## Describe common-mode rejection ratio, Electrical Engineering

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Q. Describe Common-Mode Rejection Ratio ?

When there is a common-mode input voltage, i.e., when the input signals are equal and greater than zero, the output voltage of an ideal op amp is zero because vd is equal to zero. In general, the common-mode input vC is defined as vp + vn/ 2; and the difference signal vd = vp -vn is to be amplified. Common-mode gain AC is defined as the ratio vo/vC when vd = 0. The common-mode rejection ratio (CMRR) is defined by A/AC, but is usually expressed in units of decibel (dB),

It is typically 90 dB, i.e., A/AC ∼ = 32,000. The ideal op amp, however, has infinite CMRR. To explain further, an op amp can be considered a special type of differential amplifier. The object of a differential amplifier, which is formally presented a little later, is to amplify "differences" in voltage between the two inputs, and to be unresponsive to voltage changes that appear simultaneously on both inputs. The differential-mode input signal is the difference between vp and vn; that is to say, vd = vp - vn. The common-mode input signal is the average value of the two input signals; that is to say, vc = (v1 + v2)/2. The output voltage of the amplifier vo is given by vo = vdAd + vcAc, where Ad (called simply A previously) is the differential-mode voltage gain and Ac is the common-mode voltage gain. Under ideal conditions, Ac is equal to zero and the differential amplifier completely rejects the common-mode signals. The departure from this ideal condition is a figure of merit for a differential amplifier and is measured by CMRR, which is the ratio of Ad to Ac. CMRR can thus be seen as a measure of an amplifier's ability to distinguish between differential-mode and common-mode signals. One of the practical advantages of a differential amplifier is its rejection of unwanted signals or noise.

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