# op amps voltage calculation

Discussion in 'Homework Help' started by dieggo3, Jul 6, 2016.

1. ### dieggo3 Thread Starter New Member

Jul 6, 2016
8
0
Hello, please I need a help to solve op amp circuit. Thankyou in advance.

Calculate the threshold voltage for the circuit. Assume the maximum output voltage swing of the op-amp is ± 3 V for when the supply is ± 5 V

2. ### AlbertHall AAC Fanatic!

Jun 4, 2014
6,680
1,576
I think you attached the wrong picture??

3. ### dieggo3 Thread Starter New Member

Jul 6, 2016
8
0
Sorry, thats a correct circuit.

Mod Note:
You used the wrong file type, please convert the file to the *.gif before you attach the file.

4. ### AlbertHall AAC Fanatic!

Jun 4, 2014
6,680
1,576
So you know the two values of the output voltage. What voltages does that result in on the non-inverting input?

5. ### DGElder Member

Apr 3, 2016
351
87
For the threshold voltage of post #3, you want to construct a formula to describe the feedback voltage versus the differential input voltage. Then solve for the case where the inverting and non-inverting voltages are equal, i.e. the differential voltage = zero. Vi=Vf

Last edited: Jul 6, 2016
dieggo3 likes this.
6. ### lee6282 New Member

Jun 22, 2016
3
0
With the 3V power supply biasing the Voltage divider is that giving us a upper threshold of 3V and lower of 0v seeing output swing of Amp is +- 3v?

7. ### DGElder Member

Apr 3, 2016
351
87
Yes. My formula produces the astable point halfway in between - not too relevant.

lee6282 likes this.
8. ### lee6282 New Member

Jun 22, 2016
3
0
DGElder,
So to confirm does that answer the Ops original question "Calculate the threshold voltage for the circuit. Assume the maximum output voltage swing of the op-amp is ± 3 V for when the supply is ± 5 V"

i.e. Threshold voltage is 0V and 3V?

9. ### DGElder Member

Apr 3, 2016
351
87
Yes, I think so; it depends on exactly what they meant by threshold. The use of the singular "threshold" in the question is why I suggested finding the singular astable input voltage. But speaking practically, it is going to swing to one rail or the other and resetting it would reguire the inverting input to cross above the upper threshold or below the lower threshold - depending on whether Vo was at +3 or -3V respectively.

Last edited: Jul 10, 2016
lee6282 likes this.
10. ### WBahn Moderator

Mar 31, 2012
23,194
6,989
MOD NOTE: Moved to Homework Help from Electronics Resources.

11. ### WBahn Moderator

Mar 31, 2012
23,194
6,989
The circuit is a comparator with hysteresis, so it has two thresholds. Let's give the TS the opportunity to take this information and see if they can figure out what the two thresholds are on their own and, if they can't, let them show their work and then we can see if we can give them a nudge in the right direction.

12. ### lee6282 New Member

Jun 22, 2016
3
0
WBhan thanks for the help.

My course notes show that Vthresh= Vsupply (R2 / R1+R2)
which would equate to V thresh = 3 x 0.5 which would give 1.5V.

However they have thrown a additional 3 V power supply into the original question on the bottom of the Voltage Divider (which is not covered in any of the notes).

So with my calculations When the output of the amp is +3v we will get 0V on V+ of the Amp
and when Output is -3V we will drop 3v across each resistor (both 1K) giving us 3V on V+ to the Amp

Therefore a upper and lower threshold of 3V and 0v

I have run the circuit through Pspice in Simetrix and come up with a graph showing the thresholds at 3v and 0V

Just really after a bit of confirmation I am on the right track....otherwise I am slowly losing the plot and have somehow managed to get Spice simulation to conveniently lie to me (being the first time i have used spice)

13. ### dieggo3 Thread Starter New Member

Jul 6, 2016
8
0
I have done some workout, and I came with answers:
Upper threshold voltage is 4 V
Lower threshold voltage -1 V
Is it close to the right answers ?
Thanks

14. ### WBahn Moderator

Mar 31, 2012
23,194
6,989
Please keep in mind that this is dieggo3's thread and that we ask that people not just solve other people's homework.

Leaving that aside,

(R2 / R1+R2) is the same as ((R2/R1) + R2), which is wrong. You need to pay closer attention to order of operations. What you meant was (R2 / (R1 + R2)).

The reason why your course notes yield the wrong result is that the course notes are applicable to when you have negative feedback. In this circuit you have positive feedback.

You analysis after that is correct.

I don't know what "slowly losing the plot" means.

15. ### WBahn Moderator

Mar 31, 2012
23,194
6,989
They are somewhat close, but you need to show some work for us to be able to figure out where you are going wrong.

16. ### dieggo3 Thread Starter New Member

Jul 6, 2016
8
0
Vsat=5. V4=3
UTP=((Vsat-V4)xR2/R1+R2)+V4=((5-3)x1/1+1)+3=4

UTP=((-Vsat-V4)xR2/R1+R2)+V4=((-5-3)x1/2)+3=-1

17. ### WBahn Moderator

Mar 31, 2012
23,194
6,989
How do you get that "Vsat" is 5 V? Read the information you provided in the original post: "Assume the maximum output voltage swing of the op-amp is ± 3 V"

18. ### dieggo3 Thread Starter New Member

Jul 6, 2016
8
0
I hope I'm getting on the right track this time,. so. Vsat=3

Then I get UTP=3
and. LTP=0

19. ### WBahn Moderator

Mar 31, 2012
23,194
6,989
Yep! Do you see how showing your work let us zoom in immediately on what your problem was?

The next thing you need to do is start tracking your units properly.

3 is not a voltage, it is just a number. Just like my height is no more 72 than it is 2. The units are a fundamental part of nearly all physical quantities -- 72 is not equal to 2, but 72 inches is equal to 2 feet.

You should get in the religious habit of tracking units throughout ALL of your computations -- most mistakes we make (not all) will mess up the units allowing us to catch them almost immediately, but only if they are there to be messed up in the first place. You also need to pay more attention to order of operations. Multiplication has higher precedence than addition, so

R2/R1+R2 is really (R2/R1)+R2 and not R2/(R1+R2). If you mean the latter, then the parentheses are NOT optional.

So your work should have looked something more like:

UTP = ( (Vsat - V4) x (R2/(R1+R2) ) + V4
UTP = ( (3 V - 3 V) x (1 kΩ / (1 kΩ + 1 kΩ) ) + 3 V
UTP = 3 V

LTP = ( (-Vsat - V4) x (R2/(R1+R2) ) + V4
LTP = ( (-3 V - 3 V) x (1 kΩ / (1 kΩ + 1 kΩ) ) + 3 V
LTP = 0 V

Notice that for each threshold the first line captures all the EE stuff. Everything else is just math. Then the next line (which would normally be multiple lines) is the mathematical manipulation. Finally, the last line is the answer. If you organize your work along these lines it will be much easier for everyone, including YOU, to walk through your work. But it will also be much easier for the grader to walk through your work -- and that falls under the heading of the proper care and feeding of homework graders.

lee6282 likes this.
20. ### AlbertHall AAC Fanatic!

Jun 4, 2014
6,680
1,576
So that's why my curtains never fit the windows.