An Unconditionally Stable Three-Stage OTA

A three-stage operational transconductance amplifier (OTA) stable for any load capacitor is introduced for the first time in literature. It contains a novel local ac feedback network in the intermediate stage in addition to the cascode-Miller compensation strategy. The final topology is Miller-compensated for the light and $C_{L}$ -compensated for the heavy capacitive loads, and its operation is improved by, including a feed-forward stage and a slew-rate enhancer. A prototype of the proposed amplifier was fabricated in a 65-nm standard CMOS process with 1.2-V supply voltage, occupying an active area of 0.0028 mm2 with 13.07- $\mu \text{A}$ quiescent current. A DC gain of about 100 dB was measured with a unity-gain frequency of 4000, 705, and 22 kHz for 0.03, 0.5, and 100-nF capacitive loads, respectively.