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Topic: sorted... Why have Darlingtons? (Read 1 time) previous topic - next topic

JimboZA

Jan 17, 2013, 11:38 am Last Edit: Jan 17, 2013, 02:20 pm by JimboZA Reason: 1
I understand what they do: gain the gain, so to speak.

But presumably there are technical reasons that a transistor of "Darlingtonian gains" cannot be made made to provide that gain in one pass? If so I'd be interested to hear why that is so.

Words of one syllable, please....  8)
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fungus

#1
Jan 17, 2013, 11:59 am Last Edit: Jan 17, 2013, 12:03 pm by fungus Reason: 1
Old transistors didn't have a very high beta so it was often necessary to get a high enough gain.

With newer transistors you don't need them so much, although they're still used when you need a massive gain.

http://en.wikipedia.org/wiki/Darlington_transistor

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dc42

Darlingtons are in my list of "don't use" devices and techniques, see http://miscsolutions.wordpress.com/2011/10/16/five-things-i-never-use-in-arduino-projects/. A single transistor (especially a high gain low saturation voltage transistor such as ZTX851) or a mosfet is almost always a better solution.
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JimboZA

Ah ok, so they're basically superseded.

Thanks.
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pwillard

#4
Jan 17, 2013, 04:05 pm Last Edit: Jan 17, 2013, 04:10 pm by pwillard Reason: 1
Historically, back in silicon dinosaur days, when the 2N3055 power transistor  and the 2N2222A General purpose transistor were king... the power transistors seemed to be able to gain their current capacity at the expense of Hfe (gain).  So, to create a high current solution, the darlington configuration was conceived, where a lower current/higher gain transistor would drive the base of a power transistor that had less gain... with the resulting configuration accomplishing the overall goal of hi-gain, high speed transitions, and high power.

The configuration was common enough that it eventually became a "device" you could buy, like the still popular TIP120.  There is really no compelling reason "not" to use them where it makes sense to and that is why you can still purchase them.  Using one "just because you have one" maybe not the best idea... but they have their uses... I use them to drive solenoids.

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