# LM741 Op Amp not working

#### Tiago Rocha

Joined Jul 12, 2021
30
what is the designed (expected) input voltage range?
Between 0 and -3.3v

#### Tiago Rocha

Joined Jul 12, 2021
30
Do that again, but this time make the junction of your two batteries the ground.

Bob
That was actually my doubt and what i didn't understood when i connected it, i connected the positive terminal on the + supply terminal of op amp and the negative terminal in the - supply terminal of the op amp (to create 18v potencial difference). The ground ( junction of your two batteries) was not connected to anything, but the +9V -9V are reffered to ground but i don't know where should i connect ground?

#### boostbuck

Joined Oct 5, 2017
273
The centre of the two batteries is connected to the ground points on your original diagram.

#### Audioguru again

Joined Oct 21, 2019
5,584
ANY opamp will work with a single positive supply if it is biased properly.
Your 741 opamp is 53 years old when only a positive 15V and negative 15V power supply was used.
You have the + input at 0V when its minimum is +3V in your circuit. Also, its - input minimum is also +3V but yours is a negative voltage.
Therefore a 741 opamp barely works with an 6V or 8V supply.
Why didn't you see these things in its datasheet??

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#### Ramussons

Joined May 3, 2013
1,333
So, i want to use an Op Amp with inverting topology to invert -3,3v into 3.3v (gain of -1). when i simulated it as in the figure i've used single power supplies and it worked (even with 5v in simulation). But in the practice i've tried 5v,12v and 15v single power supplies and none of them made the op amp invert the signal. I've tried also with 2 9V batteries (connected in series) but it didn't work neither (i don't have a dual power supply). Apparently some people manage to make it work like this in the past. I also tried to make a non-inverting topology and voltage follower and none of them worked.
Is this failure related to the supply voltage? do i really need to get more voltage rail to rail?

Also, can i invert an negative voltage with a single supply op amp?
If so, what are the best op amps to do it?
Any OpAmp, with this setup will give an output of 0 volts.
You have Grounded the Non Inverting Input in a Unipolar Supply.

#### MrSalts

Joined Apr 2, 2020
2,767
An op amp with positive input grounded, and -3.3v to 0v input through a resistor to the (-) input will cause the op amps output to swing between 0 and 3.3v.

You'll be fine with a 0-5v single supply. A TLC272 is a low cost.

or, here is an even more modern version... (but not available in through-hole)...

#### Alec_t

Joined Sep 17, 2013
13,337
An op amp with positive input grounded, and -3.3v to 0v input through a resistor to the (-) input will cause the op amps output to swing between 0 and 3.3v.
Only if the opamp is configured as a unity gain amp with a negative feedback resistor equal to the input one.

#### BobTPH

Joined Jun 5, 2013
6,310
An op amp with positive input grounded, and -3.3v to 0v input through a resistor to the (-) input will cause the op amps output to swing between 0 and 3.3v.

You'll be fine with a 0-5v single supply. A TLC272 is a low cost.

or, here is an even more modern version... (but not available in through-hole)...

No, the inputs are not allowed to go below-0.5V.

Bob

#### MrSalts

Joined Apr 2, 2020
2,767
No, the inputs are not allowed to go below-0.5V.

Bob
Think about it, the voltage source, -3.3v, the opAmp output +3.3v, two resistors in between in a unity gain amplifier, you get zero volts to match the voltage at the (+) input. The inputs never see a negative voltage.

Only if the opamp is configured as a unity gain amp with a negative feedback resistor equal to the input one.
True, but how much gain would you want (need) from an input signal with 3.3v range?

#### BobTPH

Joined Jun 5, 2013
6,310
In the steady state yes, but what happens before it reaches the steady state?

Bob

#### MrSalts

Joined Apr 2, 2020
2,767
In the steady state yes, but what happens before it reaches the steady state?

Bob
- one could insure they specify an op amp that is reasonably fast vs the input signal so the output never lags input by more than 0.3v. Or
- divide the input source with a pair of resistors to insure input never drops below -0.3v, then increase the size of the feedback resistor proportionality to restore amplitude lost to the divider network.

I'm sure you could have thought of those options and maybe even more if you put on your problem solving hat this morning instead grabbing your "poke holes in other people's ideas stick".

#### Tiago Rocha

Joined Jul 12, 2021
30
Just want to update you about what i did. I've tried the LM358 with 5v single power supply and it actually worked well. it can convert negative signals into positive ones.
I also tried the lm741 with dual supply and notice that it doesn't need +-15V, it worked fine with +-9V power supply.
Thanks for helping.

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#### atferrari

Joined Jan 6, 2004
4,684
Hola Tiago:

For you to keep for reference in the future, drawn many years ago by member Audioguru. Will save tons of clarifications along a thread.

Be well.

#### MrChips

Joined Oct 2, 2009
28,145
Non-inverting buffer

#### Audioguru again

Joined Oct 21, 2019
5,584
Some people were wrongly thinking that you were feeding -3.3V to the inverting input of the LM358 that exceeds the maximum common mode input rating of -0.3V. The LM358 does not have a negative supply.
But the opamp has negative feedback so then the - input is very close to its + input of 0V and its output correctly goes to +3.3V.

The LM358 can do it but the LM741 cannot because the LM358 has PNP inputs that work at 0V but the LM741 has NPN inputs that work only above +3V or +4V.

#### Tiago Rocha

Joined Jul 12, 2021
30
Some people were wrongly thinking that you were feeding -3.3V to the inverting input of the LM358 that exceeds the maximum common mode input rating of -0.3V. The LM358 does not have a negative supply.
But the opamp has negative feedback so then the - input is very close to its + input of 0V and its output correctly goes to +3.3V.

The LM358 can do it but the LM741 cannot because the LM358 has PNP inputs that work at 0V but the LM741 has NPN inputs that work only above +3V or +4V.
And i'm actually doing that sir. I thought that i should make an voltage divider to get a max of -0.3V in the inverting input and a certain gain to get the 3.3V in the output but apparently i don't need that, the LM385 can invert 0 - (-3.3V) in 0 - 3.3V (gain of <1). However invert a positive signal into a negative one is not possible

#### Audioguru again

Joined Oct 21, 2019
5,584
You do not need a voltage divider since the feedback resistor and the input resistor cause the -input to be the same 0V as the +input. That is how negative feedback works.
The input voltage to the input resistor goes negative that causes the output of the LM358 or similar opamp to go positive.

Of course if it does not have a negative supply then the output cannot go negative.

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#### sghioto

Joined Dec 31, 2017
3,948
So, i want to use an Op Amp with inverting topology to invert -3,3v into 3.3v

#### Audioguru again

Joined Oct 21, 2019
5,584
Your new VSS has no voltage. It should be -15V since VDD is +15V.
The minus of the +15V and the positive of the -15V should connect to the circuit ground.
R1 and R4 should be capacitors, use 0.1uF.
Connect all the grounds together.