Hybrid Cascode Frequency Compensation for Four-Stage OTAs Driving a Wide Range of CL

Feedback amplifiers consisting of multiple gain stages are used to establish highly accurate buffered/amplified signals that can drive a wide range of capacitive load (CL). This article models, analyzes, and presents the measurement results of a high-gain four-stage operational transconductance amplifier (OTA) that is able to handle a wide range of CL up to infinity. In addition to local compensation capacitors and nulling resistors in the intermediate stages, two high-speed feedback loops made by parallel Miller capacitors and current buffers provide Miller compensation and the consequent pole-splitting in the lower CL range. The dominant pole is made dependent on the CL for the higher CL range, enabling the maintenance of the stability conditions up to infinite CL. The proposed amplifier was integrated into a 65-nm CMOS technology, consuming 140- $\mu \text{A}$ static current under a 1.2-V supply and an active area of 0.0086 mm2. A dc gain greater than 100 dB was also perceived with a unity-gain frequency (UGF) of 4.09, 2.01, and 0.27 MHz for 4.7, 10, and 100-nF load capacitors, respectively. The average slew rate (SR) is 0.59 V/ $\mu \text{s}$ , when the OTA is formed as a buffer targeting the CLs higher than 4.7-nF.