Query in the simulation of Schmitt trigger using LM311 comparator

Hi Friends ,

I tried simulating a schmitt trigger circuit using LM311 comparator in Pspice. And i am not getting proper output. I have attached the schematic and the waveform. I have the following queries. I calculated the Vut and Vlt and they are 12.002997 and 11.97302697 respectively.


1) Why is there a delay in the output switching , when the sine input is exceeding Vut or goes below Vlt. The output should switch to low immediately right?
2) Why is the output not going to negative , when the sine input voltage exceeds Vut?It switches to zero but does not turn negative.
3) There is some pull up resistor connected to 3.3 V at output. I dont know why is that connected. But the output has to be 15 V and -15 V right? But because of the pull up resistor i think the maximum output voltage is only 3.3V. How can be that so?
4) If i remove the pull up resistor , even then the output is not 15 V , 12 V.

Thank you friends.

First **broken link removed** Next your circuit seems to have hysterisis? Compare your circuit to those in the link. Also, your negative supply looks at a glance to be backwards for negative voltage on the op amp.

Ron

datta said:

2) Why is the output not going to negative , when the sine input voltage exceeds Vut?It switches to zero but does not turn negative.
3) There is some pull up resistor connected to 3.3 V at output. I dont know why is that connected. But the output has to be 15 V and -15 V right? But because of the pull up resistor i think the maximum output voltage is only 3.3V. How can be that so?
4) If i remove the pull up resistor , even then the output is not 15 V , 12 V.

Click to expand...

The output is not 15 V and -15 V. If you look at the data sheet you will see that the LM311 has an open collector output with the emitter tied to ground. Thus the output will go between the voltage at the output (collector) resistor and ground, just as you observed.

crutschow said:

The output is not 15 V and -15 V. If you look at the data sheet you will see that the LM311 has an open collector output with the emitter tied to ground. Thus the output will go between the voltage at the output (collector) resistor and ground, just as you observed.

Click to expand...

Thanks. i didnt know that there will be a transistor at the output stage of a comparator.

datta said:

Thanks. i didnt know that there will be a transistor at the output stage of a comparator.

Click to expand...

That's what data sheets are for.

And not all comparators have an open collector, transistor output.