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Differential mode gain

  1. Oct 6, 2013 #1

    FOIWATER

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    In the following diff amp, only one output is taken from collector two.

    When I calculate the differential mode gain using the second differential input (-Vid/2) I get a positive gain. When I calculate the differential mode gain using the first differential input (Vid/2) I get a negative gain.

    A spice simulation of this circuit shows that the output is positive.

    I would then expect to get a negative gain for (-vid/2) and a positve gain for (vid/2)

    Does someone know why that is not the case?
     

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  3. Oct 6, 2013 #2

    meBigGuy

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    Not sure how you are calculating gain. The basic operation is that when V+ is greater than V-, Q2 will try to turn off.
     
  4. Oct 6, 2013 #3

    FOIWATER

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    I made a small signal circuit out of the above using a differential mode half circuit. with the input as -Vid/2, I have -Vid/2 = ib(rpi + (1+b)*Ree/2) <-- (virtual ground between Ree/2 for diff mode.)

    Vodm = -beta*ib*(Rc||Rcc/2) <--- another virtual ground between Rcc/2

    Solving for adm = vodm/vid I get a positive number.

    However, I would expect my negative input to be multiplied by a negative gain and give a positive output.

    The same is true for Vid/2, I would expect it to be multiplied by a positive gain.

    Am I missing things?

    Thanks for the feedback
     
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