741 opamp...

[Gibson486]

Is there a voltage differential on the 741 opamp? i am trying to make a comparator, but the low never goes below 2V (the negative rail goes to ground) and when the inverting side goes below 2V, it automatically ouputs the positive voltage rail (my "high") no matter what.

 

[bobsmith1492]

The outputs can only swing to within a volt or two from each rail - if you're connected between ground and, say, 12V, you'll only get outputs between ~2 and ~10 volts.

At any rate, it sounds like you may have known that. What is your circuit setup and what are you comparing? "Is there a voltage differential" doesn't really say much - there are several voltage differentials. Between what?

 

[Mark R]

The 741 has relatively limited inputs and outputs.

Outputs only reliably get within about 2-3 V of the rails. So, if powered from -15 and +15 V, the output won't usually get below -13 or about +13 (and may not get below -10/above +10).
Similarly, the inputs only work correctly when more than about 2V from the rails - i.e. you need to keep the inputs between -13 and +13 V. if they go outside this range, then the output may not be correct.

This behavior is acceptable because the 741 is designed to be run from a dual supply - i.e. + and - rails. If you're running from a single supply (e.g. 0 and +20) then you may run into problems. if you're trying to run on less than 20V, then the fact that the 741 doesn't work closer than 2-4 V from the rails, is going to lose a lot of voltage range.

If you need to work from a single supply, then you really should look for a modern replacement of the 741 which is designed to do so. These op amps often have 'rail-to-rail' outputs and 'rail-to-rail' inputs, which are fairly self-explanatory, meaning that the output can go all the way to the supply rails, and the inputs will work correctly all the way to the supply rails.

 

[Special K]

I don't know much about opamp design, so I'll just ask - is the problem the OP is describing something that could be corrected by applying a voltage to the Offset Null pins?

 

[Born2bwire]

Originally posted by: Special K
I don't know much about opamp design, so I'll just ask - is the problem the OP is describing something that could be corrected by applying a voltage to the Offset Null pins?

I think it's been pretty much stated. The OP needs to make sure that he is supplying the opamp's power rails with the range of voltages (plus some wiggle room) that he wants out of the opamp.

 

[PottedMeat]

^yep already been said - A 741 can be used as a comparator, but you're better off using a real comparator ( which are designed to be used open loop ). Like a LM393 or something.

 

[Gibson486]

Originally posted by: bobsmith1492
The outputs can only swing to within a volt or two from each rail - if you're connected between ground and, say, 12V, you'll only get outputs between ~2 and ~10 volts.

At any rate, it sounds like you may have known that. What is your circuit setup and what are you comparing? "Is there a voltage differential" doesn't really say much - there are several voltage differentials. Between what?

OK, thanks!

I just wanted to know what the voltage difference would be between the rails and the output. It look slike it is 2V, atleast for the lower rail. Do they sell op amps with inputs that can below 1 V?

 

[Gibson486]

Originally posted by: Special K
I don't know much about opamp design, so I'll just ask - is the problem the OP is describing something that could be corrected by applying a voltage to the Offset Null pins?

No, the null pins just adjust the midpoint. If you have yoru rails between 12 and -12, your mid point will be 0. You can use the null pins to adjust that.

 

[Born2bwire]

Originally posted by: Gibson486

Originally posted by: bobsmith1492
The outputs can only swing to within a volt or two from each rail - if you're connected between ground and, say, 12V, you'll only get outputs between ~2 and ~10 volts.

At any rate, it sounds like you may have known that. What is your circuit setup and what are you comparing? "Is there a voltage differential" doesn't really say much - there are several voltage differentials. Between what?

OK, thanks!

I just wanted to know what the voltage difference would be between the rails and the output. It look slike it is 2V, atleast for the lower rail. Do they sell op amps with inputs that can below 1 V?

Are you giving the negative power rail a negative voltage or ground?

 

[bobsmith1492]

Originally posted by: Gibson486

OK, thanks!

I just wanted to know what the voltage difference would be between the rails and the output. It look slike it is 2V, atleast for the lower rail. Do they sell op amps with inputs that can below 1 V?

Look for op-amps labeled "rail-to-rail output." They can do just that - swing right to the output. It would be handy if you said what you're using it for... maybe you could get better advice.

 

[herm0016]

http://www.maxim-ic.com/AmpComp.cfm

we are using maxim's in our heart monitor. you can use 1.5 volts bias ( a very low voltage for an opamp and it will go right to the rails.