Determination of stability using return ratios in balanced fully differential feedback circuits

This paper describes SPICE simulation techniques that can be used to find the return ratios for the differential and common-mode feedback loops in balanced, fully differential circuits. Since each of these loops may contain multiple feedback paths, conditions under which these two return ratios are sufficient for determining stability of the balanced feedback circuit are described. To allow the use of these techniques with switched-capacitor common-mode feedback circuits, a special simulation technique is presented that finds the dc bias for the return-ratio simulations.