This paper presents an ultra-low-power low-voltage Class-AB Fully Differential Operational Amplifier designed in 45-nm CMOS technology. The proposed circuit uses transistors operating in sub-threshold region for low-power and low-voltage operation. The proposed Op Amp offers an open-loop gain of 74.6 dB, 1 MHz unity gain frequency, 50-degrees Phase Margin, and 91.55 dB common-mode rejection ratio for 10 pF loads with a power consumption of 0.39 mu W at a +/-0.4 V supply. Simulation results illustrate a Slew Rate of 0.013 V/us and a rail-to-rail output swing, demonstrating its correct functionality.