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Transistors emitter collector flow

  1. Oct 8, 2014 #1
    Hello :-)

    I can't seem to get a good understanding about a part of transistors and how they act.

    What I think I know about transistors now is as follows:

    They're made up of npn or pnp called emitter, base and collector, and the junctions between them are emitter base junction and collector base junction.
    I'm going to use an npn transistor as an example, by the way.

    The emitter is heavily doped so it can give off many electrons, the base is thin and lightly doped so many of the electrons from the emitter can flow through to the collector which is less doped than the emitter.

    A voltage is applied between the emitter and the base, with the catode directed toward the base so the electrons pass through the depletion region at the emitter base junction.

    Here is where I'm stuck: Then a voltage is applied between the emitter and collector? Why? And in which direction does the current flow? Does the voltage between E and C need to be sufficient to get the electrons past both the eb junction and the cb junction?
  2. jcsd
  3. Oct 8, 2014 #2
    I think if you look up and get a better understanding of BIAS and Control Voltage you will find the answers to all of those questions. I'm not trying to be vague it is just that transistors can have many widely varying applications and for the most part this is a function of biasing.
  4. Oct 8, 2014 #3


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    What has been your source of information about transistors, so far?
    I suggest that you google 'theory of transistors' or something like it and read as many articles as you can - until you find one that makes sense at your level. (Warning - I have seen some that seem to want to use a convention of direction of current that is the same as electron flow direction. Madness lies there if you try to operate both conventions at once!! :w)
    One thing worth remembering is that a junction transistor only works when the voltages on e,b and c are appropriate and because the base layer is very thin. What goes on in the very thin be junction is capable of affecting what happens in the cb junction. for a start, the be junction needs to be forward biased so that there is a small but finite current flowing into the base. The thinner the base, the more the be junction controls the ce current (i.e. the higher the 'gain'.)
  5. Oct 8, 2014 #4
    My source of information is supposed to be this terrible online course offered in my country (Norway), but I mainly try to look elsewhere to actually learn anything. I get a lot of information from a University physics book and a lot online, but the book has limited information on transistors. I despise the alternation between conventional current and and electron flow, I wish we could just choose one and stick with it (preferably electron flow :-D ).

    I'm having trouble getting a clear definition of biasing, as my book does not offer such explanation. My online course doesn't even mention it of any sort, which kind of gives you a glimpse of how bad it actually is. I'll try searching some more for appropriate information, thank you!
  6. Oct 9, 2014 #5


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    It's this large collector-to-emitter current which is controlled by the much smaller base-to-emitter current to give us the amplification that is associated with transistors: a small current precisely modifies a larger current.

    C-E current flows from which ever of those regions is more positive to the other (you don't get to choose, the necessary collector battery polarity is determined by the transistor type, PNP or NPN).

    Good luck with your studies!
  7. Oct 9, 2014 #6


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    While that is true it's better to think of is as....

    A current is injected into the base and that allows the transistor to carry a collector emitter current up to a value equal to the base current multiplied by the transistor gain.

    I say "allows" because the actual current might be determined by other circuit components such as a resistors in the collector circuit. Clearly if that resistor was very large the voltage drop across it could be greater than the supply voltage and that would limit the current. Typically the collector resistor is chosen so that the voltage drop across is around half the supply voltage (assuming the transistor is being used as an amplifier rather than a switch).

    There is a lot more to it.
  8. Oct 9, 2014 #7


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    Commiserations about the low quality of your course. Hopefully, PF can supply some of the missing 'conversational' type of learning that you can get from a course where the lecturers and other students are actually in the same room!
    There is a lot to study about transistors which is not directly related (strangely enough!) to actually using them in circuits. Courses have to be a bit selective in their content. Often, Solid State Physicists are not particularly bothered about practical circuit design (and vice versa).
    The point of 'biasing' is to ensure that the transistor can be operated as a linear (or near-linear) amplifier. A transistor will 'switch off' if the Vbe is less than 0.7V and Ib is zero. Once Ib rises above zero, the transfer characteristic between Ib and Ic will be a diagonal line (corresponding to amplification. Biasing allows you to make the transistor operate over this range. It is achieved in many ways but the easiest one to describe is by 'AC coupling' the input signal to the base via a Capacitor and providing a separate path for the right bias current to feed into the base from a DC source (could be a high value resistor from the positive supply). Look at Images of Common Emitter Transistor amplifiers and you will see this is done in many cases. There are usually much more sophisticated ways of achieving bias and feedback is often used to improve amplifier linearity.
  9. Oct 9, 2014 #8


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    Having talked about single resistor biasing (which was fairly common in elementary amplifiers, way back) I found just one example of it in an extended search. HERE. It is much more sanitary to use a potential divider and an emitter resistor as you can make the amplifier less dependent upon the actual current gain of the device. But explaining three resistors is harder than explaining just one. :)
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