Circuit to exaggerate differences in current
What you are describing is an expander. This performs the opposite function of a compressor.  

Basically, it is a amplifier (or simple attenuator, see below) circuit set up so that the gain is controlled by the incoming signal level. There are various ways of implementing this in a circuit, from simple to very complex. Here is an example of about the simplest compander (compressor/exander) circuit using a light dependent resistor and LED to control the attenuation:



Here is the page of articles/schematics this came from which includes the explanation of the circuit and various other similar circuits. Google will turn loads more up.