This paper presents a single-stage ultra-low-power fully differential operational transconductance amplifier (FD-OTA) with rail-to-rail linear input range operating in weak inversion region. The input core of the OTA is comprised of source degenerated, flipped voltage follower (FVF)-based bulk-driven class AB input pair, into which a regenerative feedback loop has been inserted to boost its bulk transconductance (gib). The proposed FD-OTA has utilized selfcascode current mirror (SC-CM) loads, which increase its open loop gain from nominal intrinsic value of 42 dB to 70.4 dB. It has provided 9.24 kHz gain bandwidth (GBW), consuming 64 nW of quiescent power from a 0.51 V single power supply at 15 pF load. The proposed OTA in unity gain configuration has ensured reduced total harmonic distortion (THD) of 52.4 dB at 200 Hz frequency and 1 Vp-p signal swing. Its fully differential class AB input and output structures have ensured increased gain, GBW, slew rates and output swings with reduced nonlinearity and common mode substrate noise. The Cadence Virtuoso environment using GPDK 180 nm standard n-well CMOS process technology has been used to simulate the proposed circuit.