Synthesis of High Gain Operational Transconductance Amplifiers for Closed-Loop Operation Using a Generalized Controller-Based Compensation Method

This paper presents a systematic procedure that can be used to create operational transconductance amplifiers (OTAs) for closed-loop operation using multiple low-gain stages to realize extremely high DC gain. Such devices are necessary to realize analog functions with demanding absolute accuracy requirements, e.g., high-resolution ADCs and DACs. The principle is based on the cascade of undamped integrators to realize large DC gains and a state-space derived controller to stabilize its operation in a closed-loop configuration. A programmable OTA fabricated in the IBM 130 nm CMOS process is used as a test vehicle to prove the design principle through its 2 to 5th-order realization. Measured data reveals DC gains ranging from 50 to 150 dB with a 3-dB bandwidth of 10 kHz and a unity gain frequency of 10 MHz. While this paper demonstrates the design principles using CMOS integrated circuits, the principle is general and can be applied to any type of circuit technology in integrated or discrete implementation. Moreover, the methods are easily automated as the principles are based on closed-form formulae as opposed to iterative numerical search techniques.