Arrow-notated resistances in transistor circuits

Discussion in 'Homework Help' started by ElectricMagician, May 18, 2015.

  1. ElectricMagician

    Thread Starter Member

    Jul 26, 2012
    57
    0
    Hello,

    Can any one give me a clear, generic way to understand what resistance these arrow-notated resistors found in amplifier circuits are supposed to express?

    Also, in the example below, is Rin
    1) (Vi - VRB1)/(Current through RB1)
    2) The above paralleled with re+Re
    3) Something else

    As to Rout, is it
    1) RE
    2) RE paralleled with (re + part 1 above)
    3) Something else

    (this is not the actual problem requirement, but I find this notation highly counter intuitive so I wanted to make sure I correctly understood what is required before attempting to solve it)


    resistance_arrow_notation.png
     
    Last edited: May 18, 2015
  2. WBahn

    Moderator

    Mar 31, 2012
    17,777
    4,804
    It means the resistance seen looking into that port.

    If you increase Vi by some amount, ΔV, you will increase the current flowing out of Vi by some amount, ΔI. If you take the ratio of these, you will get a resistance. That resistance is the series sum of Rb1 and Rin.

    So imagine removing everything beyond the tip of the arrow and replacing it with a resistor (and possible a source to establish the DC operating point) and asking what value of resistance will match the slope of V/I characteristic at that point.
     
  3. ElectricMagician

    Thread Starter Member

    Jul 26, 2012
    57
    0
    So the expression for Rin in this case will be the internal resistance in the transistor + Re, whereas the expression for Rout will involve VCC, Vi and the resistances in between. Is this correct?
     
  4. WBahn

    Moderator

    Mar 31, 2012
    17,777
    4,804
    It would be best if you showed your work and then we can look it over and provide feedback.
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,052
    3,244
    The value of Rin also involves the current gain of the transistor.
     
  6. ElectricMagician

    Thread Starter Member

    Jul 26, 2012
    57
    0
    Can you please direct me to where I can read more about this notation, or preferably solved examples. I have googled it to no avail.
    The solutions to the simple questions I asked would have significantly helped me understand and saved me a lot of time.

    Thank you
     
  7. crutschow

    Expert

    Mar 14, 2008
    13,052
    3,244
    Since this is the Homework Help forum not the Do Homework forum that is what we normally do.
    We make suggestions to help you find the answer yourself.
    Sorry if that's too time consuming for you. :rolleyes:
    Doing a search for "common emitter amplifier input impedance" I found this tutorial which may help.
     
  8. panic mode

    Senior Member

    Oct 10, 2011
    1,321
    304
    arrow simply points towards part of a circuit... if you cut out part of circuit where arrow points and replace it with some equivalent resistance, what would this equivalent resistance be? It is Rin or Rout of course... (in and out is here merely description).
     
    • Rin.png
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