TL 084 can work as a comparator??

[eng_ahmed22]

hi all
i tried to make a comparator using TL 084, but failed to make it operat.
it out strange results, for example when it was suppose to out 0 v. it out 1.24 v, i dont know why??!!!
if sombody used this chip be4 as a comparator plz send me how it works.
also if sombody use another good chip as a comparator plz sent its number and schematic if possible
thanks everybody

ahmed

 

[IanP]

Some opamps don't behave well while working of single supply close to 0V output level ..
If you desperately want to use an opamp as a comparator try CA3130 or CA3140 (or their equivalents) .. The output voltage swing in these opamps is rail-to-rail ..
On the other hand if you need a comparator use one ..
Typical comparators are: LM311 (open collector output), LM319 (TTL output) or LM393 (TTL/CMOS) ..
Regards,
IanP

 

[Kral]

eng_ahmed22,
Any op-amp can be used as a comparator. However, you must be careful about oscillations and noise. When the input is equal to (or very close to) the referenece, any noise that is present on either the input or reference will show up on the output in highly amplified form. The output will probably not be symmetrical, so there will be a DC component. This is probably what you are measuring. The best way to avoid this situation is to provide some hysteresis (positive feedback) to keep the amplifier out of its linear range. This precaution also applies when you are using a "real" comparator.
~
Most comparator ICs have an open drain or open collector configuration. This is convenient for matching the input voltage requirements of the logic device that the comparator will drive. You just provide an external pullup resistor to whatever voltage you need for the "1" state.
Regards,
Kral