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Common mode voltage in op amps

Discussion in 'Electronic Basics' started by Amanda Robin, Nov 24, 2005.

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  1. Amanda Robin

    Amanda Robin Guest

    Hello all,

    I am reviewing my old electronics notes in preparation for a
    comprehensive exam.

    In my notes I have written, a couple of times, that with a non-inverting
    op amp configuration there is common mode voltage, and with an inverting
    configuration there is no common mode voltage.

    I know the definition of common mode voltage, but I look and look at the
    circuit diagrams and can't see why one would have it and the other not.
    One of the basic "rules" for op amps is that both input terminals are
    ideally held to the equivalent voltage. So it seems to me that every op
    amp in the world, however configured, would ideally have common mode
    voltage. (I'm probably confusing two different things here, but maybe
    someone can help me get unconfused.)

    Is it because, with the inverting configuration, the input terminals are
    held to ground? And zero V on two terminals doesn't count as common?

    Thanks for any tips.

    Amanda
     
  2. Bob

    Bob Guest

    Amanda,

    I think you have it right. The term 'common mode' refers to the component of
    a signal that is equal, at two nodes or two branches. This can be a voltage
    or a current, depending on what's being described. The other component would
    be (might be) called the 'differential mode'. There are various terms used
    to describe these two components of a signal. In telephony, the 'common
    mode' part is called 'longitudinal', whereas the differential part is
    referred to as the 'metallic' component. You say potato, I say potahtoe...

    In the inverting mode configuration, for an opamp, the + lead is held to
    some static voltage. Sometimes this is 'circuit common' (aka ground), but it
    doesn't have to be. So, there is no 'common mode' component, between the +
    and - inputs, because the + side doesn't move (with respect to either opamp
    supply voltage and/or circuit common). There is a differential component,
    albeit very small, between the + and - pins.

    In the non-inverting case, the + pin is driven, so it's obviously moving
    with respect to the supplies/common. The - pin moves along with the + pin,
    so since both are moving it can be said that there is a 'common mode'
    component of the two pins. Also, just as in the inverting-mode case, there
    is a small differential component between the + and - pins.

    Have fun, in school.

    Bob
     
  3. This has some truth in it. Enough to be dangerous. ;-)
    Common mode voltage is just the average voltage on the two inputs.

    It is the voltage they have in common.
    The danger is coming into focus.

    All closed loop functions made with opamps keep the two inputs at
    almost exactly the same voltage with negative feedback. The tiny
    remaining differential voltage times the very high gain of the opamp
    produces the output voltage that drives the feedback. Any voltage the
    two inputs share in common does not involve any differential voltage,
    and, ideally, does not get amplified. Every opamp powered from a
    given supply voltage has a common mode input range over which
    differential voltages can be amplified, and the common voltage shared
    by both inputs can be ignored.
    You are getting close. The inverting configuration has the + input
    held to a fixed reference voltage that may be ground. The - input
    receives both the input current and the feedback current and if these
    are not exact opposites of each other, the combination causes some
    voltage change at the - input. And any change from the reference
    voltage applied to the + input represents differential voltage and
    gets amplified largely and alters the feedback current to precisely
    match the input current and put the - input voltage back at the same
    voltage as is applied to the + input. So both inputs stay in a very
    small region of the common mode range.

    The non inverting configuration applies the input signal to the +
    input, and the - input is forced to follow it (via the feedback path)
    wherever it goes as long as it stays inside the common mode range and
    the output can swing enough in the required direction. Anything else
    would produce a difference between the two inputs, and that would
    drive the output to wherever it has to go to reduce that difference to
    nearly zero (so that nearly zero times a lot of differential gain
    amounts to wherever the output has arrived).

    So both configurations make use of the common mode range, but the non
    inverting configuration makes use of more of it.

    For this reason, an opamp has to have very low common mode voltage
    gain (it has to ignore where the two inputs are and only be aware of
    their difference) in order to work well in the non inverting
    configuration, because there is a lot of common mode swing going on,
    and it has to be ignored, and only the difference between the two
    inputs being amplified, for the circuit to perform as expected.

    Since only a tiny common mode swing (half of the tiny difference
    voltage) takes place with the inverting configuration, very low common
    mode gain is a lot less important for this configuration to work well.
     
  4. Amanda Robin

    Amanda Robin Guest

    <<snipped good explanations to question about common mode voltage>>

    Thanks, John and Bob, for your time and good explanations.

    Amanda
     
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