Abstrakt
A transimpedance amplifier has been designed for scanning tunneling microscopy (STM). The amplifier features low noise (limited by the Johnson noise of the 1 G Omega feedback resistor at low input current and low frequencies), sufficient bandwidth for most STM applications (50 kHz at 35 pF input capacitance), a large dynamic range (0.1 pA-50 nA without range switching), and a low input voltage offset.
The amplifier is also suited for placing its first stage into the cryostat of a low-temperature STM, minimizing the input capacitance and reducing the Johnson noise of the feedback resistor. The amplifier may also find applications for specimen current imaging and electron-beam-induced current measurements in scanning electron microscopy and as a photodiode amplifier with a large dynamic range.
This paper also discusses the sources of noise including the often neglected effect of non-balanced input impedance of operational amplifiers and describes how to accurately measure and adjust the frequency response of low-current transimpedance amplifiers.