Emitter Follower Voltage Regulator

Discussion in 'Homework Help' started by anhnha, Aug 14, 2012.

  1. anhnha

    Thread Starter Active Member

    Apr 19, 2012
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    Please help me with Emitter Follower Voltage Regulator.
    This is the explanation about operation of this circuit from http://www.circuitstoday.com/zener-controlled-transistor-voltage-regulators
    " Let the supply (or input) voltage increase which will cause the output voltage Vout to increase. An increase in output voltage Vout will result in decrease of VBE because Vz is fixed and decrease in VBE will reduce the level of conduction. This will lead to increase m the collector-emitter resistance of the transistor causing an increase in collector to emitter voltage and as a result the output voltage will be reduced. Thus output voltage will remain constant. Similar explanation can be given for decrease in supply voltage."
    Can anyone help me explain about when input voltage increase will cause the output voltage Vout to increase"?
    I think that when Vin increase then the voltage drop across resistor R increase. And because the R is constant, current through it Ir= Ib + Iz will increase.Because Ir increase, there are at least one of two terms Ib, Iz increse but how can I know which current increase or both of them increase.If Ib increse then Ie increse and Vout increase, but I don't know how Vout will increase if I only Iz increase.
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    The text is worded wrong.
    If the input voltage increases then the output voltage will try to increase, but the transistor keeps the output voltage nearly the same.

    The increased zener current will increase the output voltage but only a little.
     
  3. anhnha

    Thread Starter Active Member

    Apr 19, 2012
    774
    48
    Can you explain more detail about this?
    Do you mean that when input voltage increases then the output voltage will try to increase but I can't increase?
    If so, how do you explain about the operation of the circuit?
    I always think that the voltage across is constant and therefore the output voltage Vo = Vz - 0.7 volts and it is a constant.And this is the way that I explain about the operation of this circuit.But when I read the above explanation I feel confused:confused:
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    Yes, when the input voltage increases then the output will try to maintain about Vz - 0.7V. But Vz changes a little with its current and Vbe changes a little with its current.
     
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  5. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    An easy way to understand it is to see that the zener diode "locks" the base voltage at whatever the zener voltage is, and the output is always one diode drop lower than the zener voltage. Even if the input voltage changes, the zener voltage does not change by a significant amount so it "regulates" the output voltage to stay at Vzener - VBE
     
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