In this thesis some different techniques, which are usually used to achieve a constant input transconductance in terms of the input common-mode voltage variation in an op-amp are studied. A new method is introduced in order to keep the input transconductance constant against input common-mode voltage, process, and temperature variations. The design works properly regardless of the operating regions of the MOS devices used in the input differential pair (i.e., saturation or subthreshold). Furthermore, simulation results show that the input transconductance does not change appreciably with the length of these devices. These all prove that the presented method acts independent of the special drain current equation of the input devices that is generally depends on different device parameters such as channel length and gate oxide thickness. All designs are simulated in a 0.18µm standard CMOS technology. Keywords: Operational amplifier,constant- , rail-to-rail, DC level shifter
