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A basic question about current-to-voltage converters

Discussion in 'Electronic Basics' started by Jiaqi, Feb 5, 2005.

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  1. Jiaqi

    Jiaqi Guest

    I have a basic question seeming quite simple:
    An op amp with an open loop gain A, and a feedback resistor Rf, are
    connected as a current-to-voltage converter which can be found in many
    common applications. If the input impedance of the op amp is very large
    and can be regarded ideally "infinite" (as the case of FET-input op amp),
    the input resistance of the current-to-voltage converter can be estimated
    as Rf/A (Why?). With this presumption, can I estimate the voltage at the
    virtual ground pin as I*Rf/A (I denotes the input signal current)? How can
    I estimate the maximum feedback resistance Rf which the op amp can load?
    Does that value depend on the open loop gain A? If yes, please tell me the
    way or where I can find a definite answer.
  2. With the non-inverting input at zero volts and your inverting input at some
    value, V, then your output voltage is V*A, right? This produces a current
    through Rf of (V*A - V) / Rf, the voltage difference between the output and the
    inverting input divided by the feedback resistor. So:

    I = (V*A-V)/Rf = V*(A-1)/Rf, so V = I*Rf/(A-1)

    So I think the voltage at the virtual ground node (the inverting input) is
    I*Rf/(A-1), not I*Rf/A. This makes the input impedance Rf/(A-1), I think.

    For DC considerations, your Rf depends on your maximum allowed output voltage
    and your maximum input current. If you have a maximum output of 10V and your
    maximum current will be 1mA, then your Rf is limited to 10V/1mA*[(A-1)/A] or
    about 10k Ohm.

  3. Jiaqi

    Jiaqi Guest

    I think the output voltage should be -V*A rather than V*A, and the input
    current I should be expressed as (V-(-V*A))/Rf=V*(A+1)/Rf. So the input
    voltage V is I*Rf/(A+1) rather than I*Rf/(A-1). The input impedance is
    Ri=Rf/(A+1) rather than Rf/(A-1). Am I right, Jon?

  4. Ban

    Ban Guest

    The input impedance (Rf/A) is almost zero, because the opamp tries to keep
    the inverting input at gnd potential by means of the output voltage through
    the feedback resistor. With a smaller resistor the compensation current will
    need less voltage swing on the O/P.

    With this presumption, can
    Your are right with the voltage at the inverting input, additionally is
    superimposed the offset voltage.
    Rf is (almost) independent of the open loop gain. It is only dependent on
    the max output voltage swing. If your input is +1uA, with 1Meg Rf the O/P
    voltage will be -1V, so with a voltage swing of +/-13V the max. Rf to
    prevent saturation would be 13Meg.
  5. I'm a hobbyist and get my signs wrong! Yes, I like that better.

    The output would indeed be -V*A.

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