  Presented are a methodology and a DFII-based tool for AC-stability analysis
of a wide variety of closed-loop continuous-time (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 DFII?
environment. Its "push-button" graphical user interface (GUI) is easy to use
and understand. The tool can be invoked directly from Composer? schematic and
does not require 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.
