Trouble connecting Vcc, Vee and offset nulls of LM741 in Simulink

Thread Starter

ZigZag557

Joined Jan 16, 2021
8
Hello, I've been working on a project for some time and I am stuck connecting Vcc, Vee, and offset null terminals.
This is how I connected it, offset resistors are 50 kOhm, Vcc and Vee voltages are both 20V (in the max and min limits).
3333.png

I've modeled using ideal op-amps in Simulink first (ones that you don't need to connect offsets, Vcc and Vee) and it works perfectly well.
I have tried many different combinations of connecting terminals but none worked so far and I have no idea why it's not working.
 

Thread Starter

ZigZag557

Joined Jan 16, 2021
8
This is how I mean it's not working:
65656.png
First graph is the output voltage from op-amp while I expect it to be (the second graph - 500mV)
 

atferrari

Joined Jan 6, 2004
4,326
Besides that a visual inspection would reveal what is wrong or not, you could use the simulation program to check the applied voltages.
 

Thread Starter

ZigZag557

Joined Jan 16, 2021
8
I am using the LM741-TI version.

So I have inspected the voltage values and found out that I needed to revert voltage supplies. Changed how some parts were installed and this is the graph I am getting now on output voltage: 7676.png
I'm not sure why it is jaggy this time.

This is the circuit status now:
4343.png
Offset resistors are 50 kOhm while all other resistors in the picture are 10kOhm.
I have a differential amplifier system here, it worked with an ideal op-amp.

Like I said before, it was working perfectly well with an ideal op-amp so the problem should still be from how I connected the LM741.

Thanks for your help!
 

dl324

Joined Mar 30, 2015
12,871
Offset resistors are 50 kOhm while all other resistors in the picture are 10kOhm.
What's the point of using two resistors of the same value for the offset null? They do nothing to null offset.

It would be helpful if you assigned values and designators to all components.

Using a box to represent an opamp is brain dead. Is that the simulator or you?
 

Thread Starter

ZigZag557

Joined Jan 16, 2021
8
What's the point of using two resistors of the same value for the offset null? They do nothing to null offset.
I've seen a few examples that a resistor for each offset null was placed, I am not really sure why would that be a problem though.
It would be helpful if you assigned values and designators to all components.
I will be putting labels on the components from now on, should make a habit of that.

Using a box to represent an opamp is brain dead. Is that the simulator or you?
I don't think it's possible to make the component with other shapes in Simulink.



Hello,

In the datasheet this is given for the offset compensation:
View attachment 227978

Bertus
Thank you for pointing that out since I had a question about this figure.
I don't know what the arrow pointing to the resistor means. I am guessing that I should apply Vee to the middle of the resistor but that doesn't really make sense to me.

This is my first time reading component papers so I don't know the meaning of many symbols.
 

bertus

Joined Apr 5, 2008
21,364
Hello,

The resistor with the pointing arrow is a potentiometer.
It allows you to adjust the ratio between the two resistors to adjust the offset.

Bertus
 

Audioguru again

Joined Oct 21, 2019
3,190
All your circuits have positive feedback instead of negative feedback. That causes the output voltage to saturate at the negative supply voltage.
Also, you have the polarity of Vcc and Vee backwards.
Also again. You connected the offset resistors wrongly to ground instead of to the Vee negative supply.
 
Last edited:

Thread Starter

ZigZag557

Joined Jan 16, 2021
8
I think I have done all the suggested changes to the circuit, this is how it looks like now:
I'm trying to make a differential op-amp with no additional gain.
344343.png

And this is the graph I am getting:
First graph is output of op-amp and second graph is input.
6666.png

I have tried swapping Vcc and Vee, it resulted in a graph with flat -20V as shown on the first answer to this post, now it saturates at -10V for reasons I still don't know. I still need more guidance, the circuit looks perfect to my eyes at this point.
 

sarahMCML

Joined May 11, 2019
39
I think I have done all the suggested changes to the circuit, this is how it looks like now:
I'm trying to make a differential op-amp with no additional gain.
View attachment 228005

And this is the graph I am getting:
First graph is output of op-amp and second graph is input.
View attachment 228006

I have tried swapping Vcc and Vee, it resulted in a graph with flat -20V as shown on the first answer to this post, now it saturates at -10V for reasons I still don't know. I still need more guidance, the circuit looks perfect to my eyes at this point.
If you had Vcc and Vee swapped you may have already killed the IC, try another. Also, it would be safer to use +/-15 or 18 Volt supplies.
 

Thread Starter

ZigZag557

Joined Jan 16, 2021
8
If you had Vcc and Vee swapped you may have already killed the IC, try another. Also, it would be safer to use +/-15 or 18 Volt supplies.
Since I am using it on a simulator, I don't have to think about these issues!

Also, the due date of my homework has passed now. Thanks for all your help, I've learned a lot!
 
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