Simulating op-amp circuits using Multisim/Matlab

[Fatima Hasan]

Homework Statement

Find Vo for figure 1 & figure 2.
Find Vo, Vo1,Vo2 for figure 3
Figures are attached below.
Simulate using Multisim, MATLAB, or any Electrical software.

Relevant Equations

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I tried to simulate the circuit shown in fig 1 using Multisim and here's my work:

The result:

I do not know where is my mistake, could someone help me figuring out the problem?
Any help would be greatly appreciated!

 

[berkeman]

In the SPICE simulators that I use, those input sources would need explicit ground connections. Otherwise they would just be floating...

 

[Fatima Hasan]

In the SPICE simulators that I use, those input sources would need explicit ground connections. Otherwise they would just be floating...

Here's my second attempt:

However, the result is not as expected to be.

 

[berkeman]

It looks like in this version you shorted the opamp -VEE supply input to its output? I guess there is no connection dot at the intersection, so maybe they are not shorted. You should still move the -VEE power symbol off of the feedback wire, though.

Can you expand the oscilloscope trace out more so that the overall character of the startup transient can be seen? Maybe go out about 10x in timebase. Also, can you invert the colors of the oscilloscope display? It's hard to read the red trace on the black background. Thanks.

 

[Fatima Hasan]

It looks like in this version you shorted the opamp -VEE supply input to its output? I guess there is no connection dot at the intersection, so maybe they are not shorted. You should still move the -VEE power symbol off of the feedback wire, though.

Can you expand the oscilloscope trace out more so that the overall character of the startup transient can be seen? Maybe go out about 10x in timebase. Also, can you invert the colors of the oscilloscope display? It's hard to read the red trace on the black background. Thanks.

Here's what I got:

 

[berkeman]

Thank you, much easier to see the 'scope output! That looks pretty reasonable if the two sources are configured as sin and square wave sources, which was your original problem statement, right? What are your settings for XFG1 and XFG2?

 

[Fatima Hasan]

Thank you, much easier to see the 'scope output! That looks pretty reasonable if the two sources are configured as sin and square wave sources, which was your original problem statement, right? What are your settings for XFG1 and XFG2?

Yes, XFG1: sine wave

XFG2: square wave

I am trying to get a result similar to this:

 

[berkeman]

Looks like you are close. Just adjust the frequency and amplitude of the two generators to get you that desired waveform...

 

[Fatima Hasan]

Looks like you are close. Just adjust the frequency and amplitude of the two generators to get you that desired waveform...

The amplitude and frequency are mentioned in the question.
Is this result acceptable or I have to adjust the scales?

 

[berkeman]

From the picture in your Post #7 it looks like you have the frequencies and amplitudes of the square and sine backwards? Double-check the original problem statement and match those settings...

 

[Fatima Hasan]

From the picture in your Post #7 it looks like you have the frequencies and amplitudes of the square and sine backwards? Double-check the original problem statement and match those settings...

I solved all the parts, can you please check my results?
figure 1:

Figure 2 & Figure 3:

Result of fig2 :

Results of Figure 3 (Vo1,Vo2, & Vo, respectively):

I am not sure about Vo in figure 3, because the value of the rightmost resistor in figure 3 is not written. I assume it's 1k,. This is the color code of it, I did a search but did not get the right value:

Thanks,

 

[berkeman]

For Figure 1 -- does your simulator let you adjust the phase of your sources? If so, I'd play with the phase (or delay) of the sine wave to get your simulation to better match the 'scope picture. You can see how the relative phase of the sine and square waves is causing an initial offset of the sine that appears to droop across the plateaus of the square wave. Adjusting the phase of the sine so that it is mid-waveform at the square wave transitions should get rid of that.

Figure 2 is an integrator, so getting a tri wave out for the square wave input seems right.

Figure 3 has a differentiator as the 2nd stage, so the tri wave input giving a square wave output looks right. I'm not understanding the final waveforms yet, though...

 

[Fatima Hasan]

For Figure 1 -- does your simulator let you adjust the phase of your sources? If so, I'd play with the phase (or delay) of the sine wave to get your simulation to better match the 'scope picture.

In the function generator, I can change the amplitude, frequency, and the offset, if I use it as sine wave. If I use it as square wave, I can change the duty cycle too.

Figure 3 has a differentiator as the 2nd stage, so the tri wave input giving a square wave output looks right. I'm not understanding the final waveforms yet, though...

As our instructor said, the 1st stage is comparator, so the result would be a square wave ( Vo1). The 2nd stage is a differentiator, so the result would be as follows:

The 3rd stage is a rectification and the result should be as follows:

(No negative pulses)
which is the output of the time-maker.