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[[File:Darlington pair diagram.svg|frame|right|Circuit diagram of a Darlington pair using NPN transistors]]
This is a preview for the new '''MathML rendering mode''' (with SVG fallback), which is availble in production for registered users.
In [[electronics]], the '''Darlington transistor''' (often called a '''Darlington pair''') is a compound structure consisting of two [[bipolar transistor]]s (either integrated or separated devices) connected in such a way that the current amplified by the first transistor is amplified further by the second one.<ref name="TAoE">{{cite book | first=Paul | last=Horowitz |author2=Winfield Hill | title=The Art of Electronics | publisher=Cambridge University Press  | year=1989 | isbn=0-521-37095-7}}</ref> This configuration gives a much higher common/emitter [[Electric current|current]] [[gain]] than each transistor taken separately and, in the case of integrated devices, can take less space than two individual transistors because they can use a ''shared'' collector. Integrated Darlington pairs come packaged singly in transistor-like packages or as an array of devices (usually eight) in an [[integrated circuit]].


The Darlington configuration was invented by [[Bell Laboratories]] engineer [[Sidney Darlington]] in 1953. He [[patent]]ed the idea of having two or three transistors on a single chip sharing a collector.<ref name="DarlContrib">[http://andros.eecs.berkeley.edu/~hodges/DarlingtonCircuit.pdf Darlington’s Contributions to Transistor Circuit Design]</ref>
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A similar configuration but with transistors of opposite type (one NPN and one PNP) is the [[Sziklai pair]], sometimes called the "complementary Darlington."
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== Behavior ==
'''MathML'''
[[File:Darlington transistor MJ1000.jpg|thumbnail|View of the chip in an MJ1000]]
:<math forcemathmode="mathml">E=mc^2</math>


A Darlington pair behaves like a single transistor with a high current gain (approximately the product of the gains of the two transistors). In fact, integrated devices have three leads (B, C and E), broadly equivalent to those of a standard transistor.
<!--'''PNG'''  (currently default in production)
:<math forcemathmode="png">E=mc^2</math>


A general relation between the compound current gain and the individual gains is given by:
'''source'''
:<math forcemathmode="source">E=mc^2</math> -->


:<math>\beta_\mathrm{Darlington} = \beta_1 \cdot \beta_2 + \beta_1 + \beta_2</math>
<span style="color: red">Follow this [https://en.wikipedia.org/wiki/Special:Preferences#mw-prefsection-rendering link] to change your Math rendering settings.</span> You can also add a [https://en.wikipedia.org/wiki/Special:Preferences#mw-prefsection-rendering-skin Custom CSS] to force the MathML/SVG rendering or select different font families. See [https://www.mediawiki.org/wiki/Extension:Math#CSS_for_the_MathML_with_SVG_fallback_mode these examples].


If ''β<sub>1</sub>'' and ''β<sub>2</sub>'' are high enough (hundreds), this relation can be approximated with:
==Demos==


:<math>\beta_\mathrm{Darlington} \approx \beta_1 \cdot \beta_2</math>
Here are some [https://commons.wikimedia.org/w/index.php?title=Special:ListFiles/Frederic.wang demos]:


Darlington pairs are available as integrated packages or can be made from two discrete transistors; Q<sub>1</sub> (the left-hand transistor in the diagram) can be a low power type, but normally Q<sub>2</sub> (on the right) will need to be high power. The maximum collector current I<sub>C</sub>(max) of the pair is that of Q<sub>2</sub>. A typical integrated power device is the 2N6282, which includes a switch-off resistor and has a current gain of 2400 at I<sub>C</sub>=10A.


A Darlington pair can be sensitive enough to respond to the current passed by skin contact even at safe voltages. Thus it can form the input stage of a touch-sensitive switch.
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A typical modern device has a current gain of 1000 or more, so that only a small base current is needed to make the pair switch on. However, this high current gain comes with several drawbacks.
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One drawback is an approximate doubling of the base/emitter voltage.  Since there are two junctions between the base and emitter of the Darlington transistor, the equivalent base/emitter voltage is the sum of both base/emitter voltages:
*[[Inputtypes|Inputtypes (private Wikis only)]]
:<math>V_{BE} = V_{BE1} + V_{BE2} \approx 2V_{BE1}\!</math>
*[[Url2Image|Url2Image (private Wikis only)]]
For silicon-based technology, where each V<sub>BEi</sub> is about 0.65 V when the device is operating in the active or saturated region, the necessary base/emitter voltage of the pair is 1.3 V.
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Another drawback of the Darlington pair is its increased "saturation" voltage. The output transistor is not allowed to saturate (i.e. its base-collector junction must remain reverse-biased) because the first transistor, when saturated, establishes full (100%) parallel negative feedback between the collector and the base of the second transistor.<ref>Similarly, an [[emitter follower]] never saturates because of the 100% series negative feedback. Another example is an "active diode" made by a transistor with joined base and collector (e.g., the current-setting part of a [[current mirror]]).</ref> Since collector/emitter voltage is equal to the sum of its own base/emitter voltage and the collector-emitter voltage of the first transistor, both positive quantities in normal operation, it always exceeds the base-emitter voltage.  (In symbols, <math>\mathrm{V_{CE2} = V_{CE1} + V_{BE2} > V_{BE2}} \Rightarrow \mathrm{V_{C2} > V_{B2}}</math> always.) Thus the "saturation" voltage of a Darlington transistor is one V<sub>BE</sub> (about 0.65 V in silicon) higher than a single transistor saturation voltage, which is typically 0.1 - 0.2 V in silicon.  For equal collector currents, this drawback translates to an increase in the dissipated power for the Darlington transistor over a single transistor.  The increased low output level can cause troubles when TTL logic circuits are driven.
 
Another problem is a reduction in switching speed or response, because the first transistor cannot actively inhibit the base current of the second one, making the device slow to switch off. To alleviate this, the second transistor often has a resistor of a few hundred ohms connected between its base and emitter terminals.<ref name="TAoE"/> This resistor provides a low impedance discharge path for the charge accumulated on the base-emitter junction, allowing a faster transistor turn-off.
 
The Darlington pair has more phase shift at high frequencies than a single transistor and hence can more easily become unstable with [[negative feedback]] (i.e., systems that use this configuration can have poor [[phase margin]] due to the extra transistor delay).
 
== See also ==
* [[Insulated-gate bipolar transistor]]
* [[ULN2003A]]
 
== References ==
{{reflist}}
 
==External links==
* {{US patent|2663806}}&nbsp;&ndash; Semiconductor signal translating devices. (ed., "Darlington Transistor")
* [http://www.eleinmec.com/article.asp?21 A Darlington Pair motor speed control circuit]
* [http://www.tedpavlic.com/teaching/osu/ece327/lab7_proj/lab7_proj_procedure.pdf ECE 327: Procedures for Output Filtering Lab]&nbsp;&ndash; Section 4 ("Power Amplifier") discusses Darlington pairs in the design of a BJT-based class-AB current driver in detail.
 
{{Transistor amplifiers}}
{{Electronic component}}
 
[[Category:Multi-stage transistor amplifiers]]
[[Category:Transistor types]]

Latest revision as of 23:52, 15 September 2019

This is a preview for the new MathML rendering mode (with SVG fallback), which is availble in production for registered users.

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