Need URGENT help with opamp circuit

Discussion in 'General Electronics Chat' started by smithbrain, Nov 17, 2008.

  1. smithbrain

    Thread Starter New Member

    Nov 17, 2008
    1
    0
    Hello I am trying to build this circuit to give me an output of 4.5 volts when the current across the 1 ohm resistor is 0.4 amps. I have tested the differential part of the circuit and it was working but when I build the other parts of the circuit it doesn’t want to work. And another problem I found is that when I reduce the current across the 1 ohm resistor to 0 amp the out put reads 0.7 V. The part that does not want to work is the one circled in blue. I have been working on this problem for 2.5 weeks now and I seem to have run out of ideas. What can possible be the problem?
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Your first op amp is incorrect. The resistors are going to produce negative gain, and the feedback is positive. Your second op amp will also have negative gain, meaning the output will be something like 1/20th the input.

    Read this chapter - http://www.allaboutcircuits.com/vol_3/chpt_8/4.html - and the following one in our Ebook. Yhey will explain what you have done wrong.
     
    Last edited: Nov 17, 2008
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