Comparator as a current to voltage converter

Discussion in 'The Projects Forum' started by Nano001, Feb 5, 2010.

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  1. Nano001

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    Jan 12, 2010
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    Hi everyone. I recently breadboarded the attached circuit to amplify the current from a photodiode. I would now like to use LT1016 in replacment of the LM324 for high frequency photodiode current pulses (10-13MHz). I was wondering if using this comparator as an opamp is as simple as just adding a feedback resistor from the output to the inverting input. Once I get this circuit working I was told that I need a preamp due to large capacitance of the highly revered bias operation of the diode. First, however, I would like to just test this device at low frequencies until I optimize the design then I will transfer the circuit to a high freqyency PCB for testing in the 10-13 MHz range.
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    You cannot use a comparator as an analog amplifier. Comparators do not have a compensating capacitor and their output is an open collector type. You have to use a high frequency op amp if you want a high frequency amplifier.
     
  3. Nano001

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    Any suggestions as to what opamp I should get for this high frequency. I ws told on this forum that the LT1016 would work. Thank you.
     
  4. mik3

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    If you want just to amplify the pulses of the photodiode then a comparator (you can use the LT1016) will be ok. If you want to measure the intensity of the light incident on the photodiode then you will need an op amp.
     
  5. Audioguru

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    The capacitance of the diode is less when its reverse bias voltage is more. But your reverse bias voltage is only 1.15V.
     
  6. Nano001

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    Thanks Mik3. I just need to convert the current pulses supplied by the photodiode to digital pulses (0-5V) which are acheived by the output of the comparator (LM311 in schematic). How can the LT1016 be configured for this? If not, any suggestions on another suitable opamp?

    Audioguru you are right, I said it backwards. The reverse bias of the diode is actually at ground for now, until I want speed.
     
  7. mik3

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  8. Nano001

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    Last edited: Feb 5, 2010
  9. Audioguru

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    ST Micro have terrible copies of the datasheet from National Semiconductor.

    The LF355, LF356 and LF357 opamps do not work with a supply as low as 5V. The datasheet shows 10V minimum.
    The inputs will not work if they are less than +3V in your circuit.
    The fast LF357 needs a minimum gain of 5 or it will oscillate.

    Use an MC34071 single or MC34072 dual opamp that will work in your circuit and it is pretty fast.
     
  10. Nano001

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    Ok. Unfortunately, the MC34071 has a bandwidth of 4.5MHz, if I read correctly, which is too small. I need 10-13.
     
  11. atferrari

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    LM318 maybe? High slew rate as far as I recall.
     
  12. Audioguru

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    The very fast opamps from Texas Instruments have a minimum supply of 10V.
    Like any opamp, their bandwidth is less as you add gain. Your photo-diode has a very low output so you need lots of gain.
     
  13. Nano001

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    LM318 has 15MHz bandwidth, however I need single supply +5V. Can the 318 operate in single supply, I didn't see it in the datasheet
     
  14. Audioguru

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    The LM318 has a minimum supply of 10V.
    Its minimum input voltage is +3.5V in your circuit.
    Its small signal bandwidth is 15MHz when it has a gain of 1 and its large signal output is about 5MHz. Adding gain will reduce the bandwidth.
     
  15. SgtWookie

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    Jul 17, 2007
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    This thread should be closed, as it is open on another thread.
     
  16. Nano001

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