Abstract Low voltage and low power operational amplifier design has become an increasingly interesting subject as many applications switch to portable battery powered operations. The need for design techniques to allow amplifiers to maintain an acceptable level of performance when the supply voltages decreased is immense. This research work emphasizes on the study of the properties of the differential input/differential output CMOS amplifier with low voltage. A method described in this thesis is to design a two stage fully differential CMOS op-amp and analyze the effect of aspect ratios on the characteristics of the design parameters namely, nodal analysis (node voltage, node current, PD), DC sweep (ICMR), transient analysis (voltage swing, SR), and AC sweep (Open Loop Gain, UGB, CMRR) which operates at ± 2.5V power supply using 0.2µm CMOS technology. The design has been simulated with ORCAD Pspice soft ware. The op amp is designed to exhibit an open loop gain of 152.154dB (40,522,851.68 = 10 ), a unity gain band width of 4.8097MHz, output voltage swing of V is 2.0024V, CMRR of 152.165dB, the raising and failing slew rate of 12V/µs, -11.02V/µs, respectively. The design was found to dissipate very small power (≤ 0.333mW). The phase difference was 126.864 0 outpeaktopeak . Design has been carried out in mentor graph tools. Key words: CMOS, two-stage differential input/differential output op-amp, low voltage, slew rate, DC gain and ORCAD PSPICE. 7.6077