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opamp terminology

Discussion in 'Electronic Design' started by jack, Sep 11, 2003.

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  1. jack

    jack Guest

    In a text in NIM modules ,the author site several types of amplifiers
    ..
    I know what current feedback,charge sensitive and voltage sensitive
    amplifiers are but he also mentions current sensitive amplifiers . Is
    he referring to transimpedence amplifiers as he states these have
    generally low input impedances.? Also does anyone know why PMT's have
    high output impedances and what a typical value would be for a cathode
    grounded scheme? Thanks for any help. jk
     
  2. James Meyer

    James Meyer Guest

    PMTs are current sources. That makes them ideally infinite output
    impedance devices.

    The load impedance should be kept as small as possible if you want to
    preserve linearity. Any voltage developed at the output will subtract from the
    applied supply bias and it's the supply bias that determines the gain of a PMT.

    Jim
     
  3. Bill Sloman

    Bill Sloman Guest

    Sorry Jim, but this isn't quite right. The voltage between the last
    dynode and the anode doesn't directly affect the gain of the PMT
    (provided that it is higher than the the roughly 0.6V needed to ensure
    electron collection at the anode) because there isn't any secondary
    electron multiplication at the anode.

    This means that the load impedance can be as high as you like -
    provided that the anode voltage doesn't go more negative than the
    voltage at the last dynode.

    Since there is of the order of 10pF of stray capacitance from the
    anode to ground (usually via the last two dynodes) you may want to
    keep the load resistance low to take full advantage of the speed of
    your tube.

    If you are worried about linearity, you do have to worry about the
    product of the anode current and the resistance of the last resistor
    in your dynode chain, from the last dynode to ground.

    This particular voltage drop has no direct effect on the
    photomultiplier gain, because any secondary electron multiplication at
    the anode has no effect on the gain.

    It does have a marked indirect effect, if your high voltage supply
    generates a constant voltage between photocathode and ground, because
    any decrease in the voltage between the last dynode and ground
    effectively increases the voltage drop over the active portion of the
    tube, so that the gain of the tube increases with increasing anode
    current.

    This leads to the rule of thumb that the current through your dynode
    chain should be at least ten times your maximum anode current. If you
    want a lower current through the dynode chain, use a high voltage
    zener diode between the last dynode and ground.

    Henry Lush published a nice paper about this back in 1965

    H.J.Lush "Photomultiplier linearity" J.Sci.Instrum. volume 42, pages
    597-602

    G.Sauerbrey published a much more comprehensive paper (with loads of
    experimental results) in Applied Optics, volume 11, pages 2576-2583
    (1972). It is in German, but I have done a translation into English -
    e-mail me if you want a copy.

    For a more references, including fun stuff like space-charge induced
    non-linearity (which can be a real problem in fast high-gain tubes)
    see A. William Sloman, "Comment on "Computer-Aided Simulation Study of
    Photomultiplier Tubes" IEEE Trans.Electron. Devices volume 38 pages
    679-80 (1991).

    The paper I was commenting on was a classic example of a bad Spice
    simulation exercise, and I had a lot of fun rubbishing their
    predictions - badass smartass.
     
  4. Bill Sloman

    Bill Sloman Guest

    As far as I know, the effect of changing anode current on the voltage
    distribution along the dynode chain is exactly the same if you ground
    the photocathode of the photomultiplier tube, or run with the anode
    close to ground.

    Most of the other effects which make photomultiplier response a
    non-linear function of light input appear to be similarly insensitive.

    The manufacturer's advice for end-on tubes is to shield the
    cylindrical body of the tube with a mu-metal shield electrically
    connected to the photo-cathode.

    The choice of mu-metal provides magnetic shielding, but - as would any
    metal sleeve - it also provides electrostatic shielding.

    If you didn't fit such a shield you might see some funny effects, at
    least with fast focussed dynode structures.
     
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