Basic Low Dropout Voltage Regulator

This is a fundamental Low Dropout (LDO) Voltage Regulator circuit using an operational amplifier. To achieve low dropout the pass transistor used is a PNP unlike the traditional voltage regulators which uses NPN transistor. The BJT is configured in common emitter, since there is 180° phase shift in this transistor amplifier the feedback to the op-amp is through the noninverting input to achieved a net negative feedback to stabilize the loop. The disadvantage with this circuit is the feedback at the base and collector terminal of the BJT which makes it susceptible to instability.