help in designing voltage comparator

[mmnr00]

ye guys,

I'm building a comparator with hysteresis using an opamp, and I follow whats in the datasheets (LM324). Below is the circuit diagram of my design, and the output signal wave. I need to build a comparator which output voltage is +2V and -2V, and the hysteresis value Vt=1V. From the datasheets, i found that this formula is used to calculate the value of the resistors.

Vt=(R1/(R2 + R1))* (Voh - Vol), Voh is 2V and Vol is -2V. From the calculation, I got R2/R1=3. I choose R2=300Ω and R1=100Ω.

The output voltage is not like what I was expected. The output voltage is amplified by the value of 3. This means that the circuit is not working as it should as I got an amplifier rather that a comparator. I try to figure it out for hours but still got no solution. My input voltage is set 5V, and my Vref is =0.

Hope you guys can help me. I'm totally lost here. Thanks in advance for any help!

 

[abbeyromy]

So basically u building a schmitt triger...

Hmm I will try to simulate your circuit and see what is going wrong. Will take some time as I dont have my the software on this PC. Your calculation R2/R1=3 seems ok but make sure your op-amp is working in saturation mode.

Since your Vsat is +/-2V check you are connecting both supplies. You cannot work below 3v in single supply i guess. see datasheet

You are using Sin source as input or square wave?
Paste the schematics again if you have modified something..

 

[abbeyromy]

hey i tried the circuit with saw toothed wave and ua 741. Then with lm324.. Find the document attached.

Also refer

The +vsat is not acheived .. that i have to look into why.. do some study n share the analysis...

 

[JoannesPaulus]

check http://www.ecircuitcenter.com/Circuits/op_comp/op_comp.htm. This is one way you can make the schmitt trigger.