A 0.25-V Rail-to-Rail Three-Stage OTA With an Enhanced DC Gain

This brief proposes a 0.25V rail-to-rail three stage OTA. The proposed OTA improves a DC gain by inserting an NMOS gate-driven amplifier into the conventional bulk-driven OTA. In addition, it uses an asymmetric self-cascode transistor and an indirect feedback compensation to enhance a DC gain and an unit-gain frequency. At the first stage, the bulk-driven amplifier has a rail-to-rail input, but it has a low DC gain due to a small transconductance. At the second stage, the NMOS gatedriven amplifier enhances the DC gain. At the last stage, the common-source amplifier drives an output load capacitor. The proposed OTA was fabricated using a 65nm CMOS process. Its area is 0.002mm 2 . It consumes 0.026μW at the supply-voltage of 0.25V. The DC gain and unit-gain frequency are 70dB and 9.5kHz, respectively, with the phase margin of 88° at the load capacitance of 15pF.