Occasionally ordinary power sources are referred to as "negative resistances" (fig. 3 above). Although the "static" or "absolute" resistance R static of active devices (power sources) can be considered negative (see Negative static resistance section below) most ordinary power sources (AC or DC), such as batteries, generators, and (non positive feedback) amplifiers, have positive differential resistance (their source resistance). Therefore, these devices cannot function as one-port amplifiers or have the other capabilities of negative differential resistances.