I am currently examining an expired patent file of a photoelectric sensor. The file includes the circuit, I am trying to understand it with my limited analog electronics knowledge:
The circuit is on Page 3 - Fig 2. I am stuck at the first part of the circuit (the photodiode amplifier). From other photodiode amplifier circuits I have researched, none of them had the resistor in the inverting input. It's not mentioned in the PDF (not even numbered). Only thing I could think that it may be forming a series RC circuit with the parasitic capacitance of the photodiode, so it may be dampening high frequency noise by increasing time constant. Or am I thinking too deep?