The output voltage of the full-wave rectifier above varies with fluctuations in the input AC supply. Ideally we would like to have a fixed output voltage which we can use downstream. This is accomplished by a voltage regulator. Let's see how a linear voltage regulator works.
The simplest voltage regulator is a zener diode(1). This is simply a diode that is operated in its (reverse) avalanche breakdown region (the diode is manufactured for a specific breakdown voltage). Below is the current to voltage characteristic of a 1N750 zener with a breakdwn voltage of 4.7v. The current through the zener increases rapidly beyond this point.
To limit the current through the zener (so we don't destroy it), we add a resistor as below.
To increase output current capability of our regulator, we add a series pass transistor(2).
However, we know transistor characteristics vary with temperature and supply level. So we need a feedback mechanism to correct for the changes in supply and temperature. Consider the addition of a resistor divider and feedback transistor below:
Thus we have a regulator that works from 12v-30v input keeping the output constant at 10.6v. The feedback transistor helps in keeping the regulator output constant over changes in temperature and input supply. Let's analyze this circuit: