Linear regulators use transistors or FETs operating in their linear region to subtract the excess voltage from the applied input voltage to produce a regulated output voltage. The so-called voltage drop voltage refers to the minimum difference between the input voltage and the output voltage required by the regulator to maintain the output voltage within 100mV above and below its rated value. LDO(Low voltage drop) regulators with a positive output voltage typically use a power transistor (also known as a transfer device) as a PNP.