Editing Transistor (section)

==Simplified operation==
[[File: Transistor Simple Circuit Diagram with NPN Labels.svg|thumb|A simple circuit diagram showing the labels of an n–p–n bipolar transistor]]
A transistor can use a small signal applied between one pair of its terminals to control a much larger signal at another pair of terminals, a property called [[Gain (electronics)|gain]]. It can produce a stronger output signal, a voltage or current, proportional to a weaker input signal, acting as an [[amplifier]]. It can also be used as an electrically controlled [[switch]], where the amount of current is determined by other circuit elements.<ref>{{Cite book|last=Roland|first=James|url=https://books.google.com/books?id=g2qsDAAAQBAJ&q=how+do+transistors+work|title=How Transistors Work|date=August 1, 2016|publisher=Lerner Publications |isbn=978-1-5124-2146-0|language=en}}</ref>

There are two types of transistors, with slight differences in how they are used:

* A ''[[bipolar transistor|bipolar junction transistor (BJT)]]'' has terminals labeled '''base''', '''collector''' and '''emitter'''. A small current at the base terminal, flowing between the base and the emitter, can control or switch a much larger current between the collector and emitter.

* A ''[[field-effect transistor|field-effect transistor (FET)]]'' has terminals labeled '''gate''', '''source''' and '''drain'''. A voltage at the gate can control a current between source and drain.<ref name="Pulfrey">{{Cite book|last=Pulfrey|first=David L.|url=https://books.google.com/books?id=y9dYENs2SVUC&q=how+do+transistors+work|title=Understanding Modern Transistors and Diodes|date=January 28, 2010|publisher=Cambridge University Press|isbn=978-1-139-48467-1|language=en}}</ref>

The top image in this section represents a typical bipolar transistor in a circuit. A charge flows between emitter and collector terminals depending on the current in the base. Because the base and emitter connections behave like a semiconductor diode, a voltage drop develops between them. The amount of this drop, determined by the transistor's material, is referred to as ''V''<sub>BE</sub>.<ref name="Pulfrey" /> (Base Emitter Voltage)

===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>

===Transistor as an amplifier===
[[File:NPN common emitter AC.svg|thumb|An amplifier circuit, a common-emitter configuration with a voltage-divider bias circuit]]
The [[common-emitter amplifier]] is designed so that a small change in voltage (''V''<sub>in</sub>) changes the small current through the base of the transistor whose current amplification combined with the properties of the circuit means that small swings in ''V''<sub>in</sub> produce large changes in ''V''<sub>out</sub>.<ref name="Pulfrey" />

Various configurations of single transistor amplifiers are possible, with some providing current gain, some voltage gain, and some both.

From [[mobile phone]]s to [[television]]s, vast numbers of products include amplifiers for [[sound reproduction]], [[Transmitter|radio transmission]], and [[signal processing]]. The first discrete-transistor audio amplifiers barely supplied a few hundred milliwatts, but power and audio fidelity gradually increased as better transistors became available and amplifier architecture evolved.<ref name="Pulfrey" />

Modern transistor audio amplifiers of up to a few hundred [[watt]]s are common and relatively inexpensive.