A Constant-Gm, Rail-to-Rail Input Stage ultra Low Power Operational Amplifier using adaptive biasing & driven bulk topology

In this paper, a 0.9V supply , rail-to-rail operational amplifier has been designed. Because of low voltage operation we have been made to use input differential pair in weak inversion mode and consequently low frequency response is natural. To obtain a gm-constant transconductor in first stage we use from a different structure that achieves smaller gm variation than previously defined circuits.. This opamp is designed in 0.18 μm CMOS process, and simulated in HSPICE