# Need help designing a schmitt trigger using op-amp

Discussion in 'Homework Help' started by kaustubhg123, Jun 5, 2012.

1. ### kaustubhg123 Thread Starter New Member

Jun 5, 2012
3
0
Hello,
I want to design a schmitt trigger using op-amp given that Supply voltage is -15v and +15v, Upper trigger threshold must be +8v and Lower trigger threshold must be -6v.
I just need to calculate values for R1||R2 and Vref (Reference Voltage)
If possible, please post how to calculate all this step by step.
Thanks.

2. ### WBahn Moderator

Mar 31, 2012
18,093
4,920
There are many different ways to configure an opamp circuit to be a Schmitt triggered comparator. Don't you think it might be helpful if we were looking at the same circuit that you are?

What have you done on it so far?

3. ### kaustubhg123 Thread Starter New Member

Jun 5, 2012
3
0
sorry for incomplete description.
I have attached a diagram.
According to book I am using:

Upper trigger voltage Vut = ((Vref * R1)/(R1+R2))+((R2 * Vsat)/(R1+R2))

Lower trigger voltage Vlt = ((Vref * R1)/(R1+R2))-((R2 * Vsat)/(R1+R2))

As I am completely doing self study (no classes) I don't know where to start. Please let me know step by step.
Thanks.

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4. ### WBahn Moderator

Mar 31, 2012
18,093
4,920
Q1) Can you describe what "Vsat" is and do you have a specific value for it for this problem, or is it just a parameter that is to be carried along?

Q2) Can you derive the above equations for Vut and Vlt by analyzing the circuit?

Q3) What is the effective resistance of R such that R = R1||R2?

Q4) Given the answer to Q3, can you rewrite the equations for Vut and Vlt in terms of R?

Q5) Does it really matter what R1||R2 is? For instance, let's say that R1 and R2 are equal. Does it matter if they are both 1kΩ or 10kΩ?

Come back with answers (or attempts) at those, and then well take it from there.

5. ### kaustubhg123 Thread Starter New Member

Jun 5, 2012
3
0
1) Vsat is 15 as seen in circuit. The output will switch either to +Vsat or -Vsat when crossed VUT or VLT.

2)The equations are derived for calculating VUT and VLT.

3)Resistance is not give, I need to calculate that.

4)That is what I really don't know.

We need to make a combination of Vref, R1 and R2 so that VUT = 8v and VLT = -6v.
R3 will be equal to R1||R2 and used only for compensation of Input bias current and can be ignored.

6. ### WBahn Moderator

Mar 31, 2012
18,093
4,920
Okay, so we are assuming rail-to-rail opamps. That's fine.

That wasn't the question. The question was, given the circuit shown in the schematic, can YOU derive the equations for Vut and Vlt?

The answer is significant, because if you can't, then you really need to take a step back and strengthen your basic opamp circuit analysis before proceeding. If you don't, then you will only be pushing through increasingly difficult material that you will increasingly lack the skill set to understand. You will become entirely reliant on memorized formulas and recipes that you will invariably misuse because you don't understand them well enough know when and where to apply them. This is a prescription for disaster (possibly literally) at some point. The sooner you identify that you are getting into such a hole and the sooner you back up and dig out, the smoother and easier it will be.

Again, not what was asked. Given two resistances R1 and R2 (use the symbols), what is the expression for the equilavent resistance, R, if they are in parallel. This is not a trick question and, by itself, it has nothing to do with this problem. We are just trying to establish the basic concepts and relations that will be useful in solving the problem.

Fine. We'll put this one on hold until we get Q3 taken care of.

Agreed.

Don't ignore Q5. It is critical to understanding and solving the problem since your original question asked about finding two things, the value of R1||R2 and the value of Vref, that would set the thresholds at particular points. We need to be sure that finding those two values will, in fact, solve the problem. So let's break it down a bit and go step by step to gain the necessary understanding.

Q6) Let's set Vref to 0V and choose R1=10kΩ and R2=10kΩ.
Q6a) What will Vut be?
Q6b) What will Vlt be?
Q6c) What will R1||R2 be?

Q7) Now let's set Vref to 0V and choose R1=20kΩ and R2=20kΩ.
Q7a) What will Vut be?
Q7b) What will Vlt be?
Q7c) What will R1||R2 be?

Q8) Now let's set Vref to 0V and choose R1=30kΩ and R2=6kΩ.
Q8a) What will Vut be?
Q8b) What will Vlt be?
Q8c) What will R1||R2 be?

Q9) Now let's set Vref to 0V and choose R1=6kΩ and R2=30kΩ.
Q9a) What will Vut be?
Q9b) What will Vlt be?
Q9c) What will R1||R2 be?

Q10) Based on the answers to Q6 through Q9, does knowing what R1||R2 is tell you anything about the threshold values (or vice versa)?

7. ### sachinkotagond New Member

Jun 8, 2012
1
0

Dec 13, 2012
1
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Q6a) What will Vut be?
+15
Q6b) What will Vlt be?
-15
Q6c) What will R1||R2 be?
5k

Q7a) What will Vut be?
+15
Q7b) What will Vlt be?
-15
Q7c) What will R1||R2 be?
10k

Q8a) What will Vut be?
+25
Q8b) What will Vlt be?
-25
Q8c) What will R1||R2 be?
5k

Q9a) What will Vut be?
+1
Q9b) What will Vlt be?
-1
Q9c) What will R1||R2 be?
5K