# 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
6
0
Hello,

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!

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2. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
782
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
6
0
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.