Editing Transistor (section)

===Transistor as a switch===
[[File:Transistor as switch.svg|thumb|BJT used as an electronic switch in grounded-emitter configuration]]
Transistors are commonly used in [[digital circuit]]s as [[electronic switches]] which can be either in an ''on'' or ''off'' state, both for high-power applications such as [[switched-mode power supply|switched-mode power supplies]] and for low-power applications such as [[logic gate]]s. Important parameters for this application include the current switched, the voltage handled, and the switching speed, characterized by the [[rise time|rise and fall times]].<ref name="Pulfrey" />

In a switching circuit, the goal is to simulate, as near as possible, the ideal switch having the properties of an open circuit when off, the short circuit when on, and an instantaneous transition between the two states. Parameters are chosen such that the ''off'' output is limited to leakage currents too small to affect connected circuitry, the resistance of the transistor in the ''on'' state is too small to affect circuitry, and the transition between the two states is fast enough not to have a detrimental effect.<ref name="Pulfrey" />

In a grounded-emitter transistor circuit, such as the light-switch circuit shown, as the base voltage rises, the emitter and collector currents rise exponentially. The collector voltage drops because of reduced resistance from the collector to the emitter. If the voltage difference between the collector and emitter were zero (or near zero), the collector current would be limited only by the load resistance (light bulb) and the supply voltage. This is called ''saturation'' because the current is flowing from collector to emitter freely. When saturated, the switch is said to be ''on''.<ref>{{Cite book|last=Kaplan|first=Daniel|title=Hands-On Electronics|year=2003|isbn=978-0-511-07668-8|pages=47–54, 60–61|bibcode=2003hoe..book.....K}}</ref>

The use of bipolar transistors for switching applications requires biasing the transistor so that it operates between its cut-off region in the off-state and the saturation region (''on''). This requires sufficient base drive current. As the transistor provides current gain, it facilitates the switching of a relatively large current in the collector by a much smaller current into the base terminal. The ratio of these currents varies depending on the type of transistor, and even for a particular type, varies depending on the collector current. In the example of a light-switch circuit, as shown, the resistor is chosen to provide enough base current to ensure the transistor is saturated.<ref name="Pulfrey" /> The base resistor value is calculated from the supply voltage, transistor C-E junction voltage drop, collector current, and amplification factor beta.<ref>{{Cite web|title=Transistor Base Resistor Calculator|date=January 27, 2012 |url=https://kaizerpowerelectronics.dk/calculators/transistor-base-resistor-calculator/}}</ref>