OpAmp problem

Discussion in 'General Electronics Chat' started by Wtech, May 1, 2009.

  1. Wtech

    Thread Starter New Member

    Apr 27, 2009
    I am using opAmp LM358 as difference amplifier. I use single DC supply 5V and GND. I give AC signal at inputs of OPAmp. The gain of opamp is approximately 18. According to this configuration the output of opamp should have +ve side of AC signal with multiply by gain and –ve side is 0V because of supply. But opamp worked like comparator, its output is 0v and 5v. Can any help me to identify the problem. Thanks
    see the attach image,

  2. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    A difference amplifier usually has the configuration as on this web site:


    Your setup looks more like an inverting amplifier with a gain of 5.55, determined by R2/R1 or 100k/18k. However, you don't have the ground reference of the inverting amplifier.

    For simulatioin purposes, try pulling node 4, the (+) terminal of the op amp, to ground through a large value of resistance. That is, put a large resistor, >1MEG, between node 4 and GND.
  3. AdrianN

    Active Member

    Apr 27, 2009
    Wtech, you need some DC level on the noninverting input to lift the output off the ground and position it somewhere in the middle of 0-5V range. The DC input will be amplified (1+R1/R2) times, so make sure it is not too high. You can check my post about the differential amplifier here


    The schematic and calculations should help you. There are two articles, part 1 and 2, each one solving the differential amplifier in a different way.
    Last edited: Dec 6, 2009
  4. Wtech

    Thread Starter New Member

    Apr 27, 2009
    Thanks for reply,
    I am using AC source at input of OPAMP, and as my requirement i haven't add DC level at input. only i need that when node 4 goes high with respect to node 3 the output should equal to gain*voltage at node 4. and when node 4 goes low with respect to node 3 the output should equal to GND because i supply GND at pin 4 of OPAMP.
    As the basic property of AC source, when node 4 goes high with respect to node 3, conversely node 3 goes low with respect to node 4 at the same time.
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
    You need to ground the + input.