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Circuit Analysis

  1. Mar 8, 2015 #1
    1. The problem statement, all variables and given/known data
    4gL67ax.png
    Need help on part a.
    and c.



    3. The attempt at a solution
    • Part A
    Since Va = 4V then pin 2 and 3 also have 4v. Would that mean that Vin is = 4v as well?
    Im just confused how to find I1.
    Also im not sure how to find Vc.
    • Part C
    As for part c, Im having trouble with it entirely.

    Any help is appreciated!
     
  2. jcsd
  3. Mar 9, 2015 #2
    Well, yes, but you'll have to argue that based on the characteristics of an opamp with negative feedback. If you think in terms of the two golden rules of ideal opamp analysis..

    You know what the voltage is across R1. What about Ohm's law?

    Consider again the behavior of an ideal opamp with the feedback shown. It desperately wants to turn on that BJT hard, so it can reduce the voltage across its terminals to zero, but that really only requires the BJT to just barely move out of its cutoff region. You usually assume something about VBE then.
     
    Last edited: Mar 9, 2015
  4. Mar 9, 2015 #3
    The thing that threw me off, is what voltage to use with r1,
    But based off of the orientation of the current source, I use Vb?
     
  5. Mar 9, 2015 #4
    The voltage (with respect to ground) at Vb is 6 V, and you drop down to 4 V as you cross the resistor to Va.

    Does that help?
     
  6. Mar 9, 2015 #5
    so that means the voltage drop across the resistor is 2v, then using V=IR
    i get 2mA?

    or do i use Nodal Analysis?
     
  7. Mar 9, 2015 #6
    Yes. :smile:

    Since Vb is at a higher potential than Va, what does that tell you about the direction of the current?

    Nodal analysis is useful when you need to solve for unknown node voltages, but you're already given the node voltages on either side of the resistor, so all there's left to do is just apply Ohm's law.
     
  8. Mar 9, 2015 #7
    Current goes from a high to low potential so it goes from right to left!

    Thank you for all the help Miles! I might have more questions incoming lol.
     
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