Feedback question

I have two circuits working independently, one has a LM339 comparator and the other has an LM1458 Dual op-amp that is used as a comparator. There is a feedback resistor on all outputs of the 339 but not on the LM1458. I know that the feedback adds hysteresis and keeps the output from oscillating, but why aren't they needed on the circuit with the 1458?

This is the circuit with the 339:



And here's the one with the 1458:

**broken link removed**

The bottom circuit drives a flip-flip so there's no need for hysteresis resistors.

It's possible to make a flip-flop from a couple of op-amps or comparators so it would be better to use a quad (LM339 or LM324) and replace the 4001 with a comparator based flip-flop.

Hero999 said:

The bottom circuit drives a flip-flip so there's no need for hysteresis resistors.

It's possible to make a flip-flop from a couple of op-amps or comparators so it would be better to use a quad (LM339 or LM324) and replace the 4001 with a comparator based flip-flop.

Click to expand...

Thanks! Is there another use for feedback resistors other than hysteresis that would explain why they are necessary for the 339 but not for the 1458?

No, it's got nothing to do with whether the LM1458 or LM339 is used, it's purely because the second circuit is driving an RS flip-flip.

Do you know what an RS flip-flop does?

If not, I suggest you research it and you'll probably realise why you don't need a comparator with hysteresis if it's activating a flip-flip.

EDIT:
Here's a link to a circuit I designed for someone, which uses a few comparators to trigger a comparator based flip-flip. Note the LM339 and the lack of feedback resistors.