Problem with OpAmp Simulation in LTspice

Discussion in 'The Projects Forum' started by coolrox86, May 4, 2012.

  1. coolrox86

    Thread Starter New Member

    Oct 13, 2011

    Recently I have some problem with simulation in LTspice for operational amplifier (inverting amplifier) circuit. The source voltage and first resistance before opamp is 9 V and 3.6m ohm. The second resistance is 1k ohm. So, the expected output voltage should be

    Vout = 9 / 3.6m* 1000 = 2.50 V

    But instead, it shows the value of 1.21 V after running the simulation.

    I have attached my LTspice drawing here. Kindly appreciate any help and suggestion to me. Thanks in advance! :)
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    You've got things somewhat confused.

    This is an inverting amplifier design.

    With a source resistance R1 of 3.6milliohms and a feedback resistance of 1kΩ the gain will be -1000/0.0036 = -278,000! With +9V applied to the input the output voltage with an ideal amplifier would be -2.5 million volts! That's not going to happen.

    All this means is that the output will go as low as it can with the op-amp output driven towards its low (-ve) side DC supply saturation voltage - which in this case is ground - but the output cannot swing to the ground potential as expected with a real amplifier of this type.

    Change R1 to something more sensible (say 100Ω) & consider connecting the op-Amp negative supply side to an actual negative potential (e.g -9V) rather than just connecting it to ground. If you want to test amplifying behavior you would make the input voltage something much less than 9V - say ±100mV. With R1=100Ω, R2=1kΩ & Vin=+100mV and the op-amp negative supply set to -9V [not ground] one would expect an output of -1.0V. Changing the input to -100mV should give an output of +1V.

    Re-check your understanding of the inverting op-amp configuration requirements.
  3. coolrox86

    Thread Starter New Member

    Oct 13, 2011
    Finally, my problem is solved:
    1. It's my carelessness for the equation, yes it's 3.6k, not 3.6m.
    2. I've changed my voltage supply inversely.
    3. I apply double voltage supply to OpAmp.

    The output voltage is + 2.5V. Thanks for your advices!