Low-dropout (LDO) regulator is a very important bias circuit used especially in IC design . A reference voltage(1 V) is used as an input to the opamp, due to virtual short circuit caused by the feedback , both inputs will be equal to the reference voltage . For this to happen the drain of the pmos transistor (called passing device) will be adjusted accordingly based on the values of the resistors making up the voltage divider. This voltage is the output of the ldo and will remain constant regardless the load and even if the supply voltage becomes almost the same as the desired output voltage, meaning it can drive a circuit with current. The size of the transistor should be big enough to be able to carry big currents. Moreover, it offers filtering to any noise in the supply line. You can try to change the reference voltage , voltage divider, supply voltage and pmos dimensions to test the circuit 🙂
