While highly efficient switching power supplies are commonly used for long battery life in portable end equipment such as mobile phones and PDAs, the internal circuitry of some of these devices is sensitive to noise and therefore does not operate properly when powered from a switching power supply with output ripple. Audio circuitry, PLLs, RF transceivers, and DACs are just a few examples of such circuits. Linear regulators are ideal for powering these circuits.
There are quite a few applications where op amps are used as voltage followers. These include sample and hold circuits and active filters as well as general purpose buffers for transducers or other high-impedance signal sources. The general usefullness of such an amplifier is particularly enhanced if it is both fast and has a low input bias current. High speed permits including the buffer in the signal path or within a feedback loop without significantly affecting response or stability. Low input current prevents loading of high impedance sources, which is the reason for using a buffer in the first place.