  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.
