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Transimpedance Amplifier - Photodiode Reciever Circuit

Discussion in 'Electronic Basics' started by [email protected], Oct 15, 2006.

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  1. Guest

    Hi all,
    Need help with component choice!
    Trying to build a photodiode recieving circuit. Came up with the
    circuit below after reading a few things and scavenging come circuit
    please refer link for circuit schematic

    | VCC
    .-----. |
    | | |
    | | .--------o
    .-. | | |
    56k| | | | |
    | | | | |
    '-' | .-. |
    | |/ 120k| | |
    |---| | | |
    | |> '-' |
    | | | |
    | | | |
    | .-. | |
    110nf --- | | | | __68k
    --- | | 47 | .-----)--|___|---.
    | '-' 22nf | | | |
    | | | | ' |
    '-----| || | | |\| |
    o---------||-----)--o---|-\ |
    | || | | >-------------
    | o------|+/
    photodiode - | |/|
    ^ .-. |
    | 120k| | |
    | | | |
    | '-' |
    | | |
    (created by AACircuit v1.28.6 beta 04/19/05

    The op-amp converts the current signal to a voltage signal.

    The circuit worked fine with an AD817AN as the amplifier. But the
    AD817AN is too expensive, $13. I tried just pluggin in a few other
    amplifiers that were around.. None give me a clean output.

    The photodiode(BP104) is reciving pulses (square wave with "on" time of
    10us and varyin "off" times)
    the frequency of the incident pulses can vary from 14KHz to 50KHz.
    What criteria should i take into consideration while choosing the
    I thought i would be lookin at the Gain Bandwidth Product(GBP) but dont
    know if i am lookin at unity gain (becuase it is a transimpedance
    amplifier) or for gain of 68k (the feedback resistor).

    I skimmed through, but could not find something
    with the same spec's as the AD817 for a lesser cost. Do i need such a
    high spec op-amp? I am running it of single supply 5v

    BP104- photodiode
  2. Guest

    Just tried chip TL071, it seems to work. But the output is squiggly.
    Tried supplying it woth 10 v single supply.. still no luck!
    Please advise of some way to rectify this squiggly output and get a
    clean square wave
  3. Guest

    Thank you Jamie

    But decreasing the impedance on the feedback would also mean decreasing
    my gain. i want to have a fair gain.

    With the 68k feedback resistor i have at the moment, i lose signal when
    the transmitter is 5 cm away. I cannot have any lesser gain than i
    already have.

    Any other suggestions? Is this chip not suitable for this impedance?
  4. Jamie

    Jamie Guest

    it depends on the Slew/Skew timing you're looking for?

    you could lower the impedance on the feed back and input!
    that may lower the miller effects ect...
  5. Jamie

    Jamie Guest

  6. Chris

    Chris Guest

    Hi, GB. Got some problems here for sure.

    First off, I'm not sure where you got that quote of $13 for an AD817.
    DigiKey has the DIP package in stock for $3.60 USD in unit quantities.
    Although they don't have the SOIC in stock, their prices on quantity
    are $2.02 to $2.88 USD. I think you can do better than $13, for sure.

    The AD817 is a good part -- just look at the datasheet:

    It's unity gain stable, and also has a high gain-bandwidth product,
    slew rate, and settling time. These are all important. Your 22nF cap
    is a short at HF AC and an open at DC. Your application means it needs
    to be stable when there's no signal, and very fast when there is one.
    Also, the AD817 is specified for single supply operation -- the TL071

    The guaranteed input voltage range of the TL071 swamps out your 5V
    supply, too. TL071 -- bad choice here. I'm kind of amazed you got
    much of anything out of it.

    If you feel that the lower price of the AD817 is still too high, you
    may be able to get away with a lower gain-bandwidth single supply op
    amp at your lower frequency (10us pulses at <50KHz). But you're not
    going to get good results with a gumball part like an LM358. The best
    you'll be able to do is save about half the $2.02 USD cost of the AD817
    -- you'll probably not be able to spend much less than a buck unless
    your quantities are very high.

    Since you're new to this, you should know that it's considered good
    form to bottom post. Please check out Google Groups Help Topic "What's
    good 'netiquette' when posting to Usenet?"

    Good luck
  7. Chris

    Chris Guest

    You know, I'm wondering. The photodiode doesn't seem to be that
    unusual -- how about just shamelessly swiping a typical application
    from p.2 of the LM311 data sheet:
    | +5V +5V
    | | |
    | | .-.
    | ~ - 5K| |
    | ~ ^ | |
    | | '-' +5V
    | | | |
    | | | +5V .-.
    | | | | | |1K
    | | | LM311| | |
    | | | |\ '-'
    | o------------)-----|-\ |
    | | ___ | | >---o----
    | o-|___|-o----o-----|+/|
    | | 1K | | |/ |
    | .-. --- | | |
    | 50K| | --- .-.1K |===
    | | | .01uF| | | |GND
    | '-' | | | ===
    | | === '-' GND
    | === GND |
    | GND ===
    | GND
    (created by AACircuit v1.28.6 beta 04/19/05

    This should work fairly well. Your maximum dark current is supposed to
    be 2uA typ., and with the 50K resistor, that would set a max low level
    of 0.1V (probably less). The data sheet shows 34uA with 1mW/cm^2,
    which should show as around 1.7V (probably more). If you set the
    trigger value of the comparator at about 0.83V (as above), you should
    be just about done. Of course, the trigger level responds to the
    average input, so it will be somewhat less (depending on duty cycle),
    but certainly more than 0.6V with a 50% duty cycle. The only tweak you
    might add is to provide a bit of hysteresis if a little chatter at the
    output caused by the slowly rising/falling input signal bothers.
    Better sensitivity, adequate speed, gumball parts, almost rail-to-rail
    digital output signal, go home on time for less than a buck. What's
    not to like?

  8. Jamie

    Jamie Guest

    well, that would explain things, the BandWidth is 50..70 Mhz..
    so i guess all you would need is a high gain amp with high freq.
    the TL071 family is only 3 Mhz.
    that sure would make a different in shape of the wave on the output.
  9. Guest

    Thanx Chris.
    I looked up DigiKey.The shipping to New Zealand is horrendously
    expensive. About $70. I dont think i could afford that.

    Thank you for the above information, though i was using the circuit, i
    was not completely aware of how it was working.
    i think this would suit me better cost wise. Thank you very much

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