I'm new and confused about transistors

Discussion in 'General Electronics Chat' started by dougarthur, Apr 29, 2012.

  1. dougarthur

    Thread Starter New Member

    Apr 29, 2012
    I've done the basics of hooking up an LED, and calculating the proper resistor value, but when it comes time to add in a transistor, it just seems backwards to me.

    So I found this page that describes my basic understanding: http://www.kpsec.freeuk.com/components/led.htm

    Then I found this page to try and understand resistors: http://www.reuk.co.uk/What-is-a-Transistor.htm

    As I understand how resistors work (maybe I'm off here), that basic laymen's term says that an input into the base goes up to the controller, and is then output to the emitter at a higher current.

    Logic tells me (baring in mind my lack of experience/knowledge) that the emitter acts as the input for current, like the positive input from a source, but is a higher current than the original source input to base. So I would logically want to hook the emitter end to a resistor, then to the led - but that's not what I'm seeing.

    I see the positive going to positive on the led in the second link, and the emitter going to ground. This seems backwards (compared to the first link), but I find it to be consistent online, so what am I missing here?

    Thanks in advanced for the help.
  2. crutschow


    Mar 14, 2008
    A common connection for a BJT (bipolar junction transistor) is the grounded-emitter configuration where, obviously, the emitter is connected to the common (ground) terminal.

    An NPN transistor normally works with positive voltages on the gate and collector and that is what you see in your second reference.

    With no base current the transistor is off and no current flows through the collector, thus the LED is off.

    When a small current is applied to the base it is amplified by the current gain (Beta or Hfe) of the transistor to create a larger current that flows into the collector and out the emitter to ground (the direction of the emitter arrow). This collector current goes through the LED and turns it on.

    The base-emitter junction looks like a diode so you normally must add a resistor in series with the base to limit the base current when applying voltage to the base to turn it on (in the example this is provided by your skin resistance). The normal base-emitter voltage is about 0.7V when the transistor is on.

    You can also use the emitter as the output (called a common-collector or emitter follower configuration) but in that case the output voltage essentially is equal to the base input voltage minus the base-emitter voltage drop. You get current gain, since the emitter current can be larger than the base current, but there is no voltage gain.

    Does that help?
  3. dougarthur

    Thread Starter New Member

    Apr 29, 2012
    Yes, this makes sense. I totally forgot about the PNP & NPN differences. Thanks for explaining it. Now it's time to experiment.

  4. k7elp60

    Active Member

    Nov 4, 2008
    Another thing to think about is that the base current + the collecter current is = to the emitter current. If you believe that current flow is - to +. With a NPN connected to the - of the source and the the collector is connected to the + thru a resistor and LED. The total current flows thru the emitter, the base current flows out the base lead, and the collector current flows out the collecter circuit.