Why is there a difference in the bandwidth and stability of the LDO, Quasi-LDO, and standard NPN regulators?
The standard NPN linear regulator tends to have wide bandwidth and be insensitive to output capacitive loading. The LDO (PNP) regulator tends to have lower bandwidth and require specific output capacitors to maintain stability. A Quasi-LDO regulator has characteristics somewhere between the NPN and LDO. The reason for these differences has to do with the orientation and type of the output transistor. In an NPN regulator, the input voltage is applied to the collector and the load is placed on the emitter. In an LDO, or PNP-based design, the input voltage is applied to the emitter and the load is applied to the collector. If you mentally turn the circuit on its side, you will see that the standard, or NPN-based regulator forms an emitter follower. The overall circuit generally has a gain of less than 1. With the PNP LDO circuit, the load impedance acts as a collector resistance. The LDO circuit creates a common collector amplifier, with the load impedance acting as a collector gain resis
