Photoresistor provides negative feedback to an op amp

摘要

AGC (automatic-gain-control) amplifiers use the nonlinear characteristics of control devices. The magnitude of the real component in some of their differential parameters changes depending on variations in their dc operating points. A typical example is the VA characteristic of a silicon PN junction, which results in the differential conductance directly proportional to the passing dc current (Reference 1). In this form of control, the main problem is the control element's nonlinear transfer characteristic, which causes a relatively large degree of nonlinear signal distortion once the processed voltage amplitude exceeds millivolts (Reference 2).