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Noise in transresistive amplifier for photodiodes

purplehaze

Jul 14, 2013
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Jul 14, 2013
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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 (http://www.ti.com/lit/an/sboa035/sboa035.pdf).
Any comments?
Thank you

- purple
 

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john monks

Mar 9, 2012
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Mar 9, 2012
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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?
 

purplehaze

Jul 14, 2013
2
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Jul 14, 2013
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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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