Voltage regulator

Discussion in 'General Electronics Chat' started by HarrisonG, Aug 1, 2016.

  1. HarrisonG

    Thread Starter Member

    Aug 1, 2016
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    Hello. I want to learn the physics behind transistor voltage regulators. But I can't understand something. Let's say we have a constant base voltage VB, an emitter voltage VE, base emitter voltage VBE. The three are united by the formula VB=VBE +VE. I can't understand why if VE rises while VB is constant, VBE(the transistor turn on voltage) will be reduced. I know that you can increase it by reverse biasing the base emitter junction, but I dodn't know the oposite Is also true. I also can't understand why if VBE reduces, the conduction of the transistor also reduces and the collector-emitter ressistance increases. It would be great if someone help me understand the physics of this.
     
  2. hp1729

    Well-Known Member

    Nov 23, 2015
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    Design 795 b basic voltage regulator.PNG Design 795 a basic voltage regulator.PNG
    If the voltage on the base is held constant an increase in emitter voltage decreases emitter-base voltage, decreasing conduction of the transistor.
     
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  3. HarrisonG

    Thread Starter Member

    Aug 1, 2016
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    But how it decreases it. How it decreases the conduction of the transistor?
     
  4. HarrisonG

    Thread Starter Member

    Aug 1, 2016
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    I mean, as far as I remember VBE is the electric field that rises when the pn junction is formed. Also called Potential barrier. The higher it is, the harder it is for the current to flow. To me it looks like decreasing VBE (the potential barrier) will increase the conduction rather than decreasing it. I am sure missing something.
     
  5. HarrisonG

    Thread Starter Member

    Aug 1, 2016
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    Ah I get it. VBE is not the 0,7V potential barrier. It's the potential difference between the emitter and the base. Decreasing it will lower the base current, which will.lower the emitter current and the voltage drop across the load. Thank you!
     
  6. hp1729

    Well-Known Member

    Nov 23, 2015
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    If the base is held at a constant voltage and emitter voltage increases there is less voltage left to drop across the emitter-base junction. As emitter-base voltage drops so does conduction of the transistor.
     
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  7. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    In a book that was published after he died in 1824, Joseph Joubert said "To teach is to learn twice." This is because we think faster than we talk or write, and the physical act of talking or writing forces us to slow down and completely hear ourselves. This invariably leads either to realizing that what we said is not right, or to new insights into what we thought we already knew. I've learned a *lot* participating on this forum, but the fun parts are what I've learned by answering a question.

    The same is true about asking a question. You think you don't know the answer, which is why you are asking. But the act of asking out loud or in text slows things down enough to see the answer. You got there on your own. Nice feeling, isn't it.

    ak
     
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  8. HarrisonG

    Thread Starter Member

    Aug 1, 2016
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    Yep, :D, I felt great
     
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