CMOS Telescopic and Folded Cascode Amplifiers

A single stage CMOS amplifier cannot give desired performance parameters. By placing a common source amplifier stage in cascade with common gate amplifier stage, a very useful and versatile amplifier circuit is formed [1]. This is known as Cascode configuration. The basic idea here is to combine high input resistance and large transconductance of common source amplifier with current buffering property of common gate amplifier. This cascode combination gives increased output resistance, multiplied DC gain but reduced bandwidth. The load for cascode amplifier is selected a current source made from a cascode pair and is known as telescopic cascode amplifier. As all the MOSFETs in telescopic cascode operate in saturation, the range of output voltage swing gets limited. The folded cascode amplifier is formed by using NMOS CS stage and folded PMOS CG stage both fed from single MOSFET current source and loaded by cascode current source. The objective of this paper is to discuss the advantages of folded cascode over telescopic cascode amplifier. [14]