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Homework Help: OP AMP strucutres

  1. Feb 1, 2014 #1
    Hello everyone. Im studying internal structure of op amp and i dont know to solve some problems.
    The problem is that i have to specify and justify which inputs of the differential stage are inverting and non-inverting input and i dont know how to do it. I tried to find the answer on google and on many many books but i didnt find anything to give me a clearly answer. Only thing i found is that we use the maximum and minimum value of common mode voltage but i didnt understand how.
    I`ll also post some circuits if anybody can help me.

    Attached Files:

  2. jcsd
  3. Feb 1, 2014 #2
    First I must ask you a question, do understand how BJT work and also how the basic BJT amplifier configuration work?
    Also can you tell me how the output voltage will behavior if you increase voltage at inverting input?
  4. Feb 1, 2014 #3
    You can quite easy determine which input do what.
    Let as exam this circuit.


    If we increase the voltage at Vin1 (Q1 base), Q1 will turn-on more, this means that the voltage at Q1 collector will drop. Because Q4 base is connect to Q1 collector. Q4 base voltage will also drop. This of course will open more Q4 transistor. So voltage at Q4 collector and Q7 base will rise.
    This voltage rise at Q7 base will turn-on Q7 more. So the voltage at point A and at output will drop.
    So form our analysis we can tell that Vin is inverting and Vi2 is non-inverting input.
  5. Feb 2, 2014 #4
    Well, i got the idea. So if an increase in one of the input is droping the output it means it is inverting and if it increase the output it means it is non-inverting?
  6. Feb 2, 2014 #5

    Yes, exactly as you said.
  7. Feb 2, 2014 #6
    Thanks a lot!
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