Simple Photodetector Circuit

Discussion in 'Homework Help' started by maccyjj, Sep 24, 2013.

  1. maccyjj

    Thread Starter New Member

    Sep 24, 2013
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    Hi all, I am new here and just have a simple question which I need help with from the knowledgeable group on this forum. I recently built a simple photodetector circuit using an operational amplifer, using a design I took directly from a Texas Instruments datasheet. I can't seem to upload a picture, but it is the first diagram on this pdf: (http://www.ti.com/lit/ds/sbos250e/sbos250e.pdf) labelled "High Gain, 20 MHz Transimpedance Amplifier".

    My question is, what is the purpose of the resistors and capacitors connected to the non-inverting input? I undestand the point of the feedback resistor and capacitor, but cannot seem to find any information about the other passive components. If someone would be so kind to give me some advice as to their purpose or perhaps point me in the direction of a good website or book I would greatly appreciate it.
     
  2. Jony130

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    Feb 17, 2009
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  3. maccyjj

    Thread Starter New Member

    Sep 24, 2013
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    Thank you for the response. Why is the second 10 kΩ resistor required? And why do I not need to worry about it if I build on a breadboard?
     
  4. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    The purpose of 10K resistor is to reduce the voltage offset caused by input bias current. If both inputs have the same input bias current, equal current flowing through equal resistances will create equal and opposite offset voltages. Thus, input bias current will not contribute to the offset voltage of the circuit.

    As for the breadboard or poor PCB design. The noise pickup from the mains will be much larger than the resistor noise.
     
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