darlington pairs

[tama182]

i have some questions,

1. howcome where ever i see a darlington pair they always have really large resisters on the base of the first transistor, if they had small resisters then wouldn't they not need a darlington.

2. Am i right in thinking that the second transister would have to have a much bigger power handling than the first trnsister

3. rather than use a darlington would'nt it just be better to use a transister that has a high hfe and large power handling, wouldnt that do the same thing

thanks nigel cause i know you'll be the one to answer them :lol:

 

[akg]

i have some questions,
1. howcome where ever i see a darlington pair they always have really large resisters on the base of the first transistor, if they had small resisters then wouldn't they not need a darlington.

u mean the bias resistor.?? this is because the darlinton pair has very high gain , and hence it requires a very low base current , and hence a large bias resistor.

2. Am i right in thinking that the second transister would have to have a much bigger power handling than the first trnsister

need not be , darlington is used for high (voltage)gain amplifier ( are u confused with biasing transistor of push pull power amps ?)

3. rather than use a darlington would'nt it just be better to use a transister that has a high hfe and large power handling, wouldnt that do the same thing

the gain of a darlington is approx Beta*Beta , which is very huge. ,and very difficult to obtain from a single trans:

thanks nigel cause i know you'll be the one to answer them :lol:

nigel is yet to come

 

[Russlk]

1. If they could drive a small resistor, a darlington would not be needed!

2. Yes, the first transistor current is the base current of he second transistor.

3. Yes, but difficult to find. Darlington transistors have gain exceeding 1000. I have seen single transistors with gain that high, but they are hard to find and are not very stable (and were low power).