Common Mode Gain of Diff Amplifier

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

  1. Mazaag

    Mazaag Thread Starter Senior Member

    Joined:
    Oct 23, 2004
    Messages:
    255
    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

    Joined:
    Apr 20, 2004
    Messages:
    15,815
    Location:
    Missouri, USA (GMT -6)
    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

    Joined:
    Apr 14, 2005
    Messages:
    7,050
    Location:
    Idaho, USA (GMT-7)
    That is true if the amplifier is ideal.
  4. The Electrician

    The Electrician Senior Member

    Joined:
    Oct 9, 2007
    Messages:
    1,812
    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

    Joined:
    Oct 9, 2007
    Messages:
    1,812
    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

    Joined:
    Apr 20, 2004
    Messages:
    15,815
    Location:
    Missouri, USA (GMT -6)
    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.
Similar Threads
Forum Title Date
General Electronics Chat Common Mode Choke Mar 10, 2014
General Electronics Chat common-mode behavior of differential amplifier Jun 2, 2013
General Electronics Chat Common mode choke definitions Nov 20, 2012
General Electronics Chat op amp recommandation for commonly used modern op amp Nov 28, 2011
General Electronics Chat 2 transistor model of SCR: Common base?? Sep 10, 2011

Share This Page