Standard three-terminal voltage regulator ICs useanNPN transistor as the series pass element,so theinput-output voltage drop is 1.5V-2V. Low dropoutregulators using lateral PNP pass transistors havebeen available for several years, but a lateral PNPtransistorhaslow gain,sothe basecurrent is necessarilyhigh, and a low fT, so the settling time is poor.Moreover, for stability a large output capacitor isneeded.