The need for circuits that improve the properties of the DC voltage obtained by rectification and filtering is evident since one needs voltages that are not dependent on the variations of the mains voltage, load resistance, and temperature. One of the methods leading to a solution to this problem is the use of linear regulators. These circuits produce an output DC voltage of a prescribed value with the properties just listed. In this chapter, the synthesis of such circuits will be described along with thorough analysis of the obtained results. Transfer of the variation of the mains voltage and the effects of the variation of the load resistance and temperature will be part of these analyses. A specific aspect of these solutions is their efficiency. Namely, in most cases, a transistor is connected in series with the load which means that high currents are present at its terminals. Reduction of the voltage drop on the transistor is an important issue in designing linear regulators since it leads to higher efficiency. One of the solutions is the so-called low dropout (LDO) circuit which will be considered here.