We have developed a current preamplifier that operates in a liquid-helium bath cryostat. It has been optimized for the measurement of dynamical electric-current fluctuations (noise) of high-impedance sources R≳100 MΩ. A bandwidth of up to 400 kHz has been achieved by effectively minimizing the capacitance of the input transistor with a dynamical feedback. The amplifier measures current noise in a scanning tunneling microscope (STM). It enables the measurement of shot noise for currents as low as 30 pA (sampling rate 5 s) for a high-impedance source with a resistance of R≳1 GΩ, a value typical for tunneling resistances in STM.
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Birk et al. (1996) studied this question.
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