Basically there are two types of transistor voltage regulators. They are

(i) Series Voltage Regulators and

(ii) Shunt Voltage Regulators.

Each type of circuit can provide an output dc voltage that is regulated or maintained at a predetermined value even if the input voltage varies or the load connected to the output terminal changes.

Series Voltage Regulator

The basic connection of a series voltage regulator circuit is shown in the block diagram given in the figure below.

The series element controls the magnitude of the input voltage that gets to the output. The output voltage is sampled by a circuit that provides feedback voltage to be compared to a reference voltage.

If the output voltage increases the comparator circuit provides a control signal to cause the series control element to reduce the magnitude of the output voltage — thereby main­taining the output voltage. On the other hand, if the output voltage falls, the comparator circuit provides a control signal to cause the series control element to increase the mag­nitude of output voltage.

Series Voltage Regulator - Block Diagram

Shunt Voltage Regulator

Shunt voltage regulator provides regulation by shunting current away from the load. The block diagram of such a voltage regulator is depicted in the figure. The input unregulated voltage provides current to the load. Some of this current is shunted away by the control element to main­tain the regulated output voltage across the load. If the output voltage tends to change due to change in load, the sampling cir­cuit provides a feedback signal to a comparator cir­cuit which then provides a control signal to vary the magnitude of current shunted away from the load.  For example, the output voltage tends to fall, the sampling circuit provides a feedback signal to the comparator circuit which then provides a control signal to draw lesser shunt current, providing more load current, thereby keeping the regulated voltage constant.

Shunt Voltage Regulator - Block Diagram

Different types of series/shunt discrete transistor voltage regulators will be discussed in the coming posts.

Limitations of Transistorized Power Supplies

The stabilized voltage that can be obtained from transistorized power supplies is limited, to about 30 – 40 V only, because maximum safe value of V_CE is about 50 V. This puts a limit to the use of transistorized power supplies. In order to obtain high voltage regulated power supplies (say of about 250 – 300 V), voltage regulating tube (VR) is used in place of a Zener diode.