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The + sign above the photodiode, what does it mean?

  1. May 9, 2008 #1

    cks

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    I don't really have much knowledge of electronics and kindly seek your helps.

    The + sign above the photodiode, what does it mean?

    Does it mean I have to connect the cathode to the ground??

    Then, about the direction of the current, why does the current flow from cathode to anode? I mean isn't it the current should flow from cathode to anode?

    Thank you.
     

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  3. May 9, 2008 #2

    berkeman

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    Staff: Mentor

    A photodiode is used in reverse bias mode. Light hitting the junction generates a reverse photcurrent, which the circuit is converting to a voltage (the photocurrent flows through the feedback resistor to get converted to a voltage V=IR by the opamp.

    I typically connect the cathode of the photodiode to the - input of the opamp, and connect the anode of the diode to a negative supply, but the configuration shown in your circuit should work as well.

    Quiz question -- the higher the reverse bias on the photodiode, the faster the response of the circuit to light. Why is that?
     
  4. May 10, 2008 #3

    cks

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    Thank you very much berkeman.

    I now know that the + sign is to connect to the ground.

    Answer to quiz question:
    because when the reverse bias is increased, the width of the junction increases, allowing more photocurrents to be generated, which results in faster response.
     
  5. May 10, 2008 #4

    cks

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    I'm currently working on a project. My knowledge about semiconductor is zero. During this whole week, I studied semiconductor from the basic principle. I studied op amp, photodiode, but there are too many terminologies which make me to learn very slowly. I didn't know what is every negative feedback. :) but now, i know already.

    Umm,, thanks again Berkeman.
     
  6. May 11, 2008 #5

    berkeman

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    Staff: Mentor

    Close. The wider junction width lowers the junction capacitance, which provides wider bandwidth.
     
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