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Can someone explain me how a BJT works?

Discussion in 'General Electronics Discussion' started by irq20xdfr, Aug 12, 2016.

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  1. irq20xdfr

    irq20xdfr

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    Aug 12, 2016
    I have read a lot of pages about that, including the part on the Art of electronics book... but i just can't get it into my head, i am a programmer, so, electronics is not fully my area.

    Can you point me please an article that explain that in a simple way ? or the difference between a BJT and a FET, i understand that one is: emitter, collector and base, and the latter source, drain and gate, i more or less understand the basics, the thing is that i don't fully understand it, can you help me please?
     
  2. Minder

    Minder

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    This should help.
    M
     

    Attached Files:

    hevans1944 likes this.
  3. LvW

    LvW

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    hevans1944 likes this.
  4. irq20xdfr

    irq20xdfr

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    Aug 12, 2016
    Ok thank you.
    Im going to check those resources
     
  5. Amar Dhore

    Amar Dhore

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  6. irq20xdfr

    irq20xdfr

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    Aug 12, 2016
    Great video and channel I have now a more clear view about BJTs.

    I will continue investigating...

    Thank you!
     
  7. Ratch

    Ratch

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    What is it that you don't fully underestand?

    Ratch
     
  8. irq20xdfr

    irq20xdfr

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    Aug 12, 2016
    Hello,

    For example i don't understand what does reverse bias and forward bias is, and how does it relate to the BJT topic.
    I get confused with the terms vcc, vdd etc..

    and mostly i don't understand the difference between the BJT and FET besides the terms source,drain and gate
     
  9. Ratch

    Ratch

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    Not so great video. The presenter appears to believe that Ib controls Ic, which it does not. Vbe controls Ic. Without some emitter resistor, that circuit will send double the current through the fan if the beta of the BJT happens to be twice what he assumed. I wish I could talk to that presenter and set him straight.

    Ratch
     
    Arouse1973 likes this.
  10. irq20xdfr

    irq20xdfr

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    Aug 12, 2016
    you see... ? that's what i dont get...

    I read a full article about voltage controls current, i more or less understood it.

    is this http://amasci.com/amateur/transis2.html

    But the terms that i exposed above besides the depletion part... and all that stuff, i just can't find a place where it is explained step by step and don't use math in everything!
    thank you in advance
     
  11. Ratch

    Ratch

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    Mar 10, 2013
    Those terms are readily defined in any textbook about semiconductor.

    OK, a BJT if a bipolar device, in that it uses both holes and electrons as its current carriers. A FET is a unipolar device that uses either holes or electrons, but not both. A BJT controls its collector current by varying the diffusion rate with a base voltage. This diffusion control causes a highly nonlinear relationship between Vbe and Ic. A FET controls its drain current by varying the electrostatic fields that control the channel width. Electrostatic control assures a linear relationship between the gate voltage and drain current. In a BJT, the charge passes through the N-P junctions. In a FET, the charge passes along the surface of the N-P junctions that forms a charge carrier channel. Since you said you already know the basics, you should be able to find out most of the rest of your questions in a semiconductor textbook or related source.

    Ratch
     
    Arouse1973 likes this.
  12. irq20xdfr

    irq20xdfr

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    Aug 12, 2016
    Ok that explanation makes thing more clear

    One thing
    I am currently reading the book tech yourself electronics at the same time as I read the art of electronics. I know that one is not comparable to the other. Obviously I found more easy readable the first one.

    Do you know if my approach is right ?
    I use the first one to understand the concepts and the last one to get more exactly details and when I don't understand well I research the terms on Google View videos in YouTube and ask in forums. Is it ok ?
     
  13. Ratch

    Ratch

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    OK lets talk about a BJT. We start with a junction diode. The N and P slabs are physically connected together. Immediately the negative electrons from the N slab will diffuse into the P slab because they are attracted by the positive holes and will be neutralized. Simultaneously, the holes in the P slab will diffuse in to N slab because they are attracted by the negative electrons, and also will be neutralized The transfer of electrons and holes along the N-P boundary will cause a positive charge in the N slab and a negative charge in the P slab. After the boundary charge builds up high enough, The positive charge in the N slab will repel and stop holes from the P side from coming over. Same thing happens with the negative charge in the P slab. In other words, the charge carrier movement due to diffusion causes a barrier voltage to form and prevents further carrier movement. Applying a forward voltage across the diode lowers this barrier voltage and allows a diffusion carrier current to exist. The applied voltage to carrier current is highly nonlinear because it is a diffusion relationship.

    A NPN BJT has a very thin P slab inserted between two N slabs (collector and emitter). The emitter slab is highly doped and diffuses electrons readily into the thin base P slab. These electrons are easily attracted to the collector slab by the higher voltage of the collector. The thinness of the P slab does not allow the electrons to be absorbed very much on their way to the collector. The base voltage again controls the amount of charge that can transfer from the emitter to the base. Cranking up the collector voltage higher will not increase the collector current, because if no additional charge is available in the base, an increase in current cannot happen. Inevitably, some charge will "escape" from the base and form a current in the base circuit. This is "waste current" that does not control the collector current, but has a relationship to Ic called "beta". The above explanation shows that a lone transistor is a transconductance amplifier (voltage controls current).

    Ratch
     
    Last edited: Aug 12, 2016
    Arouse1973 likes this.
  14. Ratch

    Ratch

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    I am not a teacher. Therefore, I cannot advise you on what method you might find better than another. Besides, I don't have those books.

    Ratch
     
  15. irq20xdfr

    irq20xdfr

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    Aug 12, 2016
    Thank you Ratch your explanation is very straightforward.

    I'll take my time to analyse it and I will post any doubts.

    The book tech yourself electronics is this
    https://www.amazon.com/Teach-Yourself-Electricity-Electronics-McGraw-Hill/dp/1259585530

    I know there is a pizza in the cover. But it is pretty simple and straightforward in his explanations besides the writer has a very nice YouTube channel.

    The book art of electronics is most well known in the engineering world

    Thank you so much
     
  16. irq20xdfr

    irq20xdfr

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    Aug 12, 2016
    So, it seems like I could grasp all that you said. But I still don't understand what is the whole porpouse of the BJT. Is it just control the passing of current between emitter and collector increasing the power like an amplifier or is it more like a switch ? If that is the case how is distinguished from a FET ? And lastly how can you use it in a real life application ?
     
  17. duke37

    duke37

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    A BJT needs base current to raise the base voltage a little. It can be used as an amplifier or turn it on hard enough to act like a switch. Even so, the voltage drop can be up to 1V so may need a heat sink. The power will depend on the load.

    The common n-channel FET needs voltage at the gate but negligible input current. The power fets when turned on fully have a very low resistance so are prefered for switch purposes. It may not need a heat sink.
     
  18. irq20xdfr

    irq20xdfr

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    Aug 12, 2016
    Ok...
    Sorry, by the way in your explanation i found some terms that i have found in some other places but i dont understand.... What do you mean by "load" and why do you mean by "heat sink"

    thank you for your time and knowledge
     
  19. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Generally speaking, the load is the thing you are controlling with the transistor. It may be a relay, a light bulb, a LED, a speaker, or a resistor or any of a plethora of different things.

    A transistor isn't perfect. If it is controlling a large current, especially if it's not just being turned on and off (think an audio amplifier) then the product of the current through the device and the voltage across it is the power lost in it. This lost power is turned to heat. If you don't want it turned to smoke :) then you need to get the heat out of the transistor in order to keep the temperature to an acceptable level. A heat sink is something which removes heat from your device (primarily by conduction) and has a large surface area that allows the heat to be radiated and convected away to the cooler ambient environment.

    Practically speaking it is usually a piece of metal with fins. The size is determined by a number of factors including the amount of heat to be dissipated, the temperature differential, and whether there is airflow (including forced airflow from a fan. The chunk of metal and a fan on top of a CPU is a high power heatsink with forced air flow.
     
  20. irq20xdfr

    irq20xdfr

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    Aug 12, 2016
    Ok, thank you.
    Now i understand clearly the explanation of duke37

    I will continue investigating about BJT and FET.

    regards (*steve*) !
     
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