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Op-Amp current analysis

  1. Sep 19, 2011 #1
    Find all Collector currents

    I don't even know where to start, I added base resistors but am still getting stuck how can I find current in the first transistor?
     

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  2. jcsd
  3. Sep 19, 2011 #2

    berkeman

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    Staff: Mentor

    Welcome to the PF.

    Were you told to bias the inputs near ground (as you have done with the two resistors that you've drawn)? If so, that starts to define the currents in the input differential stage, right? How does connecting the two differential inputs to ground define the collector currents of the input differential pair?
     
  4. Sep 19, 2011 #3
    It doesn't state how it is biased, which is why I added those base resistors. all I am given is Betas are 100 and Vbe is 0.7V. Connected to +-10V. It says that it is a LH 0005 National Semiconductor
     
  5. Sep 19, 2011 #4

    berkeman

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    Staff: Mentor

    I think it's reasonable to assume that the inputs would be biased near ground. That's very common for split power supply opamps.

    So you have the Vbe and current gain numbers -- look at the first stage and look for some constrained voltage across a resistor. That's often the first step in figuring out bias circuits....

    Post you work so we can be sure you aren't missing any factors....
     
  6. Sep 19, 2011 #5
    Q1 has no resistors so how can I find current at its base collector or emitter?
     
  7. Sep 19, 2011 #6

    berkeman

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    Expand what you are looking at. How many transistors are in the input stage?
     
  8. Sep 19, 2011 #7
    I really have no idea how to start this. All it says is find all the collector currents. This is my first course in opamps and i'm blown away. Thanks for your help sir.
     
  9. Sep 19, 2011 #8

    berkeman

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    Start by answering my latest question. How many transistors are involved in the input stage? And then, what other components are directly connected to the input stage transistors?
     
  10. Sep 19, 2011 #9
    I think there's four. So do I calculate the impedance of them?
    I don't know why it doesn't show vin, how can I figure out the base current without voltage or resistors to put into the fundamental equation to solve for Ic.
     
  11. Sep 20, 2011 #10

    berkeman

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    Staff: Mentor

    Yes, there are 4 transistors in the input stage. What other components are there? What are the voltages at the top and bottom of the input stage? If the inputs are at ground, what can you say about the voltages and currents in the lower part of the input stage?
     
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