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 sourcesR≳100 M&OHgr;. 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 ofR≳1 G&OHgr;, a value typical for tunneling resistances in STM. ©1996 American Institute of Physics.