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How does a transistor amplify current or voltage?

Discussion in 'General Electronics Chat' started by jac4b, Jul 23, 2010.

  1. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    No I did not
     
  2. Ratchit

    Ratchit Well-Known Member

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    Look at post #436 of this thread.

    Ratch
     
  3. Ratchit

    Ratchit Well-Known Member

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    I hope you really meant "like", not "love".

    Ratch
     
  4. dave

    Dave New Member

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  5. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    Yep that was an analogy not a fib.
    I had few comments after that saying it was a good one so whats your problem?
     
  6. Ratchit

    Ratchit Well-Known Member

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    Sure had me fooled, and I thought I knew everything. But tell me, how does a switch compare to an analog device.

    Ratch
     
  7. tvtech

    tvtech Well-Known Member Most Helpful Member

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    Whoot...I love you still Ratch :)

    I can feel your presence here....

    Regards,
    tvtech
     
  8. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    What i noticed is that people that are new to anything like to find as many 'constants' as they can, and dont like to be confronted with 'variables'. They find a resistor which has a constant resistance, capacitor with constant capacitance, etc. When they find a transistor they dont like the fact that it can change it's conductance by some controlling force. If they are told it is a switch then they can start to understand how it changes from 'on' to 'off' and vice versa. After they accept that, they can then start to accept that it may turn on only a 'little' rather than completely, and thus conduct only a little current via the control mechanism. It's not lying to tel them it can act like a switch and then go on to explain this kind of operation, it's just not telling the whole story YET. Later, once they accept the switch mode operation, they find it easier to accept the more variable mode where it can be partially on instead of like a real switch, and also the fact that there is a voltage drop when it is on.
    In fact, for many this may be their first controlled device of which they have never encountered before. Before that everything was either this or that, but did not change itself within the circuit. Now they have to deal with a device that can change part of its operating characteristic, and all because of a third terminal. It's a lot to ask at first, so the switch mode operation is easier for them to get acquainted with at first and then later they can accept a more variable type of operation.

    So the switch mode operation is the "Santa Claus" of the transistor. Merry Christmas to all :)
     
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  9. Ratchit

    Ratchit Well-Known Member

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    Mr. Al,

    Did instructors teach vacuum tubes that way? Just about everyone knows that both transistor and a vacuum tube can amplify. How do they teach amplification with the on-off half-learn method? Just about anyone can learn the rudiments using a transistor in an application. But, to really comprehend how they work takes more study. I don't see why an amplifier can't be called an amplifier instead of a switch, even if it can be applied as a switch as any amplifier can be.

    Ratch
     
  10. tvtech

    tvtech Well-Known Member Most Helpful Member

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    There we go :woot:

    Love this place :)

    Regards,
    tvtech
     
  11. 4pyros

    4pyros Well-Known Member Most Helpful Member

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    They also can be used as switches.
    They are not only used for amps.
    It is easier for a novice to understand switch's then amps, that's all.
     
  12. Ratchit

    Ratchit Well-Known Member

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    Isn't that what I said in post #468?

    Studying their application as switches reveals one of their applications, but it does not teach the internal physics of how a transistor works.

    Ratch
     
  13. kubeek

    kubeek Well-Known Member

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    I don´t think a novice needs to know the internal physics of a transistor. He needs to know how it behaves, and it is easier to understand that when you start with a switch and gradually bring in the amplification.
     
  14. Ratchit

    Ratchit Well-Known Member

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    It is up to the student to decide how much s/he wants to learn and how to learn it. Most textbooks do not start out by declaring that a transistor is a switch. That is a false statement anyway. It is the complete circuit that determines if one has designed an amplifier or switch. A transistor just by itself is a transconductance amplifier. You can easily prove it by plotting Vbe and Ic.

    Ratch
     
    Last edited: Dec 25, 2014
  15. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hello again,

    What i find with people learning about transistors is they hear the word "amplify" and that gets them thinking down the line as "if i put in 1 volt then i should be able to get out 100 volts". In fact i had one person tell me this very thing. They had a power supply they wanted more voltage "out" of, so they suggested that they can use a transistor to "amplify" the voltage. They expected to use an NPN transistor powered by an Arduino (base resistor) and the Arduino connected to the base resistor and running itself at 5v, and connected a 12v computer fan up to the collector and expected it to run at 12 volts! The idea was "5 volts gets amplified to 12 volts so the fan should run".

    I believe that's a fairly logical thought for someone to have because "amplify" makes it sound like it should get higher somehow by itself. What we have to do is convert their thoughts from "amplify by magic" to thoughts about how the transistor can "control" another already present voltage supply. So the first thing is to convert their thoughts from "amplify" to "control". To do this, it is simplest to show them how it works as a switch. Once they see it acts more like a passive thing that can only control what it is already given (like an external 12v supply) then they switch thought modes, with the new understanding that it can not simply increase voltage all by itself but must have an already present power supply there already so that it can control it. Once they see this they then start to understand how it can turn on very hard or just a little, and get a variable output rather than a bang bang switch operation. So they never have to answer the question they ask themselves, "how can this dang thing increase the voltage all by itself?".

    But another thing to keep in mind is that there is a time-line to consider also. Between the time they learn it can be a switch to the time they can learn it can be a variable control device could be as long as a week or as short as two minutes. If they are very intelligent and/or with a lot of experience in similar matters they may pick it up very quickly, but if not it may take a lot longer to absorb. Some college level students will pick it up quick, but some people entering the hobby for the first time have great difficulty with it so they seem to understand the switch operation much better than any variable operation.
    I was helping a student with a bachelor's degree understand a simple current controlled model for a transistor a long time ago, and he had a very difficult time understanding how we can use a current generator to create the output signal (ie the Beta view). He got stuck on this for a couple days trying to understand it, not because he was stupid either (he picked up nodal analysis in a couple days) but because he had a basic disbelief that it could be possible so he wasnt allowing himself to absorb the new information. Some people wont give it a second thought though, they understand the concept almost as soon as they are shown ONE time only that takes like a minute or two.
     
  16. Ratchit

    Ratchit Well-Known Member

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    MrAl,

    I think it best to just answer their questions and correct their misconceptions. The time it takes to do this depends on how deep their ingrained false information is and how fast they can correct it.

    Ratch
     
  17. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hello,

    Yes, but a variable approach is still better than a hard and fast rule that says everyone must learn in a certain manner and there's no room to change that. Books are written with a certain point of view in mind and they usually dont like to present several views on how to go about understanding something.
    But i think you understand my point now.
     
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  18. tvtech

    tvtech Well-Known Member Most Helpful Member

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    Amen.
     

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