Common Mode Gain of Diff Amplifier

Discussion in 'General Electronics Chat' started by Mazaag, Apr 17, 2010.

  1. Mazaag

    Mazaag Thread Starter Senior Member

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    Hi Guys,

    Could someone please explain to me why we consider the common mode signal going into a differential amplifier (when wanting to calculate the common mode gain) is equal to the average of the inverting and non-inverting signals? I understand what common mode gain is but don't understand why the average is the common mode signal..

    Thanks guys!
     
  2. beenthere

    beenthere Retired Moderator

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    That isn't correct. A common mode signal is equally present on both inputs. A difference amplifier will not respond to a common mode signal (no difference on either input).

    http://www.edn.com/article/CA289961.html
     
  3. Ron H

    Ron H E-book Developer

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    That is true if the amplifier is ideal.
     
  4. The Electrician

    The Electrician Senior Member

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    The very first paragraph of the article you referenced says that it is correct:

    "A common-mode voltage is one-half the vector sum of the voltages between each conductor of a balanced circuit and the local ground."
     
  5. The Electrician

    The Electrician Senior Member

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    Imagine that we construct signals to be applied to the inputs of a differential amplifier.

    Let Vc be a common mode signal. This signal will be applied to both inputs.

    Let Vd and -Vd be differential signals applied to the inputs.

    To one input we apply Vc + Vd and to the other we apply Vc + (-Vd)

    The differential signal seen by the amplifier is the difference between these two, namely (Vc + Vd) - (Vc - Vd) = 2Vd

    Let's take the average of the two input signals:

    ((Vc + Vd) + (Vc - Vd))/2 = (2Vc)/2 = Vc

    We have recovered the common mode signal.
     
  6. beenthere

    beenthere Retired Moderator

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    Taking the case of an instrumentation amplifier and applying a resistive bridge (let's say strain gauges) to the inputs. With the bridge balanced and 10 volts applied as the excitation voltage, the common mode voltage is 5 volts. That is, + 5 volts present on the inverting and non-inverting inputs.

    The output of the IA is not 5 volts, but 0 volts. The classical three op amp IA appears to be a buffered difference amplifier.
     
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