Hi,
I cannot get how a transistor work.
Anyone can explain (in terms of base, collector and emitter) how current is amplified in a NPN transistor?
Thank you.
Look at the datasheet for the specific transistor you are using, you need to know the gain of the transistor. If you have a gain of 100 for example, and you give 10mA to the base - you'll get 1A across the collector and emitter.
Does [u]this[/u] help?
How much electronics do you know? Before you try to understand transistors, you need to at least know [u]Ohm's Law[/u] and [Kirchhoff's Laws](http://Kirchhoff's circuit laws).
If you were studying electronics in school, you'd have a semester of DC curcuits, and a semester AC circuits before you start learning about transistors, MOSFETs or other active devices.
DVDdoug:
Does [u]this[/u] help?How much electronics do you know? Before you try to understand transistors, you need to at least know [u]Ohm's Law[/u] and [Kirchhoff's Laws](http://Kirchhoff's circuit laws).
If you were studying electronics in school, you'd have a semester of DC curcuits, and a semester AC circuits before you start learning about transistors, MOSFETs or other active devices.
That is big factor/problem in lots of new users and their approach to electronics as they start to design and build their arduino projects. I can't blame them for wanting a head-start/leap-ahead approach to electronics much as the Arduino programming language has lots of aids to simplify the programming side. However they will always pay for their lack of ability to figure out/troubleshoot their own circuit designs. So we just keep having to ask them to post their schematic drawings and look for their fundamental mistakes. It's why we make the big bucks around here. 
Lefty
How do you get this to work the OP didn't even tell what transistor he was having problems with.
I remember when I was a kid and tube's where king I read about transistor's and I got my hands on one and it
was a big power transistor I hooked it up as a amp to a crystal set and my speaker didn't put out sound.
I wasn't driving the base with a enough current to turn it on.
How do you get this to work the OP didn't even tell what transistor he was having problems with.
Well he said:-
I cannot get how a transistor work.
Not he couldn't get a specific transistor to work. So we too it as a conceptual difficulty he was having.
I got my hands on one and it was a big power transistor
As you now probably know, power transistors especially in the 60s had a very poor gain, in the order of 10 to 20.
For NPN/PNP, think of it like a lever, or hydraulics. With a certain current (voltage and resistance) at the base, you get a proportionately lower resistance between emitter and collector, and thus a proportionately higher current through them. In this way, it "amplifies" current by a set factor, known as the "gain". Bipolar transistors are relatively linear, in that as you raise the base voltage (thereby sinking more current through the base), they just keep conducting more and more. At some point, they won't improve much. This is called "saturation" and is where they are for most purposes, fully on.
For MOSFETS, it's voltage controlled. Not immediately, but at a certain voltage on the gate, the drain begins to conduct to the source. At a certain voltage beyond that, the transistor is "fully on" and further input voltage realizes little to no improvement in the decreasing resistance between drain and source. So, it acts like a voltage-controlled resistance up to the point where it acts like a switch. Unlike NPN/PNP, the gate draws no appreciable current. (Although, the gate will have a certain amount of capacitance that will draw current until "full", but that is a short transient event when voltage is first applied to the gate, not a continuous load.)
Thank you all for helping 
You can even go through Electronics for Dummies by wiley publication..
Many people have found this to be useful:-
Transistor Circuits
This article states:
"The transistor's minimum current gain hFE(min) must be at least five times the load current Ic divided by the maximum output current from the IC."
Why the five times?
I thought, theoretically, an hfe >= lc/maximum output current from the IC would be sufficient. In practice be conservative and double it. But five times? Am I missing something?
Anyone explain how current is amplified in a NPN transistor?
We could take this question literally. Since a junction transistor is a quantum phenomenon:
Time-dependent Schrödinger equation (single non-relativistic particle)
Anyway, that's how college professors tend to answer questions 
Are you related to dhenry?
Can't we all just assume magic is involved? it's easier lol
Why the five times?
I think this is just a safety factor to reflect that in a bag of real transistors the gain changes quite a bit from device to device. Although you do say the minimum gain, I think it is just belt and braces.