A quasilogarithmic current–voltage converter has been built which allows currents spanning an exceptionally broad range from 10 pA to 10 mA with a bandwidth greater than 2 kHz to be measured. The converter is based on the virtual ground scheme employing semiconductor diodes as nonlinear elements in the feedback path. It is ideally suited for studying local interactions in scanning tunneling microscopy. ©1997 American Institute of Physics.