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Photodiode amplifier

Discussion in 'Electronic Basics' started by Ray Xu, Aug 16, 2008.

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  1. Ray Xu

    Ray Xu Guest

    Hi, I am having trouble making the receiver part for my laser range
    finder, but I do
    have a head start (FYI, I am not an analog person). Currently, on my
    I have a PNZ334 photodiode connected to a current to voltage converter
    (using a op-amp; feedback is 100K), and connected to 2 stages of
    amplification (I THINK that they are 15x and 100x). The problem is in
    current to voltage converter; it is not sensitive enough to even pick
    up the
    slightest refracted laser beam from a convex lens in a dark room. I
    did a separate test with the photodiode alone, and it was very
    sensitive to
    the small laser refracted beams. I am using photovoltaic mode and not
    photoconductive, because I tried it before, it is too hard for me
    someone can tell me how to build it). Can anyone please tell me how to
    this, or better yet, how to design a better current to voltage
  2. whit3rd

    whit3rd Guest

    Is the amplifier saturating?

    What purpose does the current/voltage converter serve?
    Wouldn't V proportional to logarithm-of-current
    output be more useful? Alternately, if you're going to
    mix down a modulated signal, can you just steer the
    photodiode current with a Gilbert cell mixer?
  3. Ray Xu

    Ray Xu Guest

    Well I was working with the Seattle Robotics Society on this too, and
    the person said I am driving it into saturation. He never said on how
    to fix it. Because the photodiode has high impedance, it'd be hard to
    interface it directly to the stages of amplification. I've never
    heard nor I have any ideas on "logarithm-of-current" (Could someone
    please tell me what it is?).

    And one more thing, I am a middle school student working on my
  4. JosephKK

    JosephKK Guest

    From this post alone i already expect that your problem area is optics
    not electronics. Try working on the optics please.
  5. JosephKK

    JosephKK Guest

    The basic diode has a log transfer function, and the transfer function
    is voltage as log of current. Make of it what you will.
  6. Ray Xu

    Ray Xu Guest

    Thanks Paul for the reply. I'll go look for some transimpedance
    opamps on Monday. Just yesterday, I re-looked on the concept of
    photoconductive and re-designed the current to voltage converter to be
    in photoconductive mode (I need it to be like this because I am making
    a laser range finder; this portion of the circuit needs to be precise
    and fast). For the stability part, I haven't seen this problem at all
    since I started building this, but I added on some 10pF caps just in
    case it ever does happen, for some reason. There's also a 50Hz
    lowpass filter I designed before and I'm planning to add it to the
    receiver's end. For now, I'm not going to be modulating the laser
    above 20Hz. And to avoid saturation in the opamp stages, would it be
    best to have more stages of opamps and to cascade them, or to try and
    put all the gain in 1 or 2 stages of amplification?

    Anyways, I'm building a phase shift-based laser range finder. On the
    receiver side, I have a 555 built oscillating at about 10Hz
    (eventually going to replace it with a MCU). That 10 Hz is used to
    drive a LA modulation-capable laser diode module from Meredith
    Instruments. For the phase shifting part, I am using a THS3202
    (mounted on a adapter board) as a high-speed comparator. I choose
    this because of it's high slew rate, bandwidth, and that the rise and
    fall times is 480 pS. The THS3202 will compare the sent modulation
    against the received modulation, and the received modulation should
    have some lag proportional to the distance from the object. On the
    comparator's output will be a pulse train, and (if I remember
    correctly) a DPRG ( said to feed it through a RC
    network which will turn it into a analog voltage and where a ordinary
    MCU can read it and determine the distance. And for tuning, I may add
    a programmable delay chip in between the laser modulation wire and the
    output of the 555 (or MCU) to "cancel out" the circuit delay. For the
    hardware, I'm going to mount the laser and photodiode parallel to each
    other but on the same "level", and the object is going to be placed
    perpendicular to the laser and photodiode. The convex lens will be
    placed on top of the photodiode. I hope this helps.
  7. Ray Xu

    Ray Xu Guest

    It needs to be fast so there is the smallest possible lag in the
    circuit. And yes, the PNZ334 has a pretty small junction.
  8. Guest

    Two questions: what comprised the "seperate test" and what do you mean by

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