The output voltage is usually regulated using a transistor. In a parallel stabilizer the transistor is connected parallel to the stabilizer load, consuming the excess of power. This type of stabilizer is seldom used. In the more popular sequential stabilizer the transistor is connected sequentially with the stabilizer load, restricting the output current.
The voltage stabilizer is the shunt regulator such as a Zener diode or avalanche diode. Each of these devices begins conducting at a specified voltage and will conduct as much current as required to hold its terminal voltage to that specified voltage . Hence the shunt regulator can be viewed as the limited power parallel stabilizer. The shunt regulator output is used as a voltage reference.
The Zener diode and avalanche diode have opposite threshold voltage dependence on temperature. By connecting these two devices sequentially, it is possible to construct a voltage reference with improved thermal stability. Sometimes (mostly for the voltages around 5.6 V) both effects are combined in the same diode.
In the simplest case, the base of the regulating transistor is directly connected to the voltage reference:
The stabilizer uses the power source, having voltage Uin that may vary over time. It delivers the relatively constant voltage Uout. The output load RL can also vary over time. For such a device to work properly, the input voltage must be larger than the output voltage and the output current must not exceed the limits of the transistor used.
The output voltage of the stabilizer is a little lower than the output of the voltage reference circuit Rv-D1 (output Uz). If the output voltage drops below that limit, this increases the voltage difference between the base and emitter (Ube), opening the transistor and delivering more current. Delivering more current through the same output resistor RL increases the voltage again.
In this case, the operational amplifier opens the transistor more if the voltage at its inverting input drops significantly below the output of the voltage reference at the non-inverting input. Using the voltage divider (R1, R2 and R3) allows choice of the arbitrary output voltage between Uz and Uin.