LM741 Op Amp not working

Thread Starter

Tiago Rocha

Joined Jul 12, 2021
30
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?
 

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Papabravo

Joined Feb 24, 2006
17,621
The 741 does not work in single supply mode. The negative input voltage is outside the common mode range. It also won't work rail to rail It will top out at 2.5 to 3.0 volts less than either rail. Why are you messing around with such an obsolete part? Pretty much every opamp has a requirement for the input signals to be with in the "common mode range" which is specified on the datasheets.
 

Ian0

Joined Aug 7, 2020
4,154
Look for the "common mode input voltage" range. For your circuit to work, it must include the negative supply. A 741 can't do that, but a LM358 can, and anything that is labelled as "rail-to-rail input"
 

Papabravo

Joined Feb 24, 2006
17,621
I think the "level shifting" problem can be solved with a discrete circuit using a BJT or a MOSFET. I don't happen to have one handy, but I'll check my archives.
EDIT: Found it. A depletion mode device like a JFET will do the job. A negative 3.3 Volt level on the gate will pinch off the channel allowing a pullup resistor determine the high level. A ground on the input will allow Idss (limited by R1) to flow pulling the positive level signal toward ground.

EDIT 2: Something like this maybe. Unfortunately your going to have trouble finding JFET. NTE electronics (notorious overchargers wan almost $9.00 for a 2N3819)

1638480558744.png
 
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Papabravo

Joined Feb 24, 2006
17,621
Alternatively you can acquire a simple low current inverting buck-boost converter that will create a negative supply for the opamp from a positive supply. the good news is that opamps do not need much current to do useful things.
 

Thread Starter

Tiago Rocha

Joined Jul 12, 2021
30
I have no doubt it doesn't work well. You're attempt to use 55-year-old technology designed for +/-15v bipolar power supply on a 3.3v single supply is beyond silly.
Not 3.3v power supply(i tried with 18v actually). -3.3v is the voltage in the inverting input.
 

Papabravo

Joined Feb 24, 2006
17,621
Not 3.3v power supply(i tried with 18v actually). -3.3v is the voltage in the inverting input.
It is the unipolar power supply that is the problem, not the magnitude of that supply. That said the opamp would probably not work with ±3.3 Volt supplies even if you had them because the output cannot come closer to the power supply rails than 2.5 to 3.0 Volts. It's a crappy part in this day and age -- leave it for the dustbin of history.

If you are still determined to use the part then take your +15V power supply and get an inverting buck-boost DC-DC converter and make a -15V supply. 100 mA on the -15V converter is more than sufficient for experimentation.
 

Thread Starter

Tiago Rocha

Joined Jul 12, 2021
30
It is the unipolar power supply that is the problem, not the magnitude of that supply. That said the opamp would probably not work with ±3.3 Volt supplies even if you had them because the output cannot come closer to the power supply rails than 2.5 to 3.0 Volts. It's a crappy part in this day and age -- leave it for the dustbin of history.

If you are still determined to use the part then take your +15V power supply and get an inverting buck-boost DC-DC converter and make a -15V supply. 100 mA on the -15V converter is more than sufficient for experimentation.
I can use other ones. In fact i just wanted you guys to name one or two newer op amps (doesn't need to be single power supply op amp ) that can do the job with lower PS voltages.
 

Papabravo

Joined Feb 24, 2006
17,621
I can use other ones. In fact i just wanted you guys to name one or two newer op amps (doesn't need to be single power supply op amp ) that can do the job with lower PS voltages.
You sir, are not listening, nor are you paying attention. You have been given at least one alternative, but you still cannot use, an LM358 for example, with a single power supply of +V and GND to invert a signal with a value below ground. Opamps in general cannot do this, no matter how much you might want them to.
 

Ian0

Joined Aug 7, 2020
4,154
Interesting that TI's LM358 has a common mode voltage that goes down to Vee, but On-Semi's LM358 has a common mode voltage that extends to -0.3V.
So, in theory, an inverting amplifier with the non-inverting input connected to ground should work. The circuit should work with a negative input voltage, but not with a positive input voltage.
 

Papabravo

Joined Feb 24, 2006
17,621
Interesting that TI's LM358 has a common mode voltage that goes down to Vee, but On-Semi's LM358 has a common mode voltage that extends to -0.3V.
So, in theory, an inverting amplifier with the non-inverting input connected to ground should work. The circuit should work with a negative input voltage, but not with a positive input voltage.
The difference between Vee and -0.3V could be the presence or absence of protection diodes. With a negative 3.3Volt input, depending on the nature of the driving source, I could believe almost any behavior. I know for example that the LM339/393 comparators have a counterintuitive output inversion if either input is outside the common mode range.
 

Thread Starter

Tiago Rocha

Joined Jul 12, 2021
30
You sir, are not listening, nor are you paying attention. You have been given at least one alternative, but you still cannot use, an LM358 for example, with a single power supply of +V and GND to invert a signal with a value below ground. Opamps in general cannot do this, no matter how much you might want them to.
Sir, i just said before that i can use a dual power supply to get that done. Maybe i'm missunderstanding something but there is newer op amps that can invert 0 -(-3.3V) into 0-3.3v right? or am i wrong?
I'm not telling that the lm358 can do that, i just want an alternative to lm741.
 

Papabravo

Joined Feb 24, 2006
17,621
What I cannot determine, according to datasheets, is the behavior of any particular opamp, when used in single supply mode, and with an inverting input outside of the common mode range. As a minimum I would choose the resistors to limit the current, if you are using a bipolar opamp. It could be that there is an opamp that will do something sensible under those circumstances. I would be uncomfortable depending on that behavior, unless it was confirmed by a statement to that effect in the datasheet.

Alternatively, you might be able to look at an equivalent circuit for some opamp, and determine that it would in fact behave in a sensible way.

@Ian0 has suggested the possibility that using an inverting configuration with the non-inverting input at GROUND might work for some opamps with negative input signals ONLY. That may be good enough for you, and if so that's great. I'd want to explore this in more detail on the bench with more than a handful of parts.

One more thing. Many manufacturer's have abbreviated the datasheets for legacy parts that are no longer manufactured. If you are going to pick a new part I would aim for one with a datasheet no more than 10 years old. Use supplier websites like Digikey and Mouser to do a parametric search
 

BobTPH

Joined Jun 5, 2013
4,306
So you want to replace one opamp that can’t do what you want with another opamp that can’t do what you want? Seems rather pointless to me.

By the way, when you used two 9V batteries in series, you had a dual supply. You just were measuring the wrong thing, or it would have worked. Do that again, but this time make the junction of your two batteries the ground.

Bob
 
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