Design of tunable folded cascode differential amplifier using PDM

Abstract

Operational Amplifiers (op-amps) are one of the most commonly used blocks in analog and mixed signal VLSI design. Designers often spend considerable time in designing op-amps analytically, and then realize that the simulated circuits do not match with analytical expectations. This is primarily due to modeling of short channel MOS devices using long channel equations. Finally an adhoc mechanism is adopted to realize the op-amp with the help of a simulator. Proposed Potential Distribution Method (PDM) is a method where the design methodology is based on actual behavior of the devices and it is free from any analytical expression. This paper demonstrates the design approach for realizing a fully differential folded cascode op-amp using PDM, which is based on simulator results obtained with predefined bias conditions. The dependency of various performance parameters, like slew rate (SR), unity gain bandwidth (UGB), phase margin (PM), etc. on potentials and current distribution at different nodes is presented. It is found that using these dependencies, the target specifications for an op-amp can be achieved with shorter design time. Also, fine tuning the performance metric can be achieved using PDM. Finally, a fully differential folded cascode op-amp is thus designed and the simulation results are presented.