US Patent 7310656 Grounded emitter logarithmic circuit

A logarithmically-responding circuit includes a differential-input amplifier that drives the control terminal of a three-terminal device that exhibits an exponential response in its output current. This arrangement allows the third terminal to be grounded. In a preferred embodiment the three-terminal device is a bipolar junction transistor (BJT). This, and other supporting circuit features described, enable single-supply, wide-range, fully temperature-compensated operation. A compensation technique significantly reduces errors caused by the finite ohmic emitter resistance of a BJT. To support use in logarithmically compressing the current generated by a photodiode, an adaptive bias signal can provided which maintains an essentially constant bias on the photodiode's internal junction.