). For low noise, it is important to minimize the input capacitance. Connecting the STM tip to the preamplifier via a long coaxial cable should be avoided, and the performance can be substantially improved by placing the first amplifier stage into vacuum. Additionally, for low-temperature STMs, the Johnson noise is reduced by placing the feedback resistor in thermal contact with the cryostat. We also discuss a source of noise in operational amplifiers usually not considered, but important for TIAs.