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The NPN Transistor

  1. Mar 27, 2008 #1
    Hey guys, sorry to keep posting things about the NPN transistor but i think im starting to understand it. I was just wondering if you could confirm these facts for me:

    1. An external powersupply is needed to turn the transistor on (by forward-biasing the emitter-base junction), or by using a resistor infront of the base to control the voltage flow into it

    2. Once the transistor is on, a signal that is 'fed' into the base-emmiter will be amplified and come out of the emmitter-collector junction

    3. If no signal is being 'fed' into the base, current still flows across emitter-collector and therefor out of the output

    4. More of a question really, what sets the queisecent point of the transistor. Is it done by the manufacturer or by an element of the circuit?
    Last edited: Mar 27, 2008
  2. jcsd
  3. Mar 27, 2008 #2
    Instead of answering yes or no, which makes me have to worry about whether your wording is exact, I'll try to say it in my own words.

    It's important to say that this conversation is about signal amplifiers, not logic gates, which use different strategies.

    There is going to be an "input side" of our circuit and an "output side". Assume a "common emitter configuration", which means that the one node that the input side and the outside side have in common is the emitter. This is the only configuration capable of having a gain greater than 1, that is, operating as an amplifier.

    The transistor is biased to a quiescent level by a simultaneous solution of several equations, which might typically be the loop equation (sum of voltages) for the input side, and the loop equation for the output side, (two equations which make your choice of values for the voltage source and the resistors important), and ALSO the nonlinear current-voltage characteristics of the transistor itself, which I believe are I_B versus V_BE, and I_C versus V_CE, which the manfacturer has published, or you can use a signal tracing device to display those characteristics on a screen.

    (I'm an old dude, and I haven't done this kind of problem since the Nixon administration, so these details might not be perfect, but this is the general idea.)

    Now that the transistor is biased, it's conducting, but not saturated. Let me say loosely that it's "partly on and partly off." It has "room" to be conducting either less or more, should the need arise. That need will arise when it does its intended job and amplifies a signal.

    Now you put a wiggly signal on the base current, that is, algebraically adding a small oscillation to the quiescent level. The collector current might be some known number multiplied by the base current. Suppose that number happens to be 100. If the base current is the sum (quiescent level + x sin omega t), then the collector current might be, for example, 100 times that time-varying function. The significant thing is you have multiplied that sine wave by that number. That's what you wanted. The whole objective was to put a small sinusoudal current into the base and produce a collector current that is a magnified copy of that sinusoid. You also magnified the quiescent base current as represented in the quiescent collector current, but you don't care about that term.

    Now a little sinusoidal voltage is seen across the B-E junction, and a big sinusouldal voltage is seen across the C-E junction. The output is usually taken to be the collector voltage with respect to ground. You attach that point through a coupling capacitor to the next circuit, which shares the ground with this circuit.
    Last edited: Mar 27, 2008
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