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How Transistors Works?

  1. Oct 6, 2004 #1
    How Transistors Works? tks!
  2. jcsd
  3. Oct 6, 2004 #2


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    This is like asking someone to teach you a new language.

    You won't learn very much from a one paragraph answer. I suggest you pick up a book, or Google "Bipolar Junction Transistor".

    Also see : http://www.allaboutcircuits.com/vol_3/chpt_4/1.html

    But I suggest you start by learning how a PN junction diode works.
    Last edited: Oct 6, 2004
  4. Oct 11, 2004 #3
    I have had big problems with finding help for this, in case the sight he showed you didn't help you, I will. Transistors are electricly-controlled switches. There's a base, a collector, and an emitter. Basicly, you put a small voltage on the base, and it closes the switch.
    I don't know how much you know about diodes, so I will explain the whole thing. A semiconductor is a material that is not an insulator nor is it a conductor, it's kind of in between. A diode is a one way gate for current, this means if current is going the wrong direction, it will not let it passed, but if it is going the right direction, it will pass. Silicon has a total of 4 valence electrons. Valence electrons are kind of electrons that are swapped between atoms, so the atom can be more stable. To understand this much better go into atoms, molecules, and matter, and take a look at my valence electrons post. Okay...a silicon, a common element used for semiconductors, has 4 neighbors it shares valence electrons with. There are no extra electrons in the lattice of silicon, and this makes electrical resistance high. So, in the lab - where the diode is made - aluminum is injected into the silicon. Aluminum has 3 valence electrons, so it can't make a connection to one of the silicon atoms, because it only has three to share. Imagine a lattice of interconnected lines. At the intersections of the lattice there are dots, these are atoms. When you look at it, one of these dots connect with 4 other atoms through the extending lines. Well, aluminum is like a dot on that lattice that does not have a fourth protruding line, so it is not connected to that atom. This is because it only has 3 valence electrons to share. So, in order to fill it's valence shell to 8 - which it will never get, but trys to get as close as possible - it steals an electron. You're saying, hey where did 8 come from, well to understand why a valence shell is filled with 8 valence electons you need to go to where I told you to go earlier. Anyway, it steals an electron from the neighboring silicon atom - SOME NEIGHBOR!!! this causes the neighboring silicon atom to only have 3 valence electron, so it still it from the next silicon atom, causing this one to steal, and so on...GEEZE, THEY'RE ALL A BUNCH OF LOOTERS!!! Alright this lack of an electron is what we in electronics like to call a whole. Now, to help clarify a little bit, imagine chineese checkers. You have a bunch of indents with marbles filling them in, but one indent is missing a marble - let's call that the whole. Move the marble left of the indent into the indent, then move the marble left of that one to fill the new one. See the process? The indent is, in a way, moving to the left. the marbles are the valence electrons, this is, basicly, what happens. This part of the silicon is called N, for negative, because there is MINUS one electron.
    Now, that we got that cleared, lets take another piece of silicon and add phosphorous to that. Phosphorous has 5 valence electrons, so it combines with all four of the silicon atoms. WAIT A MINUTE, that adds to 9 valence electrons, that can't be right!!! The final valence electron is, basicly, ejected, and is just floating around, so it adds to 8, whoo!!! This part of the silicon is called P, for positive, because there is PLUS one electron.
    Okay...Now that were a done with the hard part, to the easier part...we take the silicon pieces, and sandwhich them togethe, TADA!!! At the joint where the P and N silicons meet, there is something happening. The atoms are swapping electrons, the looters on one side are accepting electrons from the phosphorous on the other side, making them even. It's like this imagine a POOR community, and there is a RICH community, now when the neighborhoods meets, the RICH give electrons - money - to the POOR, and now they are all middleclass. If only that's how it worked in real life!!! Okay... So at the juction where the P and N meet, they are now even, no wholes, no extra electrons, but remember, this is only at the junction, the rest of the silicon is still RICH and POOR. Okay...This conjuction is no longer conductive, it is now an insulator!!! This insulator is called the depletion zone.
    So, what we do is hook a battery up, electrons flow from the negative side of a battery, unless you use conventional current, and WHY you would use that I don't know. Anyway...as current flows from the negative side of a battery, if it in connected to the N side of the the "DIODE" - that's right it is a diode - the wholes are attracted to the free electrons, they are not really attracte, it's just the wholes - or lack of electrons - kind of "moves" torwards the conjustion where the wire connects to it. This causes all the wholes to move torwards the wire, and away from the P - N conjuction. This causes the depletion zone to widen, and more resistance, basicle, it's an even bigger insulator. We call the diode reverse-biased. Okay...a bigger insulator, that has more resistance, and almost so much no current could flow through that!!! So, we change the battery around, the Positive is connected to the aluminum, and the negative is connected to the phosphous. This is called forward-biased. The electrons from the battery flow into the phosphorous silicon, causing the extra electrons to be repelled, LIKES REPELL!!! And on the aluminum the same happens. This causes the wholes and the electrons to be repelled from the wire conjuction, and reduces the depletion zone. Eventually, when 0.7 volts is applied the depletion zone, basicly, deminishes, and the diode conducts current. Wow!!!
    Now, a transistor has two possible configurations, NPN or PNP. This is, simply, the configuration of the aluminum and phosphorous. Let's use NPN. Okay...three volts is applied to the emitter, 2.3 volts must be on the base, and the collector goes to positive, through a light bult. The positive 0.7 volts in respect to the emitter, because the center is P. This causes the depletion zone between the base and the emitter to diminish, allowing current to flow through the base. Then, the emitter has to be, 0.7 volts, so that it can diminish the collector's depletin zone, this causes current to flow from the emitter to the collector. We can now add as much voltage to the emitter as we want, and therefor make an electronicly-controlled switch.
    WHOO, I hope you learn something from this, because it took me an hour to type, but...I had fun.
  5. Oct 23, 2004 #4
    think of it this way:
    a transistor is very much like a light switch, except it can also vary the amount of light.(considering a bulb load, think speaker, think amplifier, think transmitter...).with regards to a standard pnp or npn doped transistor, we have to provide at least .7 volts more to the base than the emitter (considering polarity) to place the transistor at the bottom of its operating range (we will consider the transistor already has proper collector resistance ,emitter resistance, base biasing,...)
    ok- we are at the lower end of the transistors conductivity range at .7 volts base over emitter, now we apply .72volts the light bulb lights dimly, .73 volts brighter....
    transistors also have maximum gain and power ratings [respective to size of transistor and its heat sink (chunk of aluminium attached to it)] that tell us its limitations.
    something else to consider about transistors is that; for a small change in base/ emitter current (think control current) we can obtain a very large collector/ emitter current (think working current to drive speakers). this mathematical relationship is called "gain" **********do your reading!!!!!!!!
    pnp vs npn represents respectively; positive/negative/ positive, and negative/ positive/ negative, with regards to the transistors atomic doping (or polarity configuration)(yada, yada, yada-- thats smarty pants talk for which polarity to apply to the base in order to place the transistor in its operating range to see the desired result "amplification/switching")
    Last edited: Oct 23, 2004
  6. Oct 25, 2004 #5
    "ok- we are at the lower end of the transistors conductivity range at .7 volts base over emitter, now we apply .72volts the light bulb lights dimly, .73 volts brighter...."
    *****sorry, this is wrong and should read :
    ok- we are at the lower end of the transistors conductivity range at .7 volts base over emitter, now we apply more voltage to the base [this voltage will be seen between base and gnd (or base and -vcc, or base and +vcc, whichever part of the supply is connected closest to the emitter), not base emitter junction] which will in turn increase the current through the base emitter junction, (voltage on base emitter junction remains at .7 as if it were across a fwd biased diode, therefore extra voltage drops across emitter resistor, although current through the junction will increase causing amplification) and the light bulb burns a little brighter. then we increase the base voltage a little further yet, and again an increase in light....
    Last edited: Oct 25, 2004
  7. Oct 27, 2004 #6
    ..........My brain hurts.
  8. Oct 27, 2004 #7
    take it slow, focus on the reasoning for the teachings (they are all simple concepts, and yet we all dump them from memory too fast!!!!).
    it all adds up to a bigger picture eventually.
    the few things i actually really, and honestly know are the following;
    1/ knowledge is a function of ambition (if you try hard enough the answers will come)(repetition of study is best at this point...do it, and do it, and do it, until it is correct every time.... once you have done this, please let me know if my input was correct on this post!!!!!!!!!)
    ****secondly and as important as the first;
    2/ always say to yourself, "if anyone can do this.....I CAN!!!!!!"( remove self doubt and answers come fast).
    3/ take it easy on me in your eventual correction of my input to this post, as i am going on memory, and did not refer to my texts.
    4/ if you do not understand, drive your instructors crazy with questions until you do!!!!!!!
    5/ say this to yourself; "ok... i know, i'm stupid on this problem, at this moment. however, in fifteen minutes, i will nail the !@#$%^&*er".
    ****i absolutely assure you, you will indeed nail it!
    hey, if it takes from 8 to ???? hrs, you will solve it... if you follow steps 1, through 5
    good luck and i am sure you do not need it!
    "one who says he does not know now, but will....will" don rigby (wow, i actually had an origional thought!)
    Last edited: Oct 27, 2004
  9. Oct 27, 2004 #8
    Okay...I don't know if this was a joke or not, but if it was, it was an awfully big joke. I completely understand how a transistor works. Thank you, but...I was kind of saying my brain hurts, because of the way you described it. LOL.
    Sorry, but I am not trying to be rude!!! :rofl:

    Last edited: Oct 27, 2004
  10. Oct 28, 2004 #9
    then why did you ask the question?
    are you sure what he said was wrong?
    some people think and explain above other's comprehension.
    prove him wrong, i dare you!!!!
    i, and others will correct, and be humbly corrected.
    that is my reason for putting time in here, LEARN and refresh.
    Last edited: Oct 28, 2004
  11. Oct 29, 2004 #10
    oops,thought you asked the origional question (dual opamp). made a mistake.
    thought the person asking the origional question was correcting you.(when all the while you were chastizing me)
    tried a more basic aproach, obviously it did not appeal.
    by all means, take the time to pick apart my description.
    by the way what does LOL stand for?
    Last edited: Oct 29, 2004
  12. Nov 16, 2004 #11
    LOL, I normally, don't use any form of slang, but my sister taught me that. It stands for laugh out loud, it's internet language. I tried to be as descriptive as possible, when teaching him about electronics.
    LOL = Laugh out loud.
    ROFL = Rolling on floor laughing.
    R = Are.
    U = You.
    2 = To.
    4 = for.
    Ne = Any.
    Ne1 = Anyone.
    Y = Why?
    It's a slang, I guess I shouldn't of used it.
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