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Transimpedance amplifier

Discussion in 'Electronic Design' started by [email protected], Jan 11, 2009.

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  1. Guest

    Hi,
    i've simulated a transimpedance amplifier with spice. My circuit is
    composed of an opamp and a feedback resistor; in parallel to current
    source a capacitor is present (i want to use a photodiode's equivalent
    model). I have a question: is it possible that (with an ac sweep), V/I
    transfer function is greater (at lower frequencies) than opamp open
    loop gain? Why?
    Another question: i wanto to use it as input of a sar adc. I've read
    this interesting article
    www.edn.com/article/CA6602451.html
    and i want to try it. Is it more correct to use another opamp as
    buffer and rc circuit as its load (as describend in the link) or use
    rc load directly to transimpedance amplifier?
    Thanks
     
  2. MooseFET

    MooseFET Guest

    The open loop gain spec you are looking at may be a minimum and the
    model may use the typical.

    You didn't say there was a peak but if you see a peak in the curve
    just before it falls off the answer may be the phase margin of the
    system.

    Beware of EDN articles they are too often bogus. Does your ADC have
    sample and hold in it? What sort of load does it present to its
    input?

    You may or may not need another op-amp in the system at all.
     
  3. Guest

    How can i verify it?No opamp's macromodel is present, because my opamp
    is realized with transistor...
    Yes, a peak is present....i've read about on graeme's book.I've used a
    feedback capacitor to remove it
    It has a sample and hold with r=on switch resistor=100ohm and
    c=sampling capacitor=5pF
    thanks
     
  4. MooseFET

    MooseFET Guest

    Making the peak go partly away but not completely can lead to a better
    system. A slight bump before the gain starts to fall can move the 3db
    point up.
    The output impedance of the driving circuit needs to be far less than
    the 100 Ohms. The output will actually look inductive. Since you are
    modeling the system, you can model the sample and hold action too.
     
  5. Guest

    x-no-archive:
    Well the "open loop gain" is measured in Volts out / Volts in and the
    V/I transfer function is measured in Volt out / Amps in..... so you
    have an apples and oranges problem I would think.

    Mark
     
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