Simplified design method for fully differential gain-boosted folded cascade OTA

Abstract

Operational amplifiers are an integral part of analog and mixed signal design. With the advancement of process technology, the transistor dimensions are rapidly scaling down leading to reduced ro. In deep sub-micron regime, this leads to a fairly small low frequency gain offered by amplifiers. Applications demanding high gain amplifiers suffer at design level. Even multi-stage architectures fail to provide gain greater than 60 dB. Designers now have to depend on alternate techniques like gain boosting architectures. For most designers, designing gain boosting structures using traditional design method become a trivial problem. They often find it difficult to keep all transistors in saturation, particularly in cascode structures. In this work, a simplified design method for designing a fully differential gain boosted folded cascode amplifier is presented. Potential Distribution Method (PDM) guarantees that all transistors are operating in saturation region. PDM is an extremely simple and quick methodology, which is independent of process technology, device length and complex equations governing the devices and the circuit. The implemented design provides a DC gain of around 118 dB with a unity gain frequency of 183 MHz. The design is carried out using UMC 180 nm CMOS technology and the simulation results are presented.