A two-stage low-voltage CMOS op amp with rail-to-rail input and output voltage ranges is presented. The circuit uses complementary differential input pairs to achieve the rail-to-rail common-mode input voltage range. The differential pairs operate in strong inversion, and the constant transconductance is obtained by keeping the sum of the square roots of the tail currents constant. Such an input stage has an offset voltage which depends on the common input voltage level, resulting in a poor common-mode rejection ratio. Therefore, special attention has been given to the reduction of the op amp's systematic offset voltage. Gain-boost amplifiers are connected in a special way to provide not only an increase of the low-frequency open-loop gain, but also to provide a significant reduction of the systematic offset voltage.
Gierkink, S. L. J., Holzmann, P. J., Wiegerink, R. J., & Wassenaar, R. F. (1999). Some design aspects of a two-stage rail-to-rail CMOS op amp. Analog integrated circuits and signal processing, 21(2), 143-152. [10.1023/A:1008317623471]. https://doi.org/10.1023/A:1008317623471