Low-voltage low-power ΔΣ modulators are required for many portable systems. Although SC Δ Σ modulators provide reliable high-resolution A/D conversion, the traditional SC circuit design faces many challenges as the fabrication technology moves into ultra deep-submicron CMOS technology. An OTA is not only the key building block, but also the major power consuming block in SC circuits. The traditional OTAs with differential pair have the fundamental limitation of input common-mode range, because the threshold voltage of transistors is not scaled at the rate of the supply voltage. Therefore, OTA design is the most difficult task in SC circuit design especially in scaled CMOS technologies [1, 2].