Linear regulator
Simple linear regulators may only contain as little as a Zener diode and a series resistor; more complicated regulators include separate stages of voltage reference, error amplifier and power pass element.
Linear regulators may also be made up of assemblies of discrete solid-state or vacuum tube components.
With negative feedback and good choice of compensation, the output voltage is kept reasonably constant.
Linear regulators are often inefficient: since the transistor is acting like a resistor, it will waste electrical energy by converting it to heat.
The linear regulator also has the advantage of not requiring magnetic devices (inductors or transformers) which can be relatively expensive or bulky, being often of simpler design, and cause less electromagnetic interference.
Some designs of linear regulators use only transistors, diodes and resistors, which are easier to fabricate into an integrated circuit, further reducing their weight, footprint on a PCB, and price.
It is, however, simpler, sometimes consisting of just a voltage-reference diode, and is used in very low-powered circuits where the wasted current is too small to be of concern.
The series regulator works by providing a path from the supply voltage to the load through a variable resistance, usually a transistor (in this role it is usually termed the series pass transistor); it is in the "top half" of the voltage divider - the bottom half being the load.
The image shows a simple shunt voltage regulator that operates by way of the Zener diode's action of maintaining a constant voltage across itself when the current through it is sufficient to take it into the Zener breakdown region.
Here, the load current IR2 is supplied by the transistor whose base is now connected to the Zener diode.
This circuit has much better regulation than the simple shunt regulator, since the base current of the transistor forms a very light load on the Zener, thereby minimising variation in Zener voltage due to variation in the load.
Note that the output voltage will always be about 0.65 V less than the Zener due to the transistor's VBE drop.
Finally, it is occasionally made microadjustable by adding a low value pot in series with the Zener; this allows a little voltage adjustment, but degrades regulation (see also capacitance multiplier).
Two example methods are: An adjustable regulator generates a fixed low nominal voltage between its output and its adjust terminal (equivalent to the ground terminal in a fixed regulator).
They offer the capability to adjust the output voltage by using external resistors of specific values.
The ratio of resistances determines the output voltage using the same feedback mechanisms described earlier.
Single IC dual tracking adjustable regulators are available for applications such as op-amp circuits needing matched positive and negative DC supplies.
It provides separate positive and negative regulated outputs, simplifying dual power supply designs.
Linear regulators can be constructed using discrete components but are usually encountered in integrated circuit forms.
The most common linear regulators are three-terminal integrated circuits in the TO-220 package.
Robust automotive voltage regulators, such as LM2940 / MIC2940A / AZ2940, can handle reverse battery connections and brief +50/-50V transients too.
Some Low-dropout regulator (LDO) alternatives, such as MCP1700 / MCP1711 / TPS7A05 / XC6206, have a very low quiescent current of less than 5 μA (approximately 1,000 times less than the LM78xx series) making them better suited for battery-powered devices.
