# I'm stuck at couple points in the problem

#### ericgibbs

Joined Jan 29, 2010
13,640
hi,
That is not good, the OPA is saturated, the Va,b,c are much to high a value.
Dont forget the OPA has Gain of 1+(50k/10k) = ~6

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#### cikalekli

Joined Dec 10, 2020
103
hi,
Your teacher is incorrect it saying that Va,b,c and the currents ia, ib ,ic can be determined in using LTS, without knowing Va,b,c.

The actual values of ia,ib,ic depend upon the applied voltages Va,b,c
If Va,b,c are 0V then Vout of the OPA is also 0V.

If you wish, ask him to contact me and explain how he can solve this without Va,b,c.

E
View attachment 225533
Excuse me sir but how could you separate each waveform just like that. When I run it, just I was not enabled to seperate them just as you did...

#### cikalekli

Joined Dec 10, 2020
103
hi,
That is not good, the OPA is saturated, the Va,b,c are much to high a value.
Dont forget the OPA has Gain of 1+(50k/10k) = ~6

E
I'm very new to hear that "the OPA is saturated" term. I could not understand that what does this lead to?

and my second question is the equation which you gave 1+(50k/10k) = ~6 does it mean that if the input voltage is 5V in magnitude, the output voltage will be 30V in magnitude?

I do not fully understand the meaning of this either, how can I learn them?

#### ericgibbs

Joined Jan 29, 2010
13,640
hi,
The Vout of an OPA is limited by the supply voltages plus the limitations of the OPA to get close to the supply voltages.

So yours with a~5Vout means the OPA cannot go as high as you expected.

eg:
a 0.5Vinp would give 3Vout
a 1.5Vinp would want give a 6Vout but it cannot, it would be 5Vout max.

Also most common OPA's will only go between +/-1V of the supply voltages

So set Va.b.c around 0.1V
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Image shows the limitation of the OPA

Update:

To get get multiple plot panes.

Right click on the plot, from the pop up Menu select 'Add Plot Pane'

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#### cikalekli

Joined Dec 10, 2020
103
hi,
The Vout of an OPA is limited by the supply voltages plus the limitations of the OPA to get close to the supply voltages.

So yours with a~5Vout means the OPA cannot go as high as you expected.

eg:
a 0.5Vinp would give 3Vout
a 1.5Vinp would want give a 6Vout but it cannot, it would be 5Vout max.

Also most common OPA's will only go between +/-1V of the supply voltages

So set Va.b.c around 0.1V
E

Update:

To get get multiple plot panes.

Right click on the plot, from the pop up Menu select 'Add Plot Pane'
Now I see my fault and you just made me truly happy by giving these further examples to clarify that term. I don't know how to thank you but really now I understood the importance and the real meaning of the Vout..

I set now for Va, b and c are 0.1, 0.2, and 0.3 and Vout is around 1.1143V

but I cannot see the exact values for them on the graph, how can I see their exact values with the table sir?

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#### ericgibbs

Joined Jan 29, 2010
13,640
hi,
You have to use the .meas ure command
or
Right click on the plotted label and a drop menu will show select one cursor, look at these clips.

E
It is not practical to teach you this way how to use the LTS functions.

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#### ericgibbs

Joined Jan 29, 2010
13,640

#### atferrari

Joined Jan 6, 2004
4,425
Sorry but I fear the OP, even if getting the correct values and consequently, correct results, would end having no idea of what is going on in the divider on the NI input. Much more in his benefit would be if he manages to calculate the input voltage as determined by the three voltages on the three resistors, by hand.

In my quest to learn the basics of op amps, once I got that clear, it became my eureka moment. Sure, LTSpice was not then available, at least to me.

#### cikalekli

Joined Dec 10, 2020
103
hi,
You have to use the .meas ure command
or
Right click on the plotted label and a drop menu will show select one cursor, look at these clips.

E
It is not practical to teach you this way how to use the LTS functions.
no no you explained way clear for it. Finally, I understood that thing ^^

#### cikalekli

Joined Dec 10, 2020
103

#### cikalekli

Joined Dec 10, 2020
103
Sorry but I fear the OP, even if getting the correct values and consequently, correct results, would end having no idea of what is going on in the divider on the NI input. Much more in his benefit would be if he manages to calculate the input voltage as determined by the three voltages on the three resistors, by hand.

In my quest to learn the basics of op amps, once I got that clear, it became my eureka moment. Sure, LTSpice was not then available, at least to me.
Ah, I just lived that moment when sir ericgibbs clarified my questions on this op-amp design ^_^

#### cikalekli

Joined Dec 10, 2020
103
hi,
You have to use the .meas ure command
or
Right click on the plotted label and a drop menu will show select one cursor, look at these clips.

E
It is not practical to teach you this way how to use the LTS functions.
One last question is my teacher also said this and I could not understand it here:
"You should analyze the op-amp circuit manually (calculate) and obtain the output in terms of input voltages."

I just only measured them and do you think that which of them needs to be calculated by hand?

#### ericgibbs

Joined Jan 29, 2010
13,640
hi,
Your teacher means, use pencil and paper with your calculator to solve the problem values.

Watch this video, good teacher.
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#### cikalekli

Joined Dec 10, 2020
103
hi,
Your teacher means, use pencil and paper with your calculator to solve the problem values.

Watch this video, good teacher.
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Thank you very much for sending me this video.

Yeah, really the instructor on this video is explaining clearly. I'm now working on that

Thank you so much again ^^

(I need to also mention that English is not my native language. Hence, sometimes I make grammatical mistakes and I'm really sorry about that...)

#### cikalekli

Joined Dec 10, 2020
103
hi,
Your teacher means, use pencil and paper with your calculator to solve the problem values.

Watch this video, good teacher.
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Hi again, I found Ia Ib and Ic with reversed signs:

my Vp value was 0.18571 V

and my voltage values were Va = 0.1, Vb =0.2 and Vc = 0.3

and my resistor values were Ra = 20kOhm, Rb = 10kOhm and Rc = 40kOhm

here i did that Ia = (0.1-0.18571)/20 = -4.2855 µA
Ib = (0.2-0.18571)/10 = 1.429 µA
and Ic = (0.3-0.18571)/40 = 2.85725 µA

but... here is my question, in the ltspice, I found those sign with reversed. You can look the image of it to see this:

Why am I getting reversed sign values in ltspice sir I have been looking through of it to find the reason but i could not find it... Could you help me out please if you are not busy....

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#### ericgibbs

Joined Jan 29, 2010
13,640

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#### cikalekli

Joined Dec 10, 2020
103
hi c,
In LTSpice the current direction can be shown the 'wrong' way to what you would expect.

Your answers are very close to correct values as shown by LTS.

Look at this plot and ask any questions you may have.
E

I just wanna ask that in the future as an engineer, can it be any problem to find those directions reversed just like that?

Also, my second question is, I'm sorry but I just wondered how could you paste your outcomes onto the circuit near the cables? I have looked to find this feature in ltspice but I could not find it...

Here is my asc file as you requested:

EDIT UPDATE: From your website that you suggested, someone said that " putting the minus sign in the label of the waveform in the graph is the easiest way to change the looks of the graph. "

so how is it possible to put a minus sign in the label of the waveform, please?

EDIT UPDATE 2: Oh my god I just rotated the resistors of Ia, b, and c, and yeah now I'm getting the right outcomes with the right directions. Sorry but as far as I knew resistors don't have any directions but I'm shocked that in LTSpice the direction of the resistors does make sense wow... Is this a software problem sir?

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#### ericgibbs

Joined Jan 29, 2010
13,640
hi c,
Unzip this MP4.
Right Click on wire of interest.

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#### cikalekli

Joined Dec 10, 2020
103
hi c,
Unzip this MP4.
Right Click on wire of interest.

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Omg thank you so much, finally, I've learned to do this trick on ltspice, I'm considerably a lot happy now ^^

here I truly accomplished:

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#### ericgibbs

Joined Jan 29, 2010
13,640
hi c,
So do you understand how to do the .meas command.?
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