Real Newbie Question

Discussion in 'Homework Help' started by GlennWelker, Sep 5, 2014.

  1. GlennWelker

    Thread Starter New Member

    Sep 5, 2014
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    Just started reading a couple of Electronics books and I am confused in general once the books move past the truly basic descriptions.

    Here is an example of my confusion. Make - Electronics pg 81
    This has a diagram depicting a transistor circuit. My confusion stems from understanding the direction of flow. My understanding is that DC only flows in one direction, although it could flow in either direction. I also understand the normal flow is from negative to positive. However in this book they depict flow from pos. to neg.

    In this example, they have added resistors on both sides of the transistor. If the flow moves from pos. to neg. then what purpose would the resistor on the opposite side provide? For example, why does R3 exist?
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    The charge carrier in electrical circuits is the electron which flows from one point in a circuit to another point that is more positive in electrical potential.

    Conventional current flow assumes a positive charge that flows to a more negative electrical potential.

    Stick to one convention and don't confuse the two, while respecting the other.

    A current flowing through resistor R3 produces a potential difference across R3. The same applies to all other resistors, R1-R4. This establishes operating voltages on the base, collector and emitter of the transistor. Each resistor serves a specific function in the circuit.
     
  3. GlennWelker

    Thread Starter New Member

    Sep 5, 2014
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    Thanks for the reply. I'm still lost :)

    It just seems as though there wouldn't be any current flowing through r3 until the transistor is triggered. Of course the same is true on the other side. If R2 is protecting P1 then why R4. If I am just trying to set the total voltage drop on that side, why not just add a larger resistor ahead of P1.

    I know that I have to be thinking of it wrong. Just trying to understand how to visualize this in my head :)
     
  4. ScottWang

    Moderator

    Aug 23, 2012
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    This kind of circuit almost use it as amplifier, because when we using the amplifier to amplifying the sine wave, the voltage has a limited range from 0 to +12V as from 1V to 11V or 2V to 10V, etc..., it depends on what you want to setting the voltage level, so it can not like the bjt using in logical circuit that it can only jump from 0V to 12V, and from 12V to 0V.

    The R3 was used to set the lower voltage as 1V or 2V, etc..., R4 was used to get the middle voltage as 4V to 8V, all the value of resistors are according to the gain of amplify and range of voltage to make choice.
     
  5. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    This may be wrong as I'm learning too. R2, P1, and R4 form a voltage divider circuit for the transistor base. R1 limits the available current to Q1. R3 is the "load", without it as Q1 turns on, there would be a short circuit between Q1 and ground. R3 could be any other devise or component.

    This is just a example circuit, to show the effect of changing the current on the base of Q1 and the effect it has on it's output.. That would be A1, A2.
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    Don't always think of resistors as limiting current or protecting a circuit element.

    Fundamentally, a resistor creates a potential difference (voltage drop) across the resistor.

    Two resistors or resistive elements (which would also include components such as diodes and transistors) connected in series will create different voltage drops across each element and hence will establish a given voltage at the node.

    Here is a simple exercise:

    [​IMG]

    Calculate the voltage V2 at the node shown between R2 and R4 given that the supply voltage applied is V and GND.

    By "Calculate" I mean express V2 as a function of V and R2 and R4. Show how you arrive at your answer.
     
    Last edited: Sep 6, 2014
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