Six Decade Current Sensing Log Amplifier

The low pA's of bias current of CMOS amplifiers make them suitable for high input impedance current sensing applications. By taking advantage of the logarithmic relationship of a transistor base-emitter voltage to it's collector current a very wide range of measured input current can be converted to a voltage. In this circuit the input current can span six decades from 10nA to 10mA. The positive tempco resistor compensates for the 0.3%/C tempco of the logging transistors scale factor coefficient (kT/q).