Op amp output different from calculated?? Any help appreciated!

Thread Starter

Jambu

Joined Sep 15, 2021
15
Hi all!
I am new in op amp, and currently studying the basics of it..

So I designed this LM741 circuit with a single supply, I made the input at the V+ to be 1.5v supplied by a voltage divider, and a gain of 3 with feedback resistor being 20k and R1 being 10k.
My problem is, according to the op amp rule, V- will try to be the same as V+, but in my case, it is about 0.35v different.
Also the V out I calculated with the gain is suppose to be 4.5v, but it measured a 5.5v.
Am I doing anything wrong?

Any help would be so appreciated guys!
Thanks in advance!!
Ps: I've tried searching in the thread for any related post but I couldn't find.
 

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Sensacell

Joined Jun 19, 2012
2,919
The 741 is an ancient part, with limitations that many modern parts don't have.

The input voltage of 1.5 volts is too close to your negative supply, which is zero volts in your circuit.
This would work if you had a negative supply voltage but beware also that the 741 output cannot go all the way up to the positive supply.

In a traditional, old-school opamp circuit, you have both a +15V and -15V power supply, the signals are typically constrained to +10 to -10 V to stay away from the input and output voltage limits.
 

dl324

Joined Mar 30, 2015
13,532
Welcome to AAC!
I am new in op amp, and currently studying the basics of it..
Aside from being an old design, the LM741 is still quite useful. You just need to understand how to use it.

Non-ideal opamps have a limitation on the range of input voltages allowed. For the 741, that's:
1631725289665.png
1631725309101.png
With a V- of ground, the lowest voltage guaranteed to be allowed on the inputs is 3V, with 2V being allowed for a typical part (which are most of the parts you'll have). So the 1.5V you have on the non-inverting input isn't within the allowed input voltage range. What the opamp will do with invalid inputs depends on the design. Some will experience phase inversion (also called phase reversal) and some won't. See the attached application note MT-036 for more information.

Most opamps from the 1970's were intended to be used with +/-15V supplies.
I made the input at the V+ to be 1.5v supplied by a voltage divider, and a gain of 3 with feedback resistor being 20k and R1 being 10k.
You need to be careful about saying V+ when you really mean the non-inverting input. V+ is what the positive supply is called:
1631725203648.png

The LM741 is still a very good opamp to learn with. As recently as a decade ago, people working on their PhD in Computer Engineering still used/studied them. Because of its limitations, it forces you to learn about them and how to work with them. The number of engineers and designs that were able to use the LM741 successfully are countless. When the LM741 was first introduced, it was the best thing since sliced bread. I think it was the first integrated opamp that didn't require external compensation.

I just bought a dozen or two of them from someone selling parts from an estate sale of a longtime Tektronix employee because Tektronix liked to use them in their designs unless something better was required.
 

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Last edited:

LowQCab

Joined Nov 6, 2012
1,341
The only "good-thing" about a 741 is that
"they're giving them away down at your local corner Gas-Station".

Is ~$0.35 cents really too expensive for a reasonable modern Chip that always works ?

I'd pay ~$8.oo for excellent specs and manners,
and zero frustrating, never ending, troubleshooting.
.
.
.
 

Thread Starter

Jambu

Joined Sep 15, 2021
15
The 741 is an ancient part, with limitations that many modern parts don't have.

The input voltage of 1.5 volts is too close to your negative supply, which is zero volts in your circuit.
This would work if you had a negative supply voltage but beware also that the 741 output cannot go all the way up to the positive supply.

In a traditional, old-school opamp circuit, you have both a +15V and -15V power supply, the signals are typically constrained to +10 to -10 V to stay away from the input and output voltage limits.
I see! thank you so much for you help.
 

Thread Starter

Jambu

Joined Sep 15, 2021
15
Welcome to AAC!
Aside from being an old design, the LM741 is still quite useful. You just need to understand how to use it.

Non-ideal opamps have a limitation on the range of input voltages allowed. For the 741, that's:
View attachment 248031
View attachment 248032
With a V- of ground, the lowest voltage guaranteed to be allowed on the inputs is 3V, with 2V being allowed for a typical part (which are most of the parts you'll have). So the 1.5V you have on the non-inverting input isn't within the allowed input voltage range. What the opamp will do with invalid inputs depends on the design. Some will experience phase inversion (also called phase reversal) and some won't. See the attached application note MT-036 for more information.

Most opamps from the 1970's were intended to be used with +/-15V supplies.
You need to be careful about saying V+ when you really mean the non-inverting input. V+ is what the positive supply is called:
View attachment 248030

The LM741 is still a very good opamp to learn with. As recently as a decade ago, people working on their PhD in Computer Engineering still used/studied them. Because of its limitations, it forces you to learn about them and how to work with them. The number of engineers and designs that were able to use the LM741 successfully are countless. When the LM741 was first introduced, it was the best thing since sliced bread. I think it was the first integrated opamp that didn't require external compensation.

I just bought a dozen or two of them from someone selling parts from an estate sale of a longtime Tektronix employee because Tektronix liked to use them in their designs unless something better was required.
Thank you for your detailed explaination! Will take not on my wordings next time!
 

Audioguru again

Joined Oct 21, 2019
3,849
Before buying an opamp, read its datasheet.
1) The TLC272 has two opamps in it.
2) Its maximum supply voltage is only 16V or 18V.
3) Its output current is low.
4) Its output high voltage cannot go as high as its supply voltage.
 

Juhahoo

Joined Jun 3, 2019
250
Will definitely take note to buy TLC272 in my next purchase! Thank you!
It is not a recommendation, you need to understand what you want and select the component based on your criteria. You can ask here when you have an application is it good option for the use.
I use different OP amps for different purposes, whether the reason is drive capability, component size, current consumption, voltage range, price, availability etc etc.. You obviously can't get everything in one single package.
 

dl324

Joined Mar 30, 2015
13,532
Thank you for your detailed explaination!
That's what most of us are here for...
Will take not on my wordings next time!
You can call the inputs IN- and IN+ if you don't want to type out the names, but making up your own names or misusing others will just cause confusion.

The output voltage range also has limitations. For the LM741 operated from a single 9V supply, the guaranteed output voltage range will be 3-6V lightly loaded. It isn't guaranteed to work with a heavier load.
1631805912663.png
1631805931710.png

My go to opamp for single supply operation is LM358. Since the input devices are PNP, the input voltage range includes ground. Since the opamp was designed for single supply operation, the output can get close to ground.
1631806277669.png
1631807197502.png
1631806305831.png
 
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Audioguru again

Joined Oct 21, 2019
3,849
The old 741 has all of its datasheet specs when the supply is +15V and -15V. Some of them do not work when the supply is only 9V.
The old LM358 uses a low supply current then it produces crossover distortion and has a poor high frequency response and a low slew rate.

Both these old opamps produce lots of noise.
 
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