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Why are the resistors in parallel?

  1. Jun 18, 2012 #1
    Resistor Rb1 and Rb2? How come?
     

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  3. Jun 18, 2012 #2
    They are in series to provide 5 volts at the base of the transistor.
     
  4. Jun 18, 2012 #3
    They are not in parallel, they are essentially a voltage divider in order to provide biasing.
     
  5. Jun 18, 2012 #4

    AlephZero

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    If depends whether you are talking about DC or AC (small signal) analysis.

    For the DC conditions they are not in parallel, as #3 said.

    For the small signal analysis they ARE in parellal, because the impedance of the power supply for AC signals is very small. Most real-world power supplies will have a large value capacitor between the supply rails, to give a low-impedance path for AC signals.

    For most AC circuit analysis, you assume that the power supply AC impedance is very small compared with everything else, so components connected to any supply voltage line are effectively connected to ground.
     
  6. Jun 18, 2012 #5
    Wow thanks! Especially for AlephZero, I didnt know it made a difference whether u are in AC or DC
     
  7. Jun 18, 2012 #6

    sophiecentaur

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    Yes - that's one way of looking at it but don't jump to the conclusion that it's always different for AC and DC. The word 'parallel' is only a short hand. If you really want to calculate what is going on, you have to do the sums and the sums don't pay attention to the words series and parallel.
     
  8. Jun 19, 2012 #7
    On the diagram it's marked as +15V, suggests that it's DC.
     
  9. Jun 19, 2012 #8

    NascentOxygen

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    As far as the supply rail is concerned, the two of resistors are, of course, in series. But as with any two-resistor potential divider, the pair of resistors can be represented by its Thevenin equivalent circuit, and this amounts to it being a voltage source with an impedance = R1||R2. This analysis applies regardless of the source being AC or DC.

    If we now focus on your example, it is this Thevenin equivalent circuit that the base of your transistor "sees" as supplying its base bias.
     
  10. Jun 19, 2012 #9

    sophiecentaur

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    Suggest 'what' is DC? Clearly, the only stated voltages are zero and +15V.
    That circuit, as it stands, could be part of a bigger circuit - an amplifier or a voltage reference (on the emitter). As it stands it has no input or output - there is just a status quo.
    You need to consider the reason for the statement that the two resistors are in parallel. It would have to be to decide on the effect of some perturbation of the system in the form of an unspecified input signal at one of the transistor terminals, perhaps.

    But I have already made the comment that the term 'parallel' is really not relevant to the circuit performance. It is a waste (as usual) to spend too much time in worrying about the terminology / classification. If you want to know what happens, then you need to do the full analysis.
     
  11. Jun 20, 2012 #10
    These resistors are not in parallel, they're essentially a voltage divider.
     
  12. Jun 20, 2012 #11

    sophiecentaur

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    Stop getting hung up on the definition. If you were to feed that base with an AC signal, via a capacitor, those two resistors would appear as a load equal to the two in parallel (and also in parallel with the transistor input). Just put it down to Engineers' shorthand and 'get over it'. You'll never reform us.
     
  13. Jun 20, 2012 #12
    Errr....what? This is a basic question asked in a circuits class for the soul purpose of learning about the functionality of the BJT. You find the voltage for the Base region and in order to do that...voltage divider. Of course...if this circuit was a piece of a much larger one...i would expect changes and would need to evaluate what else is happening. But, this is a homework problem. So...voltage divider, meaning they are in series.
     
  14. Jun 20, 2012 #13

    sophiecentaur

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    Jeeeez! You guys.
    Voltage divider means they are a voltage divider - that's all. Why this obsession about giving things names? Does the word "series" or the word "parallel" appear in the formula for working out the volts out of a potential divider? Just get on and do the sum and have some confidence that it's right.
    Just consider this - if you are sitting on that transistor base, looking out, would you see two resistors, both connected to you, or would you see one resistor and then another resistor, connected to the other end of it? Where is there anything at all in series when you look from the base's point of view?
    Who cares? If you're dealing with transistors, you have advanced beyond the stuff that nice lady told you about when you were 13 yrs old.
     
  15. Jun 20, 2012 #14

    psparky

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    Guys, stop arguing with Sophie.

    Pay attention....he is the smartest man alive:)
     
  16. Jun 20, 2012 #15

    sophiecentaur

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    OMG, it's you again.
    Hi.
    (I really do have a point there, you know!)
     
  17. Jun 20, 2012 #16
    I agree that you have a point. But, I also think you forgot the question.
    "Why are these in parallel?"
    The answer:
    They're not...they're considered to be in series.
     
  18. Jun 20, 2012 #17

    psparky

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    You do have a point....but since you are so smart....I think you forget the learning process.

    Most students struggle with parallel and series....it is quite confusing at first. What is obvious to a man of your infinite wisdom...is not so obvious to us mere mortals.

    For aspiring students....remember, for all parallel loads...."the voltage across is the same".
     
  19. Jun 20, 2012 #18

    Averagesupernova

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    For aspiring students....remember, the resistors that form the voltage divider to bias the base voltage to 5 volts DC will see the same AC voltage. Sooooo, "the voltage across is the same". They are in parallel concerning the AC voltage on the base of the BJT.
    -
    See it now DailyDose?
     
  20. Jun 20, 2012 #19

    sophiecentaur

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    Considered to be in series with what ? - is the question.
    They certainly aren't in series with the be junction.
     
  21. Jun 20, 2012 #20
    Ah yes, I see. It is all clear to me now. Thank you.
    I think we should all celebrate. And by celebrate I mean helping me understand this circuit.
     
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