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

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  • #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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  • #2
berkeman
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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?
 
  • #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.
 
  • #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.
 
  • #5
berkeman
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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.
Close. The wider junction width lowers the junction capacitance, which provides wider bandwidth.
 

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