Editing Linear regulator (section)

==Overview==
The transistor (or other device) is used as one half of a [[voltage divider]] to establish the regulated output voltage. The output voltage is compared to a reference voltage to produce a control signal to the transistor which will drive its gate or base. With negative feedback and good choice of [[Frequency compensation|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. In fact, the power loss due to heating in the transistor is the [[current (electricity)|current]]  multiplied by the [[voltage]] difference between input and output voltage. The same function can often be performed much more efficiently by a [[switched-mode power supply]], but a linear regulator may be preferred for light loads or where the desired output voltage approaches the source voltage. In these cases, the linear regulator may dissipate less power than a switcher. 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.

All linear regulators require an input voltage at least some minimum amount higher than the desired output voltage. That minimum amount is called the [[dropout voltage]]. For example, a common regulator such as the [[78xx|7805]] has an output voltage of 5 V, but can only maintain this if the input voltage remains above about 7 V, before the output voltage begins sagging below the rated output.  Its dropout voltage is therefore 7 V − 5 V = 2 V.  When the supply voltage is less than about 2 V above the desired output voltage, as is the case in low-voltage [[microprocessor]] power supplies, so-called [[low dropout regulator]]s (LDOs) must be used.

When the output regulated voltage must be higher than the available input voltage, no linear regulator will work (not even a low dropout regulator). In this situation, a [[boost converter]] or a [[charge pump]] must be used. Most linear regulators will continue to provide some output voltage approximately the dropout voltage below the input voltage for inputs below the nominal output voltage until the input voltage drops significantly.

Linear regulators exist in two basic forms: shunt regulators and series regulators. Most linear regulators have a maximum rated output current. This is generally limited by either power dissipation capability, or by the current carrying capability of the output transistor. 

=== Shunt regulators ===
The shunt regulator works by providing a path from the supply voltage to ground through a variable resistance (the main transistor is in the "bottom half" of the voltage divider).  The current through the shunt regulator is diverted away from the load and flows directly to ground, making this form usually less efficient than the series regulator.  It is, however, simpler, sometimes consisting of just a voltage-reference [[diode]], and is used in very [[Low-power electronics|low-power]]ed circuits where the wasted current is too small to be of concern. This form is very common for voltage reference circuits. A shunt regulator can usually only sink (absorb) current.

=== Series regulators ===
Series regulators are the more common form; they are more efficient than shunt designs. 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 power dissipated by the regulating device is equal to the power supply output current times the ''voltage drop'' in the regulating device. For efficiency and reduced stress on the pass transistor, designers try to minimize the voltage drop but not all circuits regulate well once the input (unregulated) voltage comes close to the required output voltage; those that do are termed '''low dropout''' regulators,  A series regulator can usually only source (supply) current, unlike shunt regulators.