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Discussion in 'General Electronics Discussion' started by Dylan Wisdom, Apr 10, 2014.

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  1. Dylan Wisdom

    Dylan Wisdom

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    Apr 10, 2014
    On a normal NPN transistor, how do you calculate the Emitter Voltage? Is the voltage of the Collector & Base just added together to equal the output of the Emitter or is it something else. In either case could you provide a formula? I am having a hard time understanding the transistor. Thank you.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    It all depends on the circuit.

    For common emitter circuits the answer is that it's always 0V (kinda)
     
  3. Harald Kapp

    Harald Kapp Moderator Moderator

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    A voltage is always between two points in a circuit. Therefore asking for emitter voltage doesn't make sense unless you ask "voltage from emitter to point X" and show the circuit where point X is clearly visible.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Yeah, and my answer basically says that the emitter on a common emitter amplifier is at the point you reference other voltages.

    The same is not true for other configurations. That's why we need to see the circuit.
     
  5. Dylan Wisdom

    Dylan Wisdom

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    Apr 10, 2014
    Ok, I will upload the circuit eventually, I honestly don't understand the amplifying property of transistors.
     
  6. BobK

    BobK

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    Think of it this way. You have a hose nozzle that has a very sensitive level that controls the flow of water coming out out of the hose. You "press" that lever by shooting it with a squirt gun. The small flow of water from the squirt gun then controls a large flow of water out of the hose. That is amplification. A transistor does basically the same thing, with a small current at the base controlling a large current at the collector.

    Bob
     
  7. Dylan Wisdom

    Dylan Wisdom

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    Apr 10, 2014
    I've heard that analogy before, but that is probably the clearest way I have seen it worded. But is there any way to actually calculate that relationship in general, or do you have to look for the relationships off of data sheets?
     
  8. BobK

    BobK

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    The beta of a transistor is the ratio of the collector current to the base current. But beta is not a constant, it changes depending on whether the transistor is in cutoff (basically no collector current), saturation (the most collector current you can get given the voltages) or the linear region. The stated beta is for the linear region with typically a small percentage of the max collector current. The characteristic curves you might find in the datasheet shows the relationship over a wide range of conditions.

    Bob
     
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  9. Dylan Wisdom

    Dylan Wisdom

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    Apr 10, 2014
    I am sorry for all the questions but i am very new to transistors, what exactly is the beta of a transistor. Thank you so much for your cooperations and help.
     
  10. BobK

    BobK

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    Read the first sentence of my last post. If beta is 100 and the transistor is operating in the linear region, then if the base current is 0.1mA, the collector current should be 10mA.

    Bob
     
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  11. Dylan Wisdom

    Dylan Wisdom

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    Apr 10, 2014
    Okay this is all making really good sense, but if you could just explain "linear region" i think i will finally understand it all.
     
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Imagine a tap.

    When the tap is turned off, it doesn't matter how tight you turn it, no less water flows. That's cutoff for a transistor.

    When you turn the tap hard on, no matter how much further you try to turn the tap, no more water flows. That's saturation for a transistor.

    Between these points, the amount of water coming out is controlled by your turning of the tap. A bit more turn, more water. A bit less turn, less water. That's the linear region of the transistor.

    In practice the linear portion (that's closest to linear) starts after the cutoff region (which extends through until a small current flows) and ends before the saturation region (which is where the rate of increase of current starts to rapidly fall -- it eases into saturation, it doesn't bang into it)
     
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  13. Dylan Wisdom

    Dylan Wisdom

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    Apr 10, 2014
    Okay so this all makes a lot of sense. Beta is a ratio of the Base and Collector currents that are dependent upon the state of the transistor in cutoff, saturation or in the linear region. I understand that, but i still don't understand the relations of the Emitter. Or is there something that I'm still not getting?
     
  14. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    The base current flows between the base and the emitter.

    The collector current flows between the collector and emitter

    The base current is normally small compared to the collector current so the emitter current is often almost the same as the collector current (the implication is that where this condition is not satisfied the transistor must be in saturation)
     
  15. Dylan Wisdom

    Dylan Wisdom

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    Apr 10, 2014
    If the Collector current and Emitter current are almost proportional then how does amplification occur?
     
  16. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Compare base and collector, not collector and emitter.

    However note that even with the same current you can have amplification. Maybe the collector voltage changes more than the emitter voltage.

    You can have voltage, current, or power amplification.

    To talk more sensibly about this you need to define whether you're using a common base, common collector, or common emitter circuit. You may be using something which is a combination like cascode or darlington or Sziklai arrangements. Each of these will differ in their explanation as to how amplification occurs, what type of amplification, the input and output impedance, and the magnitude of amplification.
     
  17. Dylan Wisdom

    Dylan Wisdom

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    Apr 10, 2014
    Okay i can answer this one. I am basing my questions off of a common-collector circuit.
     
  18. BobK

    BobK

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    A common collector circuit has current amplification but no voltage amplification. In fact, it is also known as a voltage follower, because the voltage at the emitter follows the voltage at the base - the Vbe drop.

    Bob
     
  19. Dylan Wisdom

    Dylan Wisdom

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    Apr 10, 2014
    Oh ok, so what type of transistor circuit amplifies Voltage?
     
  20. BobK

    BobK

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    Common emitter.

    Bob
     
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