5.3 PRACTICAL PROPERTIES OF OPERATIONAL AMPLIFIERS 237−− −+++
+−vovn
vpRi+−A(vp − vn)Figure 5.3.3Op amp as a circuit component
block.Common-Mode Rejection Ratio (CMRR)
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 becausevdis equal to zero. In general, the
common-mode inputvCis defined as
(
vp+vn)
/2; and the difference signalvd=vp−vnis to be
amplified. Common-mode gainACis defined as the ratiovo/vCwhenvd=0. The common-mode
rejection ratio (CMRR) is defined byA/AC, but is usually expressed in units of decibel (dB),
CMRR=20 log 10A
AC(5.3.1)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. Thedifferential-mode input signalis the difference between
vpandvn; that is to say,vd=vp−vn. Thecommon-mode input signalis the average value of the two
input signals; that is to say,vc=(v 1 +v 2 )/2. The output voltage of the amplifiervois given byvo
=vdAd+vcAc, whereAd(called simplyApreviously) is the differential-mode voltage gain andAc
is the common-mode voltage gain. Under ideal conditions,Acis 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
AdtoAc. 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.
INPUTOFFSETVOLTAGEVOS
When both inputs are tied to ground, i.e., both differential-mode and common-mode inputs are
zero, the output should be zero. In practice there will be mismatches in amplifier components, and
if there is a mismatch in an input stage, the effect will be amplified, leading to a significant output
voltage. The input offset voltageVOSis the differential input voltage required to make the output
zero, and is typically 1 mV. With some op amps, e.g., the 741, two voltage offset terminals are
provided, and by connecting a potentiometer between them and taking the slider to the specified
dc supply rail (or bus), as in Figure 5.3.4, the potentiometer can then be adjusted to zero the offset
voltage.