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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).