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14th January 2014, 19:35 #1
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Bode plot for opamp circuit different depending whether in LTspice or Excel..why?
Hello,
Please can you advise on why the Bode plot for an opamp circuit is different depending on whether I use LTspice to get it , or when I calculate it in Excel?
(If I do the Bode plots for a simple, passive RC circuit, then they are the same whether I use Excel or LTspice)
The below shows the opamp schematic in question, and the bode plots, plus LTspice simulation.
For example, at 50.31KHz…
Excel gives a gain of 6.74dB & phase of 163 degrees
LTspice gives a gain of –9.202dB & phase of 155 degrees
Also, as you can see, at low frequencies of around 1Hz, the gain and phase are massively different between Excel and LTspice.
The LTspice bode plot is the red one and the phase is a broken line.
Do you know the reason for the differences?

14th January 2014, 19:47 #2
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Re: Bode plot for opamp circuit different depending whether in LTspice or Excel..why?
I don't see the Excel plot. How are you doing this in Excel? The Excel calculation likely is a more crude approximation as compared to the LTspice calculation.
Zapper
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14th January 2014, 20:01 #3
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Re: Bode plot for opamp circuit different depending whether in LTspice or Excel..why?

14th January 2014, 20:05 #4
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Re: Bode plot for opamp circuit different depending whether in LTspice or Excel..why?
Hint: check the DC operating points of the circuit.
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14th January 2014, 21:23 #5
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Re: Bode plot for opamp circuit different depending whether in LTspice or Excel..why?
I did an .op analysis and they all came out very low numbers....like 3e9 etc

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14th January 2014, 21:37 #6
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Re: Bode plot for opamp circuit different depending whether in LTspice or Excel..why?
Okay, I see the plots. (Working on simulation)...
Last edited by crutschow; 14th January 2014 at 21:53.
Zapper
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14th January 2014, 21:49 #7

14th January 2014, 21:53 #8
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Re: Bode plot for opamp circuit different depending whether in LTspice or Excel..why?
Excel is correct, LTspice is wrong.
The opamp has no negative feedback so its gain is millions at DC and low frequencies but LTspice says its DC and low frequency gain is only about 5.
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14th January 2014, 22:02 #9
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Re: Bode plot for opamp circuit different depending whether in LTspice or Excel..why?
Yes, the problem is the circuit is an integrator and has no DC feedback so the DC operating point is not correct in LTspice (as it would be in the real world). Below is the simulation with two added resistors for feedback to give the proper operating point. I also added a large capacitor to ground at the junction of the two resistors so the AC response is not significantly affected. The LTspice plot now looks quite close to your Excel plot I believe.
Last edited by crutschow; 14th January 2014 at 22:29.
Zapper
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14th January 2014, 22:09 #10
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Re: Bode plot for opamp circuit different depending whether in LTspice or Excel..why?
Guys, you are genius's, and thanks Crutschow for your two resistors and cap!

15th January 2014, 09:58 #11
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Re: Bode plot for opamp circuit different depending whether in LTspice or Excel..why?

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15th January 2014, 13:08 #12
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Re: Bode plot for opamp circuit different depending whether in LTspice or Excel..why?
Thanks LvW, the error amplifier is seen here in the flyback....(LTspice simulation and schematic attached)
(although this is the opto isolated version of that type 2 error amplifer being used here)
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