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:

Patent no: US6455839B1

Circuit diagram

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?


1 Answer 1


Correct, it is not needed, yet more parts are needed for Vref above CM minimum and Cf for matched RfCf = RshCpd = diode values at desired optical current levels for best step response.

Then gain ratio may be selected to trade-off with rise time and sensitivity.

https://www.digikey.com/en/articles/techzone/2017/jul/design-transimpedance-amplifiers-for-precision-opto-sensing enter image description here

  • \$\begingroup\$ Thank you for the source. I will be designing a photoelectric object sensor though, I think I won't be needing this much of complex calculations. Still, I would appreciate book/article recommendations on the topic. \$\endgroup\$
    – C K
    Jan 4, 2019 at 15:22
  • \$\begingroup\$ Under rare conditions of high impedance Rf.>Rsh adding Ri input may reduce overshoot. \$\endgroup\$ Jan 4, 2019 at 15:24

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