Reversing the output voltage in photodiode + opamp circuit

Discussion in 'Homework Help' started by hwoarang, Dec 30, 2015.

  1. hwoarang

    Thread Starter New Member

    Dec 28, 2015
    13
    0
    Hello, guys.

    I am a physicist and a very beginner in electronic circuits. I have a very simple question.
    Now I am using a UDT455 photodiode (OSI company) which includes op amp inside like a left picture (photoconductive mode). But I am trying to get the reverse sign of the output voltage by modifying some connections. From the LTspice simulation, I found that it is not possible unless I disconnect the photodiode (cathode) from the op-amp (-) input (red x mark in the picture). Is it true ? Or, if you have other solutions, could you let me know the way ?
    upload_2015-12-30_22-55-58.png
    Another question is.....
    Now I am going to use other type of photodiodes (UDT020D right picture). Here, the photodiode (cathode) and the op-amp (- input) can be disconnected (7 and 5 leads). To reverse the sign of the output, if I connect 7 to 12 (for + voltage) and 5 to 9 (also Cf and Rf filter will be connected to 9), do you think that the output voltage can be reversed ?

    Thank you very much in advance.

    Happy new year!
     
  2. panic mode

    Senior Member

    Oct 10, 2011
    1,321
    304
  3. sailorjoe

    Member

    Jun 4, 2013
    361
    63
    Panic mode is right. If you connect the existing op amp as a non-inverting amplifier, you will either lose gain or stability. But you want both. So add another op amp after the one you have, hook it up as an inverting amplifier with unity gain, and Bob's Your Uncle.
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,026
    3,236
    Yes, you can reverse the output polarity in the second device by reversing the photodiode connections (anode to [-] op amp input) as you suggested.
     
  5. hwoarang

    Thread Starter New Member

    Dec 28, 2015
    13
    0
    Thank you all,

    The reason why I do not add another op-amp is that I wanted to use PCB which has been already made for UDT455 previous version.
    Thank you. :)
     
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