# Homework Help: Threshold Voltage Question

1. Jul 4, 2016

### Enochfoul

• Poster has been reminded to post schoolwork in the Homework Help forums
Hi,

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

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

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2. Jul 4, 2016

### LvW

At first - using the term "threshold voltage" you should know the definition of that term.
Otherwise you cannot find the values.
So - what is the definition?

3. Jul 4, 2016

### Enochfoul

A threshold is an amount, level, or limit on a scale. So in this case a threshold voltage should be a voltage limit within the circuit.

4. Jul 4, 2016

### LvW

Enochfoul - having a look onto your circuit, I see an opamp with positive feedback.
Question: Do you know what such a circuit does? Is it an amplifier or something else?
You must know and understand the function of the circuit - without that you will not be able to know the meaning of "threshold voltage" for THIS kind of circuits.

5. Jul 4, 2016

### Enochfoul

This is a comparator which is a decision making circuit that makes use of an operational amplifiers very high gain in its open-loop state. It compares one analogue voltage level with another analogue voltage level.

6. Jul 4, 2016

### jim hardy

Hmmmm

Be more literal.
Where'd you get 8 ?
IF the opamp has only 5 volts available to it, how can it make its output go beyond 5 ?
Most opamps need some "headroom" between supply and output. There exist special ones that do not, they are called "rail to rail " . 741 has been around since 1967 long before rail-to-rail came about.
So its output can only approach ±5, and they're telling you how close it can come.
Taking them at their word they say your 741 needs 2 volts of headroom, look again at your quote above.

That's a natural mistake to make when just starting out.

Are you sure you have that right ?

Last edited: Jul 5, 2016
7. Jul 5, 2016

### LvW

Yes - that is correct.
Now - based on this description - I think the meaning of the term "threshold" is clear. When the input voltage reaches a certain value something happens. More than that, because of the positive feedback (a kind of memory) it is important if the input voltage is approachng this "certain" value from higher or lower voltage levels.
Further comment: I think, it is not necessary to further discuss opamp 741 properties and other rail-to-rail questions. The task requires to consider +- 3V as output limits. That`s all.

8. Jul 5, 2016

### jim hardy

beginners need to comprehend the concept of 'headroom", that's all. Else the limits makes no sense and will not be accepted on faith.

That he mentioned 8 volts indicated confusion. I remember when opamps were not yet intuitive to me.

But i agree, needn't go further into them than that.

old jim

Last edited: Jul 5, 2016
9. Jul 5, 2016

### Averagesupernova

It seems hard to believe that there was a time when it was not intuitive to me. Although there was such a time.

10. Jul 5, 2016

### Merlin3189

I wonder if it would help Enochfoul to think what the output is for the given triangular wave input? Then to relate the two graphs.

11. Jul 5, 2016

### Staff: Mentor

I think you mean half of these values?

It's sufficient to have a basic idea of what you'll be needing the comparator to do, so don't worry about the exact values of the threshold levels at this stage.

What you need first is to determine some design equations for the circuit: you need equations relating each threshold voltage to circuit parameters such as R1, R2, V4 and the OP-AMP's ± output (max). Have you studied this type of squaring circuit in your course?