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In this paper, a low-voltage low-power two stage rail-to-rail CMOS opamp is presented. The input stage is composed of the complementary differential pairs and a pair of modified current-mode maximum selecting circuits (Maximum Current Selecting Circuit II) in order to achieve rail-to-rail operation and constant-gm. Using the modified constant-gm methodology, this input stage co-operated with a family of two class-AB output stages can drive a wide-range of capacitive loads with a large bandwidth-to-supply-power ratio. These opamps reach a high DC gain (≥84 dB), and have a bandwidth-to-supply-power ratio of 13 MHz/mW and 9.5 MHz/mW with capacitive loads 20 pF and 100 pF respectively.