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Understanding Transistor

Discussion in 'General Electronics Chat' started by sdmuashr, Mar 24, 2012.

  1. sdmuashr

    sdmuashr New Member

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    Hi,

    I have huge problem understanding circuit involving transistors. Therefore, I would be very thankful if anyone here can guide me to any online material or a book (preferrably low cost that can be bought) which is about the pratical use of transistors (BJT, FETs) and how they work in circuit about their biasing. The material should be down to earth.

    I also saw capacitor being used in transistor circuit though I am not 100 % sure about their use with transistor.

    Thanks
     
  2. AlainB

    AlainB Member

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  3. tvtech

    tvtech Well-Known Member Most Helpful Member

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    Good link. Basics are so important. ;) You cannot build a house without foundations. Or progress in ANYTHING unless you understand the very basics. Life or electronics...all the same. Get the basics right and you are good to go ;)

    Cheers,
    TV Tech
     
    Last edited: Mar 24, 2012
  4. dave

    Dave New Member

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  5. WTP Pepper

    WTP Pepper Active Member

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    There is little in basic electronics that cannot be explained with water.

    A NPN BJT. Normally no water can flow from (C)ollector to (E)mitter with the (B)ase valve switched off or no water flowing into the B pipe. What does flow from C to E is leakage from the valve or C to E current leakage

    Start injecting a little water into the Base pipe and it starts the C to E valve to open. The amount of water flowing out of the E pipe is B pipe amount + C pipe amount of water flow.

    Too much water flowing into the B pipe causes the C to E flow to reach a maximum due to water pressure and pipe size . This is saturation. To much B flow will kill the valve. A base resistor acts as a pressure reducing valve reducing the flow of water into B to prevent damage. C and E flow also need a resistor to limit the flow to prevent too much flow causing damage.

    The amount of gain is the tiny amount of flow in the B that causes a fixed amount of water to flow C to E. This is the DC gain of the BJT. How rapidly the valve between C & E responds to changes in flow into the B pipe is the AC gain of a transistor. Eventually the C to E valve will not be able to keep up with changes in the flow into the B pipe and the resulting flow will slow down and not respond. This is the bandwidth of the transistor coming into effect. Keep changing the amount flowing into B fast enough and the flow into B will eventually equal the C to E amount. The gain has become 1. Known as the transition frequency.

    In N type MOSFETS it's much the same except it's the amount of water sitting behind a dam of the B (or (G)ate) stream and not the flow of water that causes flow from the (D)rain to (S)ource.

    This can then be extended to negative amplifiers. Put a resistor valve between E and ground and when the flow becomes too much into E, it pushes back reducing what not only flows from C, but what is allowed to flow into B.

    I have used N types as an example as this is easy to get into your head about what is happening. With P devices, it's much the same but upside down. Think about it.

    Kirchoff's laws can be explained in exactly the same way.

    Think of a current breaker. It trips if there is a current imbalance between live and neutral - i.e current flowing into the earth due to a fault.
    If the water flowing into a radiator is more than the water flowing out of a radiator, then you have a leak in your radiator.

    My old Polish teacher who struggled with his English explained this is how current works and from then we had a class of dozy 16 year olds fresh from school who understood how basic electricity worked within an hour and I will never forget this simple but basic analogy.

    Hope this helps.
     
    Last edited: Mar 24, 2012
    • Like Like x 1
  6. tvtech

    tvtech Well-Known Member Most Helpful Member

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    Excellent +1

    Basics are the secret.

    Thank you WTP

    Cheers,
    TV Tech
     
  7. colin55

    colin55 Well-Known Member

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    <mod edit: Self-promotion deleted. If you have information to share, please post it directly to ETO. Do not link to your own personal website.>
     
    Last edited by a moderator: Sep 16, 2014
  8. ljcox

    ljcox Well-Known Member

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    This is a reasonable anology, but it needs qualification.

    A BJT is a voltage controlled current source.

    The collector current is determined by the charge in the base/emitter which is a function of the base/emitter voltage.

    Certainly, the collector current is proportional to the base current (Ic = β * Ib) while the transistor is in the active region, but it is not fundamental to the BJT operation.
     
    Last edited: Mar 24, 2012
  9. Winterstone

    Winterstone Banned

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    Yes, unfortunately it happens very often (even in textbooks with good reputation) that the BJT is described and understood as a current controlled device. And this is not in accordance with the physical effects inside the device. A small current never can be the physical cause for a larger current. It's only a practical rule of thumb to assume that Ic depends on the ampunt of Ib.
    The fact that the BJT is voltage controlled is important - for example - for understanding the current mirror concept as well as barry Gilberts "Translinear Loops".
     
  10. WTP Pepper

    WTP Pepper Active Member

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    Hey guys. he wanted a little bit of an explanation of how they work. I could blind him with the science if needed,but if you read the OP, it's not what he wanted.
     
  11. tvtech

    tvtech Well-Known Member Most Helpful Member

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    You did a brilliant job with the learning link you posted:)

    It does not help to provide lot's of technical info if the OP doe's not understand the basics....

    Hence, the difficulties of forums like this..

    Cheers,
    TV Tech
     
    Last edited: Mar 25, 2012
  12. ljcox

    ljcox Well-Known Member

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    I understand that but the OP is not the only reader.

    Besides, the OP may re-read the thread once he is further up the learning curve.
     
  13. Ratchit

    Ratchit Well-Known Member

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    ljcox,


    Winterstone,

    Wow! I thought I was the only one in this forum who believed that a transistor is a voltage activated device. There was a long thread about this some time ago.

    Most folks who write textbooks can't wrap their mind around the difference between causality and functionality. Any good transistor book will show that the Vbe voltage nullifies some of the barrier voltage of the BE junction, thereby increasing the collector current. That makes it the causal voltage controlled current source. This is a exponential relationship. Both the base current and collector current have this exponential relationship. So when the collector current is divided by the base current mathematically, the exponential relationship cancels out, and it appears as though the base current is controlling the collector current in a somewhat linear manner. That is the functional current controlled current source. Therefore, a BJT mimics a current controlled current source, but is really, physically, at the basic level, a voltage controlled current source.

    WTP Pepper,

    I believe he said the wanted to learn how to use them, and how to bias them, and not necessarily how they work.

    sdmuashr,

    I hope the links you have been given help you. Bias is used to set the operating point, and minimize against fluctuations of temperature, voltage supply, and β. That is a study all by itself.

    Ratch
     
  14. Mr RB

    Mr RB Well-Known Member

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    How a transistor works;
    Give it >0.6v at the base, and it turns ON.
    :)
     
  15. Ratchit

    Ratchit Well-Known Member

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    Mr RB,

    That is how it operates, not how it works.

    Ratch
     
  16. atferrari

    atferrari Well-Known Member

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    Mod, please delete
     
    Last edited: Mar 26, 2012
  17. colin55

    colin55 Well-Known Member

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    To keep the discussion simple, we should think of a BJT as a current controlled device as it definitely operates as a current amplifying device once you have provided the base with the necessary voltage to get it into conduction-mode.

    Once this basic premise is accepted we can consider the FET as a voltage controlled device and thus differentiate the two.

    It is absolutely pointless confusing a beginner and making the situation more-difficult.
    It’s hard enough understanding all the possible circuit variations and a little latitude is needed to simplify the operation of a device they thought “would never get off the ground.”
     
  18. Winterstone

    Winterstone Banned

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    Colin, do you really believe that telling the physical truth will "confuse a beginner" ? I rather think, your false statement ".....it definitely operates as a current amplifying device" will contribute to confusion.
    I agree that in some cases it is helpful to "keep discussions simple" - but not for the price of wrong explanations.
    Regards

    Here you can find the detailed explanation of the working principle of the BJT (for everybody who is interested):
    http://amasci.com/amateur/transis.html
     
    Last edited: Mar 26, 2012
  19. Mr RB

    Mr RB Well-Known Member

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    The problem of course when you get too pedantic is that it reduces ability. In most cases, the ability to communicate effectively with a beginner.

    So "works" is a perfectly adequate term. If a beginner asked "how does a diode work?" the simplest answer would be "a diode works by only allowing electricity through in one direction".

    That would have a number of errors to a pedantically imparied person like yourself, but is an extremely effective communication.

    Always insisting on absolute correctness is a crippling disability that I an sorry you are afflicted with. ;)
     
  20. Ratchit

    Ratchit Well-Known Member

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    Mr RB,

    If someone asked me how a diode operated, I would say it allowed charge flow in one direction, and restricted flow in the opposite direction.

    If someone asked me how a diode worked, I would say that it restricted charge flow by increasing the barrier voltage in that direction and assisted charge flow by opposing the barrier voltage in the opposite direction. If they ask further questions, like "What is barrier voltage?, I would say it is the voltage caused by uncovered charges (ions) left behind when the electrons and holes diffuse into the PN barrier. The discussion could go on from there.

    It is not enough to be an effective communicator, one should be correct also. Otherwise one is effectively wrong.

    At least I acknowledge that proclivity. But being correct is not a problem if one can explain something simply and clearly. Analogs are a crutch, and I use them as little as possible--and usually only for making a point, not explaining how things work. I will leave it to others to judge my skill in explaining things.

    Ratch
     
  21. colin55

    colin55 Well-Known Member

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    He is asking about undestand how a circuit using a transistor, works. Not the PHYSICS of transistor operation.
     

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