# op amp problem

Discussion in 'Homework Help' started by ninjaman, May 10, 2014.

1. ### ninjaman Thread Starter Member

May 18, 2013
307
1
hello

I am simulating an op amp using a 741 on multi-sim.
IT HAS TO BE MULTISIM. (just to get that out of the way.)
I have an input of 2v pk at 1kHz function generator, inverting op amp 741
10Kohm input resistor and 500kohm feedback resistor. I should have a gain of -50. so 2v * 50(gain) = 100v
however, I have a +15v and a -15v giving me 30vpk-pk. so the maximum output should be 30v pk-pk. there should be clipping, an almost square wave. I have to show the spectral content of the waveform using the spectrum analyzer. I have a fundamental wave and odd harmonics.
The fundamental frequency is 1kHz with a value of 19v.
This is where I am confused.
Should it be 19v?
I cant figure out if this is correct.
any help would be great
thanks
simon

2. ### AnalogKid AAC Fanatic!

Aug 1, 2013
7,558
2,142
Please post your schematic if you expect any kind of detailed response. Beyond that, two things.

Read the 741 data sheet section about output voltage.

Read web pages about the Fourier series, Fourier Transform, and the harmonics of a square wave. The p-p value of a square wave and the p-p value of its fundamental are not the same thing.

ak

3. ### t06afre AAC Fanatic!

May 11, 2009
5,936
1,227
In a real world the output of your 741 would never have been 30 volt peak to to peak. What kind of output voltage do you get?

4. ### enggricha Member

May 17, 2014
68
1
unless ur using an ideal op amp model the output will never swing to the supply voltage levels. I would say you should get about 25-26 volts for different varients of 741.

5. ### crutschow Expert

Mar 14, 2008
21,372
6,123
A 1kHz, ±15V square-wave has a value for the fundamental (1kHz 1st harmonic) frequency of about 12.6Vrms. Don't know how you got 19V.

Last edited: May 19, 2014
6. ### AnalogKid AAC Fanatic!

Aug 1, 2013
7,558
2,142
4/pi x 15 = 19.09.

ak

7. ### crutschow Expert

Mar 14, 2008
21,372
6,123
That can't be the RMS value of the 1st harmonic since it exceeds the RMS value of the square wave.

Last edited: May 20, 2014
8. ### AnalogKid AAC Fanatic!

Aug 1, 2013
7,558
2,142
Exactly. That is one of the fundamental truths of Fourier analysis, and a conceptual hurdle for many. The p-p value of the fundamental is greater than the p-p value of the composit waveform. There are many Fourier websites that show the incremental contributions of the lower harmonics, and you can see that the harmonics combine with both constructive interference to speed up the sides, and destructive interference to pull that fundamental peak down to the flat level.

http://en.wikipedia.org/wiki/Fourier_series

Note that in the bottom image of the graphic the flat top is starting to form but it still has little peaks on the ends. This is the Gibbs phenomenon, and still is present after 125 harmonics. Among other things, this is why people exaggerate the end of motion when emulating a robot. A well programmed robot doesn't move that way; its hard on the bearings. We perceive it incorrectly due to limited processing bandwidth in the brain. (Used to build hardware for experimental/cognitive psychologists, learned cool stuff.)

ak

9. ### crutschow Expert

Mar 14, 2008
21,372
6,123
So you are talking Pk values, not RMS for the sinewave? Spectrum analyzers usually give the values in RMS.

10. ### AnalogKid AAC Fanatic!

Aug 1, 2013
7,558
2,142
True. I was only explaining where the 19 came from.

ak