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How does a transistor amplify current or voltage?

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

  1. jac4b

    jac4b New Member

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    Isn't that like creating something from nothing?
     
  2. BrownOut

    BrownOut Banned

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    If you put your thumb over the end of a water hose to spray your friend with the resulting high pressure water, you then can control the water with small movements of your thumb. You're amplifying small thumb movements and getting large differences in water output as a result, but have you created something from nothing?
     
  3. colin55

    colin55 Well-Known Member

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    .
    A transistor can amplify current and it can amplify voltage and it can do BOTH at the same time.
    It doesn't actually amplify the current entering the transistor but it "looks at" the current entering the base lead and allows a higher current to be passed from the power rail and through the collector-emitter leads. The transistor is said to be in emitter-follower mode for this to occur. (and other modes too)
    A transistor can also amplify the voltage is "sees" on the base.
    It does this in common-emitter mode where the emitter is connected to the 0v rail and the collector has a load resistor.
    When the voltage on the base is very close to 0.6v, the transistor is just at the point where it is turned ON and the voltage on the collector will be say 8v for a 9v supply.
    If the voltage on the base rises by 50mV, the transistor will turn ON more and the voltage on the collector will drop to say 3v.
    We call this voltage amplification, as 50mV has produced a change of 5,000mV and this is a gain of 100:1 or amplification of 100 times (100x)
    At the same time we may only need a current of 0.1mA to deliver 50mV rise into the base of the transistor and the collector may be able to deliver 1mA and 3v to an external load. This is a 10 times current gain along with the 100x voltage gain.
     
    Last edited: Jul 24, 2010
  4. dave

    Dave New Member

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  5. Ratchit

    Ratchit Well-Known Member

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    jac4b,

    No, it is not. The transistor is modulating and converting the energy supplied by the bias voltages in accordance with the signal voltage/current it receives as input. There is no free lunch involved.

    Ratch
     
  6. tvtech

    tvtech Well-Known Member Most Helpful Member

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    LOL...now that is funny. And an excellent way to demonstrate the obvious in practical terms.....

    Cheers
     
    Last edited: Jul 24, 2010
  7. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    Transistor are current controlled devices which control the flow of current thru a load, not voltage.

    The voltages measured in a simple single stage amplifier are due to the current flowing from the positive terminal to the negative terminal of the current source that is powering the amplifier.

    If the current source is say a 9V battery, the transistor amplifier is simply controlling the current flowing around the loop.
    The loop being the battery and transistor, the battery is supplying the current, not the transistor.

    The transistor is able to control a high current flowing thru the transistor from collector to emitter by a small current current flowing from base to emitter, so its said to have a current gain.

    The transistor does not create the current, it simply controls the current flowing around the loop.
     

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  8. jac4b

    jac4b New Member

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    I think I get it, but in this example you're not really amplifying the current right? I mean the water's shooting out faster in a smaller stream but it's still the same amount of water leaving the hose. I could imagine calling this increasing the voltage because it would cause the water pressure to build up in the hose but only inversely related to current, as in the case of a dam. I'm still trying to understand how it can increase voltage and current at the same time.
     
    Last edited: Jul 24, 2010
  9. jac4b

    jac4b New Member

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    Is it kind of like the transistor allows you to draw more voltage or current from the battery than it's rating? Obviously it can't be just increasing the total energy in the circuit or there would be no energy crisis.
     
  10. Ratchit

    Ratchit Well-Known Member

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    ericgibs,

    BJT transistors are voltage responding devices, as are FETs and tubes. The only basic current responding device I know of is a magnetic amplifier. As any transistor book will show, collector current Ic = Is*e^(Vbe/Vt), where Vbe is the base-emitter voltage, Is is the saturation current, and Vt is the thermal voltage, of which both the last two are relatively constant. This shows that the Vbe controls the current in the transistor. The base current Ib is (Is/β)*e^(Vbe/Vt), where β is relatively constant for a particular transistor. It also shows that both Ic and Ib are exponentially responsive to Vbe. Dividing Ic by Ib cancels the exponential terms and gives the ratio β . So a BJT mimics a current amplifier, but its Ic is really and truly controlled by Vbe. The base current is the unavoidable "waste current" that is present during operation in the active region. If the β were super high or infinity, there would not be any base current at all to worry about. Therefore, the Ib is a linear indicator of what the collector current is, but it is not controlling the collector current. Now one can make voltage amplifiers out of basic current amplifiers and current amplifiers out of basic voltage amplifiers by the using external circuitry, but that does not define what the basic active element is.

    current = flow of charge
    flow of current = flow of flow of charge

    Ratch
     
  11. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    If the current source is a 'dry' battery, the chemical reactions in the battery producing the current will eventually become depleted and the current will cease.

    With a rechargeable battery its possible by charging the battery to reverse the chemical reactions and the battery will become charged.

    On a mains powered current source as long as the power supply is connected to the mains, current will flow.

    The transistor does not allow more current from the battery than is stored in the battery, the more current you draw the faster the battery will discharge.

    All batteries have a design amp hour rating, which roughly gives you an idea how long a battery will last when drawing a certain current from it.
     
  12. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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  13. Ratchit

    Ratchit Well-Known Member

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    jac4b,

    Nope, you are not getting it. A transistor acts like a fast changing variable resistor in a circuit. A transistor's name comes from the words "transfer resistance". It cannot deliver more voltage than the battery voltage, or more than the short-circuit current of the battery. It simply changes the energy flow from the battery according to a control signal. The control signal uses little energy and controls a relatively large amount of energy from the battery, but all energy sources have finite limits which are never exceeded. Stay dry and don't get confused by hydraulic analogies. Just remember that small amounts of control signal energy control large amounts of electrical energy to a load. That's amplification.

    Ratch
     
  14. crutschow

    crutschow Well-Known Member Most Helpful Member

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    Ratchit, I agree with Eric. Bipolar transistors are current controlled devices. You go through great mathematical contortions to show that it is voltage controlled, because the diode equation of the base input has a voltage component, but that is a secondary result of the current operation of the transistor. For normal amplifier design you are only concerned about the current gain of the bipolar transistor not its voltage gain (transimpedance, which is only sometimes used in RF design). If you want to still think of bipolar transistors as voltage controlled, go for it, but don't try confuse the beginners by promoting a viewpoint that is yours alone.

    By your logic a magnetic amplifier is also voltage controlled if you include the resistance of the control winding.

    If you want to use the water analog for a transistor, think of a pipe with a valve. The pipe is the current through the collector-emitter. The valve is the base. Adjusting the value with a small amount of power can control the flow rate of a stream of water with a much larger amount of power. That's amplification.
     
    Last edited: Jul 24, 2010
  15. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    I am getting it
    I got it back in the late 1960's when studying with lecturers who were working on developing rationalised transistor theory, so if you don't mind I will stick with what they taught me, its served me well for the past 50 years.:rolleyes:

    My advise to any newbie studying transistors is to think of them in terms of a current controlled device, this will make your life and design calculations much easier.
     
  16. Ratchit

    Ratchit Well-Known Member

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    ericgibbs,

    OK, I have read the link, which is a summary of transistor operation. I read many of these types of articles, and this one in particular does not prove your point. If fact it says:
    It is correct that a BJT can be viewed as a current controlled device for design and calculation purposes, but I am averring that it is physically a voltage controlled device. Nowhere does the article say that a BJT is a current controlled device. To successfully refute my assertion, you have to show where I am wrong in my statement which describes the equation showing Ic is dependent on Vbe, and that Ib is a waste product that has to be taken into account, but does not control the Ic. It only indicates what the Ic is. I know that a lot of literature says that a BJT is a current control device, but when the external circuitry is removed, the Vbe to Ic equation says otherwise.

    Ratch
     
  17. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    Hi,
    I can see that this thread is going to spiral out of control, so if you want to believe a transistor is best considered as a voltage controlled device makes you happy, I don't have problem with that.
    I think you will realise that there are a lot learned and experienced members who will try to convince you otherwise.

    Eric
     
  18. Ratchit

    Ratchit Well-Known Member

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    crutschow,

    Just saying it is so does not make it so. I expounded on my assertion. Can you do the same for yours?

    How so? What great math contortions? Which diode equation were you looking at? I am looking at the Shockley diode equation, which has only one main variable, voltage across the diode Vd. Diode - Wikipedia, the free encyclopedia The other "variables" are controlled in manufacture, and one is temperature dependent. They could be considered constants. In a transistor, Vbe controls Ic, not the other way around. Any good text like Sedra and Smith will confirm that.

    And what is the current gain of a BJT without any external circuitry? It is β of course. I said that already. How does knowing that a BJT is a voltage-controlled confuse the beginner? Anyone would be a fool not to use the useful relationship of Ic to Ib in calculations and design. That does not refute my assertion. The truth is not confusing.

    No, the winding is a relatively small impedance, so responds well to current input.

    I am a electrical engineer, not a hydraulics engineer. I know how a BJT works. Do you find hydraulic engineers using electrical analogies?

    Ratch
     
  19. Ratchit

    Ratchit Well-Known Member

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    ericgibbs,

    It does not make me happy or sad. After all, it is just technical knowledge.

    Bring them on. But I hope they can show me their reasoning, and not just throw out some links that only parrot the false viewpoint that has been published before.

    Ratch
     
  20. Mikebits

    Mikebits Well-Known Member

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    I always liked this explanation of a transistor. Picture says it all.

    [​IMG]
     
    Last edited: Jul 24, 2010
  21. Ratchit

    Ratchit Well-Known Member

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    Mikebits,

    Good one. The man in the picture represents Vbe.

    Ratch
     

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