Photodiode question

Discussion in 'General Electronics Chat' started by robby991, Jan 15, 2008.

  1. robby991

    Thread Starter Active Member

    Dec 17, 2007
    79
    0
    Hi everyone. I have a photodiode, photoconductive mode, and I need to amplify the signal comming out of it. How is this done? Can I just use an op-amp? Thank you.
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    You can definitely use an opamp.

    What exactly is your application?

    hgmjr
     
  3. robby991

    Thread Starter Active Member

    Dec 17, 2007
    79
    0
    Hello hgmjr. I am using pulsed LEDs into a photodiode as an input into a logic block. When hardwired, the pules are sent directly and the circuit operates as it should. I then added the LED and photodiode and the circuit does not work. This makes me think that the signal is not strong enough. I have not recorded the output voltages of the photodiode yet, I was just wondering if amplifying the output of a photodiode is common. I was thinking a non-inverting opamp would work? Thanks for your help!
     
  4. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Can you provide us with a schematic of your circuit so that we can try to help you diagnose your problem?

    hgmjr
     
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    A photodiode has an extremely low output current. Of course an opamp is needed to amplify it to a useable level.
    The photodiode can be used in two different modes:
     
  6. robby991

    Thread Starter Active Member

    Dec 17, 2007
    79
    0
    Hi hgmjr, I don't currently have a schematic drawn up yet. It is a fairly large circuit so it would take much to do so. I will do so if the amplifier does not solve the problem.

    Audioguru - this looks like what I need. Could you provide me with the link to the website you got this from? So a LM741 would work for A1 in the photoconductive mode? Also, why is the photodiode in this picture connected to -30V rather than ground? I am a newbie to photodiodes so I do not understand this. Thanks again for the help.
     
  7. Distort10n

    Active Member

    Dec 25, 2006
    429
    1
    The photodiode is connected to -30V to reverse bias the pn-junction. Since the pn-junction is reversed biased (larger junction), then the capacitance decreases. This is called photoconductive mode. As a rule of thumb, the amplifier will remain stable over a larger bandwidth. Capacitance at the summing junction is bad.

    I would not suggest the LM741, as I believe it has high bias currents and could swamp your photodiode signal. The OPA129 is what I would suggest.
     
  8. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    robby991,

    What supply voltage or voltages are you proposing to use in your design?

    hgmjr
     
  9. robby991

    Thread Starter Active Member

    Dec 17, 2007
    79
    0
    The pulses into my logic block are 1.4V (hardwired), which is what I need the output of the photodiode to be. The photodiode I am using is this:

    http://www.osioptoelectronics.com/products/13-14_PhotoCondSer_OSIOpto.pdf

    It is the PIN-10D.

    To test it what I did was ground it by tying a wire around the base of it, and then connected a wire to the output pin in the back of it where i recorded the voltages. When doing this I got .3V output with just the light around me shining in, and .324 V when I shine the red LED I am going to use into it. Is this the correct way to use a photodiode? How do I supply 30V to it if there is no place to power it?
     
  10. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The opamp in the Photoconductive Mode for the photodiode has a positive and negative supply. Simply connect the anode of the photodiode to the negative supply instead of to -30V.
     
  11. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    What power supply voltages do you have available to work with?

    hgmjr
     
  12. robby991

    Thread Starter Active Member

    Dec 17, 2007
    79
    0
    Hello. The supply voltages for the opamp are around +/-18V, the required input to my logic block has to be around 1.3V, and 5V powers the logic gates. I think the power supply I am using goes up to 30V. Is this what you are looking for hgmjr? I am going to build an amplifier tomorrow and let you know how it works. Thank you!
     
  13. robby991

    Thread Starter Active Member

    Dec 17, 2007
    79
    0
    Hi everyone. I ran a preliminary test using an LM741 opamp and my photodiode (with BNC connector) and I would like to hear what people think about what is going wrong. My set up is the photovoltaic mode from page 3 of the following document.

    www.aptechnologies.co.uk/PDF/PD Theory of Operation.pdf
    LM741 data sheet: http://www.datasheetcatalog.com/datasheets_pdf/L/M/7/4/LM741.shtml

    I am supplying +15 V into pin 7 and -15 into pin 4, as well as a 47k resistor between pin 2 and the output pin 6.

    Even before I attach the photodiode to the opamp I get about .3V when I probe it with the multimeter in a well lit room.

    Following the circuit diagram above, I attach the anode to pin 3 and the cathode to pin 2. My output voltage is -13.72, which is incorrect. When I reverse the anode and cathode I get 15V, the supply voltage. I am new to electronics so I feel like I am missing a crucial element in this set up. Anyone have any thoughts?!



    p.s. I also set up the photoconductive mode circuit in the same document (using -15V as Vr) and I got the supply voltage, 15V, and with the anode and cathode reversed I got 10.6V

    Thanks for your help!!!
     
  14. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The 0V of the power supply must connect to pin 3 of the opamp.
    In the photo-voltaic mode, the photo-diode is a solar cell.

    With the 47k resistor connected but no photo-diode then the output of the opamp should be 0V.
    A 741 opamp is too old and has an input current much too high for this function. Its bandwidth is very narrow.

    I think you got the anode and the cathode of the photo-diode mixed up. In the photo-conductive mode it is supposed to be reverse-biased then it leaks a small current when light shines on it. Yours must have been forward biased for it to conduct hard enough for the output to be +15V then it worked normally when you reversed it.
     
Loading...