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Noise in transresistive amplifier for photodiodes

Discussion in 'General Electronics Discussion' started by purplehaze, Jul 14, 2013.

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  1. purplehaze


    Jul 14, 2013
    Hi guys, I was dealing with modeling noise sources in transresistive amplifier for photodiodes.
    In a BB application note I found this schematic (attached). I do not think that the at DC the noise voltage gain is 1 + R1/RD, as they write in the AN (
    Any comments?
    Thank you

    - purple

    Attached Files:

  2. john monks

    john monks

    Mar 9, 2012
    Maybe I'm not clear enough with this datasheet but if you place a signal on the + input of the op-amp then the reported gain of 1 + R1/RD would be correct. Therefore if the datasheet is referring to noise generated with the inputs of the op-amp then formula would be correct. Why do you believe the formula is incorrect?
  3. purplehaze


    Jul 14, 2013
    You are right, but 'en' is the input noise voltage (I would consider shot noise of the photodiode) and it is on the - input. However, applying superimposition, in DC, it could be right, but still the transfer function is not clear to me, I would obtain for en Vout = (R1/RD)*((1+CD RD s)/(1+CS R1 s)) and not ((1+CD R1 s)/(1+CS R1 s)) , as indicated in the graph. :confused:
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