Hello, Where Does this 2V difference come from in Op-Amps

[Armia]

So, I wanted to try simulating a virtual Op-Amp as a comparator, and noticed something bothering me and have no clue why it happens

When the supply voltage +Vcc = 15V and -Vcc = 0 and the inverting input is 3V and non inverting input is 7V, the output I assumed The output would be 15V or slightly less, but the output was 13V.

when I switched the Inputs, the output was 2volts and not 0, and when changing -Vcc to -15V
the output was about 13V

Why is it always 2 Volts difference from the supply voltages no matter what I've changed in the Circuit ? is the simulation is broken or there is some constant in each junction due to the voltage on the semiconductor junctions inside ? I Know a proper IC for this stuff will fix the issue, but I want to know the juice of how and why this happens? Use as complicated math as you want if needed.
the above simulation was in NI MultiSim on DC sweep for VDC1 [ 0:10V, 0.01V step] and didn't change the default parameters of it.

 

[bertus]

Hello,

What happens when you make R1 1K?
It can be that there are calculations made with an ouput resistance.
I do not know the properties of the "OPAMP_RATED" in NI.

Bertus

 

[Armia]

It can be that there are calculations made with an ouput resistance.

Same results when changing the resistor,
the parametrs (the changeable ones ) are the following

 

[Armia]

I used OPAMP_5T_Virtual which have more variables, the results were like this

 

[Armia]

and the parameters were the following btw

 

[AnalogKid]

I'm not familiar with this simulation program, nor its library structure and model complexity level. But ...

It could be that the opamp model has +/-2 V of headroom built into its output characteristics to more accurately simulate real-world devices. It is common for an opamp's output stage to clip when it is within xx volts of either supply rail. This is because the circuitry that drives the output transistors needs a minimum amount of voltage to operate. On datasheets, the maximum output voltage often is specified as the difference between the output voltage and the closest supply rail, such as "Vcc-1.5 V".

ak

 

[Armia]

I'm not familiar with this simulation program, nor its library structure and model complexity level. But ...

... On datasheets, the maximum output voltage often is specified as the difference between the output voltage and the closest supply rail, such as "Vcc-1.5 V".

ak

So some of the source voltage has to be consumed by the Op-Amp circuitry inside it making the output saturation is less than supply, Is this consumed value constant with most Op-Amps ( meaning doesn't change with different supply voltage or outside circuitry)? thank you btw.

 

[hevans1944]

National Instruments (NI) LabView software is now available for free for non-commercial use by students and hobbyists. It and similar SPICE-based software programs are yet another reason why I prefer actual circuits, made with real components, to software simulations. If money is a problem, there are plenty of free or inexpensive alternatives. Let Google help you find them.

Years ago, in the previous century, the company I worked for purchased LabView hardware to facilitate building an analog data acquisition system. The LabView simulation software was a necessary part of the whole package, because IIRC that software was also used to "wire up" LabView hardware to make a working system. The graphic user interface (GUI) was fairly sophisticated for its time, but NI products, whether hardware or software, were never exactly cheap nor inexpensive, but they were reliable. Once the systems integrator got over the steep learning curve and assembled a working system, it worked fine and lasted a long time without problems.

There are op-amps whose outputs are rated "rail-to-rail" and you might even find a simulation model for one or more of them. If the link doesn't work, try Google with this search string: "rail-to-rail" op amps list. The quotation marks around rail-to-rail may be helpful.

@Armia: Your stated intent was to simulate a comparator using a "virtual op-amp," whatever that is. Be aware that an op-amp generally makes a piss-poor comparator. You would be better advised to use an integrated circuit designed as a comparator, not one designed as an op-amp. The NI database probably has several comparator models available.

It would help us to help you if you described exactly what you are trying to DO. Do you have LabView hardware you are trying to use?

 

[Harald Kapp]

Is this consumed value constant with most Op-Amps ( meaning doesn't change with different supply voltage or outside circuitry)?

No. You'll have to look up the limits in the opamp's datasheet.

 

[AnalogKid]

So some of the source voltage has to be consumed by the Op-Amp circuitry inside it making the output saturation is less than supply, Is this consumed value constant with most Op-Amps

Of course not - that would be waaaay too easy.

Some newer opamp designs are called "rail-to-rail". At low output currents, the output stage can swing very close to the rails, sometimes within 0.1 V. But most of the standard garden-variety opamps people think of need at least 1 V of working space. A common datasheet example says something like at 15 V operation, the typical max output is 13.5 V, but the worst case value is 12 V. Ouch.

And it varies with the amount of output current.

And it varies with the power supply voltage value.

ak

 

[Armia]

National Instruments (NI) LabView software is now available for free for non-commercial use by students and hobbyists. It and similar SPICE-based software programs are yet another reason why I prefer actual circuits, made with real components, to software simulations. If money is a problem, there are plenty of free or inexpensive alternatives. Let Google help you find them.

I agree with you, I prefer using actual circuit rather than simulation, only downside is that one can not undo his mistakes especially when he's still learning :/ . Fortunately my college provided me with a free NI MultiSim copy, although I use LT spice more now since there are now plenty of tutorials on it, but NI MultiSim has much wider variety of parts than LT spice and has more friendly UI in testing.

Your stated intent was to simulate a comparator using a "virtual op-amp," whatever that is. Be aware that an op-amp generally makes a piss-poor comparator. You would be better advised to use an integrated circuit designed as a comparator, not one designed as an op-amp. The NI database probably has several comparator models available.

It would help us to help you if you described exactly what you are trying to DO. Do you have LabView hardware you are trying to use?

My only interest at the moment is knowing how op-amps works and its limitations, when I tried using it in a simulation I noticed that the voltage saturates below the source voltage, also I tried using 741 op-amp in both simulation and on a breadboard, and I wanted to know why this limitation happen since I recently learned how to analyze Op-Amp circuit (Ideal op-amps) and noticed that real op-amps are much more complicated, and I practically know as little to nothing about them especially after the generous answers from all of you. thank you all btw, Hope I find some more materials to know about them more and its similar device the comparators

 

[bertus]

Hello,

but NI MultiSim has much wider variety of parts than LT spice

Then you probably do not know this wiki:
http://bordodynov.ltwiki.org/
https://disk.yandex.ru/d/6nSSUmZiT9oCoQ

Bertus

 

[Armia]

Hello,
Then you probably do not know this wiki:
Bertus

Yup I didn't know about it, It will come in handy soon! thank you!!

 

[hevans1944]

... only downside is that one can not undo his mistakes especially when he's still learning...

It's not necessarily a downside to make mistakes, especially while learning!

Most op-amps are dirt cheap. If you accidentally destroy one, and if you can learn from your mistakes, just consider them to be lessons and plug in a replacement.

My only interest at the moment is knowing how op-amps works and its limitations

There is a mountain of literature, much of it free to download, that explains how op-amps work. Here is a pretty good recent text book to get you started.

The best way I have found to learn about new op-amps is to first carefully read their datasheet (assuming you already know what their parameters mean) and then purchase a few to bread-board some test circuits to measure their properties in actual circuits.

Buy some decent test equipment (digital multi-meter; digital storage oscilloscope, dual channel with at least 1 MHz bandwidth; variable audio-frequency sine, triangle, square. and pulse wave-form generator with output from about 0.01 Hz to perhaps 1 MHz) and have a go at it.

You would need some special-purpose test equipment (such as noise generators, spectrum analyzers, and nano-current meters) to measure some device parameters, but it is probably not worth your time and the money required to purchase or rent, and learn how to use, this equipment unless you are actively employed in the design of circuits that require verification of certain parameters. The casual learner or hobbyist will just have to trust the manufacturer's datasheet when it comes to more esoteric op-amp specifications and choose devices that, at least on paper, appear to be adequate to the task at hand.

I also recommend powering test and prototype circuits with either alkaline dry cells or rechargeable NiMH (nickel metal hydride) cells to avoid power supply noise problems. Lithium polymer or LiPO batteries are excellent for higher current applications but require a special charging protocol. Later you can add a low-noise, linear, regulated bench power supply. Switch-mode power supplies are more efficient, but they can introduce switching noise onto the power supply rails unless their output is carefully filtered.

If you can find appropriate parameters (from datasheets) to plug into an op-amp model-specification, then simulation can help verify your understanding and possibly demonstrate limitations of the device. Unfortunately, errors are easily made in "setting up" simulation parameters, as well as the device parameters, that can lead to misleading and confusing results, especially for dynamic simulations with energy-storing (reactive) components.