photodiode output amplification

Discussion in 'General Electronics Chat' started by Chinmaya kulkarni, Sep 9, 2016.

  1. Chinmaya kulkarni

    Thread Starter New Member

    Sep 8, 2016
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    Guys i need some help. So i have this application in which i use an infrared led at sending end sfh 4845 osram and at receiving end i have a photodiode sfh 2400 fa osram in reverse bias. I need a simple amplification circuit using any bjt so that when i shine led, i get some amplified voltage at output. I had tried common emitter amplification. As it amplifies current. But somehow haven't gotten any result. I would really appreciate your help. Thank you.

    upload_2016-9-9_6-14-30.png
     
  2. be80be

    Senior Member

    Jul 5, 2008
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    The output is going to be low when the photo diode is conducting. It turns on the base and sinks your output to ground. You need a emitter follower
    2000px-NPN_emitter_follower-s.gif

    https://upload.wikimedia.org/wikipe...lower.svg/2000px-NPN_emitter_follower.svg.png
     
    Last edited: Sep 9, 2016
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  3. be80be

    Senior Member

    Jul 5, 2008
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    Maybe the moderator can fix the pic I couldn't get smaller
     
  4. ScottWang

    Moderator

    Aug 23, 2012
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    The svg file type is not good for the forum to display, and I compressed it to smaller and have a links to the original picture.
     
  5. Sinus23

    Member

    Sep 7, 2013
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    Are you sure that you have tried this?

    The red line is when the current source is turning the transistor hard into saturation making the voltage drop over R2 dominant. The blue one is when there is almost no current going to the base, turning it pretty much off making the voltage drop over collector-emitter dominant.

    The R2 is probably not the best calculated value but as an example it works...

    1234.png

    Also you might want to read this if you haven't already.

    http://www.osioptoelectronics.com/application-notes/AN-Photodiode-Parameters-Characteristics.pdf
     
    Last edited: Sep 9, 2016
  6. dl324

    Distinguished Member

    Mar 30, 2015
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    Welcome to AAC!
    What voltage do you measure on the collector of the transistor under no light and full light conditions? What voltage change do you want?
     
  7. Chinmaya kulkarni

    Thread Starter New Member

    Sep 8, 2016
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    First of all thank you guys for your replies. So first I will try the emitter follower (in spice simulation) and see what output I get. @dl324 I got the same output 11.4/11.5 V with or without light. That's the problem. My output with the circuit remains independent of the IR LED input. It made me wonder whether my amplification circuit is wrong. I hope I am making myself clear, as I am a newbie in electronics. Thank you.
     
  8. dl324

    Distinguished Member

    Mar 30, 2015
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    Try adding another transistor to increase current gain:
    upload_2016-9-9_13-37-29.png
     
  9. HW-nut

    Member

    May 12, 2016
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    I suggest getting rid of the BJT's and use an op-amp configured as a current to voltage converter. The circuit is simple and has excellent control of the gain. The BJT approach will have poor gain control.
     
  10. be80be

    Senior Member

    Jul 5, 2008
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    I don't no what spice shows but the emitter follower in real life with photodiode inside a 1 inch straw covered with black tape so light only comes in at one end. Gives you about 1.88 volts off I mean as in no led pointed at it and 4.88 volts turned on and if you move away it can change lower from the 4.88 volts down to the 1.88.
    This is using 5 volts so I can read it with a ADC pin of a arduino.
    But at 12 volts you'd be in the 4.5 volt to 11.5 volts
    I don't see how you could test this with spice I didn't see any light setting last time I looked Joke but I still
    don't think spice is what you want to test it.
     
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  11. HW-nut

    Member

    May 12, 2016
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    I have simulated optical sensors with spice using voltage controlled current sources.
     
  12. Sinus23

    Member

    Sep 7, 2013
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    If it's not an industrial secret would care to show us? Besides I don't think that simulation is the problem.

    I think be80be hit the nail on the head with the real world problem. I just made the simulation to prove that emitter follower would most likely be better than common emitter in this case.
     
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  13. be80be

    Senior Member

    Jul 5, 2008
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    I guess some miss the Joke part I know you can test with spice just I'm about sure what you come up with will not match the real hardware.
    Which I no it true cause I just did some test in last week and it didn't.
    And the emitter follower works good a opamp is better. This week is going to be phototransistor week when I get into it.
    I just got a bunch of cheap laser pointers to shine on them.
     
    Chinmaya kulkarni likes this.
  14. Chinmaya kulkarni

    Thread Starter New Member

    Sep 8, 2016
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    Hi Guys, sorry for my late answer. @be80be I agree with you, spice just shows the simulation for electronics signals/ circuits. I also tried emitter follower and my application worked. Thanks a lot.
    @Sinus23 My application is to make a 'proof of concept' design for frustrated total internal reflection. but I have to replace camera and use photodiodes instead; which is now working (yeyy!!). I can not take pics to show the whole assembly but I will get the spice file.
    Thanks all.
     
  15. be80be

    Senior Member

    Jul 5, 2008
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  16. ScottWang

    Moderator

    Aug 23, 2012
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  17. be80be

    Senior Member

    Jul 5, 2008
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    That's what it's for (video imaging ?) scanning but you could do a big area with one and the op still be using photodiodes.
     
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