A low drop regulator (LDO regulator) is a DC linear voltage regulator that can regulate the output voltage even when the supply voltage is very close to the output voltage. Voltage regulators are used to provide a stable power supply voltage independent of load impedance, input-voltage variations, temperature, and time. Low-dropout regulators are distinguished by their ability to maintain regulation with small differences between supply voltage and load voltage.
Nowadays the only reasons for not using a voltage regulator in a power supply are: I have not got one: I need an ‘odd’ voltage; I want to keep the current drain very low. The regulator shown in the diagram is suitable for currents of 5-10 mA. The two transistors draw only a tiny current. The drop across the regulator depends on the load current and lies between 0.5 V and 1.4 V. The output voltage may be preset between 1.8 V and 8 V. On power-up, there is no voltage at the source of T1, so that the FET conducts. Current amplifier T2 then draws base current and is switched on. This arrangement means that the reference (gate) voltage may be taken from a high-resistance potentiometer. The quiescent current depends on the level of the preset output voltage: at 5 V. it is a mere 1 pA. When T2 is switched on. the output voltage will rise to its present level. The base poten-tial of T2 and thus the source potential of T1. remains about 0.6 V higher than the output voltage, so that it rises in step with the output voltage. The gate of T1 is, however, connected to the wiper of P1, whose voltage rises more slowly than the output voltage, because the present is a potential divider. Consequently, the gate of T1 II
becomes more and more negative with respect to its source. An equilibrium is soon reached, whereupon the FET reduces the base current of T2 to a degree that entires stability of the output voltage. In normal circumstances, the output volt-age vs load current ratio is of the order of 9 mV/mA.