Problem with an exercise on the Differential Amplifier

Discussion in 'Homework Help' started by Helljoke, Nov 2, 2017.

  1. Helljoke

    Thread Starter New Member

    Nov 2, 2017
    Hi guys! Sorry for my english wrote :).
    I don't understand why the value of the Opamp's Vout is too different from the calculate value.
    Vout = (R7/R5) * (Vpos - Vneg), where Vpos=Vneg=2.5V. I know that, in the really case, there isn't the complete rejection of common mode, but i see that the different between ideal and really result is important. There's a Vout about of 2.40 mV, it's impossible!
    Can you help me, please? Circuit OPAMP.jpg Simulation circuit opamp.jpg
  2. WBahn


    Mar 31, 2012
    What is it that you are disturbed by? You seem to be trying to achieve an output of 0 V by applying the same voltage to both inputs. You aren't quite getting matched inputs and you aren't quite getting 0 V out.

    Have you explored what output you get if you truly drive both opamp inputs with the same voltage? Sweeping a hard common mode signal across the entire input range might give you some insight.

    And remember, this is all in simulation. When you start looking at effects down at this level, simulation and reality can differ by quite a bit.
    Helljoke likes this.
  3. crutschow


    Mar 14, 2008
    No, it's quite possible.
    Look at the input offset of the 741 op amp in its data sheet.
    It's not that great and is likely what you are seeing at the output.
    If you connect the 741 as a non-inverting follower, you should see a similar offset at the output.

    If you want ideal results then you should use an ideal op amp.
    absf, Helljoke, MrAl and 1 other person like this.
  4. Helljoke

    Thread Starter New Member

    Nov 2, 2017
    Thank very much guys!
    I'm studying an application report of TI to better understand this problem and solve it.