# Common Mode Gain of Diff Amplifier

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

1. ### 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 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 AAC Fanatic!

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That is true if the amplifier is ideal.

4. ### The Electrician AAC Fanatic!

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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 AAC Fanatic!

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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 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.