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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.
     
  22. Jun 20, 2012 #21

    sophiecentaur

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    I think I see the perils of learning in a non-optimal way. Hanging on to words which you think are well defined but which are not, can't be a good idea. If there is a node in a circuit and you are interested in what is happening at that node then using a word like "series" to describe the way two of the components may not be appropriate. Yes- of course those two resistors are in series with the power supply but they are not in series with the base junction of the transistor. So why use the word at all unless you include all the qualifications about how you are using it?

    In so many posts, I read that people want to categorise rather than to understand. This is a prime example. The Sums give you the answer and sometimes the result can be summed up with simple terminology. When it can't, there is no point in trying to make it.

    This is a prime example of over-simplification, which is dodgy. There are two paths for current to flow from that node point - round the top and round the bottom and through the base circuit. This is what Kirchoff 2 tells you and Kirchof will give you the right answer.

    People who hold on to handy, elementary, terms through thick and thin will always risk falling over. For instance, take a circuit of three resistors, connected in a Delta (not an overly complicated circuit). Are they in series or are they in parallel? Which is which? You can't answer until you supply which two connecting points you are discussing.
     
  23. Jun 20, 2012 #22

    psparky

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    Hey cool, always happy to learn.

    If I state something that is correct...somebody else learns something. If I state something that is wrong....somebody else learns something along with myself. Either way...win-win in my opinion. Learning is the main goal here.

    The people on this forum are some of the smartest and most experienced I've ever had the pleasure to correspond with. At my job I work mostly with high voltage components for factories and so forth. Primary substations, Factory substations, transformers, big 500 MCM IAC, Cable ladders, HVAC motors and controls, MCC's, fused disconnect....etc.

    I like to follow electronics as a hobby. I did pretty well in electronics in college and even took my PE in electronics.

    That being said, it is clear some of the "basic" or "ideal" things I learned in college are not always correct. So I can't know what is wrong unless I post on this forum...and get corrected as I often do. When I say something on here....I am mostly repeating what I was taught in school. When I comment on things that I do at work...I am speaking from experience. My ego is in check on this site...and I will continue to post what I honestly believe is correct.

    And I appreciate anyone who corrects me.

    I look at it this way...because I fail...I succeed.
     
  24. Jun 20, 2012 #23

    sophiecentaur

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    @psparky
    You have an excellent attitude to this stuff. You must be soaking it up and really enjoying it.
    btw, my Dad was a transmission engineer with the old 'Central Electricity Generating Authority' (UK). (also on RAF Radar in WW2) He was full of talk of Bus Bars, Transformers and Trips. I learned to drive a car on the perimeter road of an electricity substation down in Cornwall.
     
  25. Jun 20, 2012 #24

    psparky

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    Thank you sir. Funny you should mention bus bars....I am just now running bus bar from the 2400 volt secondary of a transformer to a primary substation. Learning all the components, making material lists and writing scope of work for it....it's all mostly new to me, but I'll get it.

    You should see some of the scopes of work that are written for 12 KV switchgear that power 11,000 HP motors with autotransformers and a bunch of parts that I don't even recognize. It's unbelievable what some of the guys at work can come up with....although these guys all have at least 30 years experience. Cant wait to get there...to the knowledge anyways...not so much the age:)
     
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