Comparator circuit with +/-5v saturates to +4v and -2v.

WBahn

Joined Mar 31, 2012
29,979
I am trying to use lm741 IC as a comparator.
+/- 10V is my supply to the rails. It's output saturation voltages are 9 and -7 Instead of 9 and -9. How do I fix this problem?

I know the lm741 is not a great IC but I want to use it.
Then you will have to live within its limitations.

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Notice that with rails from ±15 V, even with a 10 kΩ load the 741 may only be able to swing within 3 V of either rail. With a heavier load, it may not get any closer than 5 V. So your opamp is operating well within spec.

You need to learn to read data sheets. It's not the easiest thing to do. After decades as a working engineer, most data sheets still have information that I'm not sure exactly what it means (because it relates to applications or issues that have just never been important in what I do). As a student or new engineer, that will be extra true. How to tackle it is that, when you have a problem in which a part doesn't seem to be acting like you think it should, do a deep dive into the data sheet and ask yourself which of those specs might possible have any bearing on your situation. Don't expect your answer to always be explicitly spelled out, either. You often have to infer things from the information that is there. That is the case here -- the datasheet doesn't explicitly say how close to the rails it can get (some datasheets do), instead you have to look for specs that yield that information indirectly, in this case for the "normal" opamp supply voltages.

As for how to fix the problem, the proper solution is to use a part that meets your needs. In this case, that would be a comparator (not an opamp used as a comparator) that has the desired output range.

But if you absolutely insist on using the 741, then you should look at adding a couple of transistors to the output, along with a bit of biasing circuitry to make them do what you want.
 

Ian0

Joined Aug 7, 2020
9,671
I am trying to use lm741 IC as a comparator.
Don't.
Use a comparator (such as LM339 or LM393) as a comparator, don't use an op-amp as a comparator. It's a bad habit to get into, as you will come unstuck one day, as only some op-amps can be used as comparators. Quite a lot don't work as comparators.
 
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