A tool and methodology for AC-stability analysis of continuous-time closed-loop systems

Presented are a methodology and a DFII-based tool for AC-stability analysis of a wide variety of closed-loop continuous-time systems (operational amplifiers and other linear circuits). The methodology used allows for easy identification and diagnostics of AC-stability problems including not only main-loop effects, but also local-instability loops in current mirrors, bias circuits and emitter or source followers without breaking the loop. The results of the analysis are easy to interpret. Estimated phase margin is readily available. Instability nodes and loops along with their respective oscillation frequencies are immediately identified and mapped to the existing circuit nodes, thus offering significant advantages compared to traditional "black-box" methods of stability analysis (transient overshoot, bode and phase margin plots, etc.). The tool for AC-stability analysis is written in SKILL™ and is fully integrated in the DFII™ environment. Its "pushbutton" graphical user interface (GUI) is easy to use and understand. The tool can be invoked directly from a Composer™ schematic and does not require an active Analog Artist™ session. The tool is not dependent on the use of a specific fabrication technology or process design kit customization. It requires OCEAN™, Spectre™ and waveform calculator capabilities to run.