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 v_p + v_n/ 2; and the difference signal v_d = v_p -v_n is to be amplified. Common-mode gain AC is defined as the ratio v_o/v_C when v_d = 0. The common-mode rejection ratio (CMRR) is defined by A/A_C, but is usually expressed in units of decibel (dB),

It is typically 90 dB, i.e., A/A_C ∼ = 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 v_p and v_n; that is to say, v_d = v_p - v_n. The common-mode input signal is the average value of the two input signals; that is to say, v_c = (v₁ + v₂)/2. The output voltage of the amplifier vo is given by v_o = v_dA_d + v_cA_c, where A_d (called simply A previously) is the differential-mode voltage gain and Ac is the common-mode voltage gain. Under ideal conditions, A_c 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 A_d to A_c. 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.