One of the more common features of a differential amplifier is its ability to cancel out or reject certain types of undesired voltage signals.such undesired signals are referred to as "noise" and may occur owing to voltages include by stray magnetic fields in the ground or signal wires,as voltage variations in the voltage supply.Here the important point is that these noise signals are not the signals that are desired to be amplified in the differential amplifier.Their distinguishing feature is that the noise signal appear equally at both inputs of the circuit. It means that any undesired (or noise) signals that appears in polarity,or common to both input terminals,will be largely rejected,or cancelled out at the differential amplifier output.The signal that is to be amplified appears at only one input or opposite in polarity at both inputs.Now what is important to be considered is that,if undesirable noise does occur up to what extent it is rejected out by the differential amplifier ? A measure of this rejection of signals common to both inputs is referred to as the common-mode rejection of the amplifier and a numerical value is assigned,which is called the common-mode rejection ratio(CMRR).
CMRR is defined as the ratio of differential voltage gain common-mode voltage gain and it is given as
CMRR =Ad/Acm
CMRR is defined as the ratio of differential voltage gain common-mode voltage gain and it is given as
CMRR =Ad/Acm
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