Rail-to-rail CMOS opamp with stabilized operating point

The paper presents a new solution to achieve rail-to-rail common mode input voltage swing for CMOS operational amplifiers. The common mode input range of a MOS differential stage is limited by the threshold voltage of the input transistors. With a pair of complementary input stages the whole range from GND to VDD can be covered. Their drain currents are summed up but the resulting current as well as the DC voltage of the summing node drastically change when one of the input stages turns off near the power rails. The new method applies a built-in feedback loop with an auxiliary opamp for stabilizing the voltage at the summing node. This way a very stable operating point and ideal CMRR can be assured. With selected transistor aspect ratios nearly constant voltage gain can be achieved for the whole common mode input voltage range.