Voltage swing question

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TunerPhish

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I need to calculate the threshold voltage for an op amp.

I am given "The maximum output voltage swing of the op-amp is ± 3 V for when the supply is ± 5 V."

Would this mean that I should calculate the upper threshold voltage as 8V and the lower threshold at 2V?

I have added an image of the Waveform and circuit I am using.



Thanks
 

Hi TunerPhish,

If the datasheet says that on a 10V supply (-5 to +5) the max. output is the supply voltage minus 2V at either end, that that is the case. Perhaps the simulation doesn't account for the Op Amp model you are using (Is it the same type/model as the physical one, or a "close match"?), which is why it looks like it reaches ~4.5V either end. I'd trust the datasheet, so yes, I imagine you are correct about upper and lower threshold being -3V up to +3V, or that you have 6V headroom.
 

The threshold is the voltage at which V- = V+. We know V- = Vin.
Let's calculate V+

V+ = V4*R1/(R1+R2) + Vo*R2/(R1+R2)

Where:

Vo = -Vsat if V- > V+
Vo = +Vsat otherwise

with the values of your circuits

V+ = (3+Vo)/2 that is

V-(low-->high) = (3-3)/2 = 0V
V-(high-->low) = (3+3)/2 = 3V
 

You show a very old 741 opamp. It is designed to be powered from a plus 15V and minus 15V supply. Some do not work with only plus 5V and minus 5V like you have.
The datasheet of a uA741 shows its output has a positive 3V loss and a negative 3V loss when its load is 10k to ground. Its loss with a 2k load is 5V positive and negative.
You are simulating it but the model might not be correct and if it is, it shows a "typical 741 opamp" that you cannot buy. The one you buy might have more loss than a typical one.

Why do you talk about "the threshold voltage"? The datasheet does not say anything about a threshold voltage. A Mosfet has a threshold voltage.
 

Hi

I have a 3 part question to answer. The first part states that I need to calculate the threshold voltage for the circuit I have posted above. I am told to assume the maximum output voltage swing of the op-amp is ± 3 V for when the supply is ± 5 V. My question is what value should I use for the upper threshold and what value should I use for the lower threshold. So if the supply voltage is at +5V would the swing be 3V over that value at 8V and if the voltage is at -5V would the lower value be at -8V. Or would I use the ± 5V value and have my upper limit at 8V and lower limit at 2V. Thanks

- - - Updated - - -

Or neither that I've stated. Thinking about it if the supply voltage is 5V it can't go over that limit. Am I going off in the completely wrong direction and are the values I should be using -3 and +3 to be used in my calculation.
 

Audioguru said:
Why do you talk about "the threshold voltage"? The datasheet does not say anything about a threshold voltage. A Mosfet has a threshold voltage.
Click to expand...

...because - I suppose - the questioner does not know what the circuit (opamp with positive feedback) is doing (see the thread title: voltage "swing" question)
 

The circuit topology is that of a voltage comparator, so I presume the threshold is the voltage at which the output goes from high to low or from low to high. This means when V+ equals V-.
What we know is that the saturation voltages are -3V and +3V, that is the maximum negative and positive swing,
The calculation of the corresponding thresholds were given in my previous post.
 

albbg said:
The circuit topology is that of a voltage comparator, so I presume the threshold is the voltage at which the output goes from high to low or from low to high.
Click to expand...
Why do you suppose it? Of course it is, in comparators with positive feedback we speak about positive threshold and negative threshold or High Threshold and Low Threshold.
 

CataM said:
Why do you suppose it?.
Click to expand...
Yes, of course the meaning of threshold of a comparator is unique.
My "presume" was referred to the original answer. It seemed to me that the question could also be relative to the minimum supply voltage to have operations between +/-5V (also reading other comments), calling this minimum voltage, improperly, threshold.
Sorry if I generated confusion.
 

Of course it is the threshold voltages for a comparator that has positive feedback. But an opamp was shown, not a comparator IC.
A school kid would use ohm's law to calculate the threshold voltages.
 

Audioguru said:
Of course it is the threshold voltages for a comparator that has positive feedback. But an opamp was shown, not a comparator IC.
A school kid would use ohm's law to calculate the threshold voltages.
Click to expand...

Sorry, I didn't understand you answer.
What is shown is an op-amp configured as a comparator. You have to consider the whole schematic.
You have to consider ohm law and superposition as well as the op-amp parameters (ideal except the output voltage swing)
 

Audioguru said:
I think the teacher wants the students to see that a 48 years old opamp makes a poor comparator.
Click to expand...

I think, we shouldn`t discuss here how old an opamp is or if there are "better" opamps (for the described purpose).
More than that, the opamp type was given with the task description - so, why not discussing the properties of the given circuit?
 

To summarize the previous discussion, the first question has been clearly answered by albbg, but apparently TunerPhish didn't yet get the answer. The two other questions are yet unknown.

It seems obvious to me that the exercise problem focusses on the calculation method and basic circuit properties rather than real LM741 parameters. +/- 3V output swing is a datasheet dummy value, not what you really expect with the given asymmetric load. Similarly you would need to consider worst case common range if you analyze a real circuit, in this case the exact behavior is unpredictable. The only meaningful way to deal with the problem is to use the given output swing and assume that +3V is still within common mode range.

Audioguru's comments about LM741 are appropriate for real circuit design, but the exercise problem apparently ignores related questions.
 

The next stage of the problem is to sketch the waveform of Vo. Would this be as simple as sketching a square wave using the values 0v and 3v. Thanks
 

TunerPhish said:
The next stage of the problem is to sketch the waveform of Vo. Would this be as simple as sketching a square wave using the values 0v and 3v. Thanks
Click to expand...
Maybe. First you must calculate the frequency then look on the datasheet for the opamp to see if its frequency response can produce a squarewave at that frequency.
 

Would this be as simple as sketching a square wave using the values 0v and 3v.
Click to expand...
Triangle voltage crossing of 0 and 3 V thresholds causes the output to switch between minimum and maximum level.
 

The input is a triangle wave so would the output generate a square wave with peaks and troughs of 0V and 3V?
 

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