Document Type : Review Article
Integrated Circuits Design Laboratory, Department of Electrical Engineering, Amirkabir University of Technology (Tehran Polytechnic), Tehran, Iran
In this paper, a tutorial review on several structural improvements of the CMOS folded-cascode operational transconductance amplifier (OTA) is presented. After a brief discussion on the structure and operation of the conventional folded-cascode amplifier, its several architectural improvements are reviewed. These improvements include advances on class A and class AB folded-cascode amplifiers and recycling folded-cascode amplifier. Then, several improved class A and class AB recycling folded-cascode operational transconductance amplifiers are discussed and finally some two-stage operational transconductance amplifiers based on the current recycling technique are reviewed. As it is seen, many architectural innovations have been proposed to improve both small-signal parameters including DC gain, unity-gain bandwidth, and phase margin, and large-signal operation, which is usually characterized by slew rate, in the proposed OTAs compared to the basic folded-cascode amplifier. Current recycling, shunt current sources, cross-coupled transistors to realize local positive feedback paths, local common-mode feedback, flipped voltage follower cells to realize the class AB operation, active current mirrors, and several other techniques have been utilized in the structural improvements of the basic folded-cascode operational transconductance amplifier. The achieved results are more promising and demonstrate substantial achievements in design of operational transconductance amplifiers in low-voltage and more scaled nano-meter CMOS processes.