The darlington pair (Electronics)

Could someone please explain how this works, it's becoming something of irritant.

I understand the idea of it being better than a single transistor as you benefit from having more gain than a single transistor, though am baffled as to why on an example of application of the DP that the collector is connected in a similar way to this -

http://ourworld.compuserve.com/homepages/g_knott/darling.gif

This may sound simple, though I thought that the gain related to the increase in the emitter current. Currently I am able to fathom the physics of the process, though the principle I understand.

I'd be grateful to anyone who could advise.

Reply 1

meatstu

1

studentee

Could someone please explain how this works, it's becoming something of irritant.

I understand the idea of it being better than a single transistor as you benefit from having more gain than a single transistor, though am baffled as to why on an example of application of the DP that the collector is connected in a similar way to this -

http://ourworld.compuserve.com/homepages/g_knott/darling.gif

This may sound simple, though I thought that the gain related to the increase in the emitter current. Currently I am able to fathom the physics of the process, though the principle I understand.

I'd be grateful to anyone who could advise.

The gain or hFE of a transistor refers to the current gain of the device

Where Ic = hFE x Ib, and Ie = Ic + Ib.

So if we had a single npn transistor with a gain of 50, for 1mA of base current we would get 50mA of collector current and 51mA of current flowing in the emitter.

Lets say the DP in your diagram is consists of two of the above transistors.

Now for a 1mA input to the the first transistor we have 50mA in its collector and 51mA at its emitter.

51mA at the the emitter of the first transistor = 51mA at the base of the second transistor, therefore with a gain of 50 for the second transistor we get 2550mA at its collector and 2601mA flowing through the emitter. Hence lots of total current gain.

So the total collector current for the device for 1mA input = 50 + 2550 = 2600mA.

This is all very general and its been a long time since I've done much of this stuff but does that answer your question?

Regarding the way collectors are connected, this allows a common collector within the device package saving space I think.

Now, how you apply this to whatever circuit you want to create is another matter, but I think that covers the basic principle.

Reply 2

studentee

OP

Thank you for your assistance. That covers exactly what I wanted and the fairly detailed explanation has enabled me to grasp the concept fully. The explanation that I received from the teacher was vague .. "use of two transistors amplifies the current significantly which may be necessary in order to turn on some devices". It may be correct, but it feels a great deal better to have a solid understanding in my experience.

As you have been so helpful on this topic, would I be able to ask you further questions relating to aspects of the course I am unsure about? If you aren't, no worries, you have been very helpful and shall be "repped" in due course, though if yo are able to help that would be fantastic.

Thanks

Reply 3

meatstu

1

studentee

As you have been so helpful on this topic, would I be able to ask you further questions relating to aspects of the course I am unsure about? If you aren't, no worries, you have been very helpful and shall be "repped" in due course, though if yo are able to help that would be fantastic.
Thanks

I'm normally very busy but I'm happy to help when I have time so no problem.

If you plan on studying electronics may I suggest "The Art of Electronics by Horowitz and Hall". I'm sure you'll be able to find one cheap on ebay. Its a very popular textbook and would provide an excellent understanding of the basics before attending uni.

Reply 4

studentee

OP

meatstu

I'm normally very busy but I'm happy to help when I have time so no problem.

If you plan on studying electronics may I suggest "The Art of Electronics by Horowitz and Hall". I'm sure you'll be able to find one cheap on ebay. Its a very popular textbook and would provide an excellent understanding of the basics before attending uni.

Thanks for the advice.

I've sent you a PM regarding the stuff I'm a bit light on.

Thanks again.

The darlington pair (Electronics)

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