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. The converter is based on the virtual ground scheme employing semiconductor diodes as nonlinear elements in the feedback path. It can expand its band of frequence through additional circuits. Its dynamic range of input is broadened by using several pairs of diodes. It is ideally suited for studying local interactions in scanning tunneling microscopy.