Butterfly curve
Beginner
A graph engineers draw of a memory cell’s two halves. It looks like a pair of wings, and bigger wings mean the cell holds its bit more firmly.
Novice
One inverter’s transfer curve plotted together with the other inverter’s curve flipped across the diagonal. The two curves cross three times: two stable states and one balance point between them. The open ‘wings’ show stability.
Expert
Plot of and the inverse of in the (Q, QB) plane. Each lobe corresponds to one stored state; its largest inscribed square gives that state’s SNM, and the smaller lobe sets the cell’s SNM. Under mismatch the lobes become unequal; when one closes the cell is monostable.
Explained in Memory cells (Transistors).
See also: Static noise margin (SNM), Voltage transfer curve (VTC).