The op-amp operates as described in Section 1.2. The differential input pair of a typical op-amp is composed of P-channel MOSFETs as shown in Figure 3-6. As the VIN(+) and VIN(-) voltages increase, the drain-source voltage of Qp3 in the current source decreases, causing the current flowing to the differential input pair and the current mirror to decrease slightly. Figure 3-7 shows an example of ID-VDS curves for a discrete P-channel MOSFET. Suppose, for example, that VDS = -1.5 V and ID = 80 mA initially. As the drain current of the MOSFET decreases, its drain-source voltage changes nonlinearly in the saturation region.
Although the internal devices of an op-amp IC are placed and fabricated in such a manner as to make the MOSFETs uniform, they are not perfectly symmetrical at the micro level. In addition, semiconductor chips are mounted on a metal frame in a package and soldered onto a printed circuit board. Therefore, the mechanical stress applied to each element of an op-amp IC differs slightly. These factors cause a slight difference in threshold voltage among the P-channel MOSFETs of the differential input pair. Therefore, a decrease in drain current causes different nonlinear changes in their drain-source voltage. As a result, the input offset voltage has a slope with respect to the input voltage (VIN) in the common-mode input voltage range as shown in Figure 3-8. Therefore, the CMRR values are calculated at the maximum and minimum common-mode input voltages.