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# Opamp help

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#### Integrate

##### New Member
Hello. I need a little help with an opamp problem I'm having. I'm using the opamps as a comparator to see if the opamps are functioning. I'm using an LM358, supplying 12V, and I'm applying 5V positive/ 0V negative voltages to the inputs, and the output goes through a resistor and to an led. By switching the inputs around from 5V to ground in various combinations, I can get the led to be on or off, which is what I'd expect.

The problem is when I connect a high speed opamp such as LM833 in the exact same circuit. The opamp seems to only want to output high even when it should be producing a low. I also tried with other high speed ones such as TL082, and they all do the same thing. Is there something different about these opamps that I'm missing?

Thanks!

You are missing the fact that the LM358 is designed to operate from a single supply and the others are not. If the common-mode input range does not go to the most negative rail, then you can not operate them with a single supply voltage with the input going to zero volts (the negative rail).

Thank you for the quick response. I did a quick google search and I looked at some stuff about rail to rail, but I don't quite understand it.

What do you mean by "common-mode input range"? Do you mean that in order for my opamp to work, I need to supply a lower ground voltage than 0V, or keep the ground at 0 but have my digital lows at something like 1V?

Edit: I just tested the comparator with the lows at 2.5V through a voltage divider and it seems to be working. Thanks!

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The "common-mode input voltage range" of an opamp is the range of input voltages where the input transistors work normally.

The "common-mode input voltage range" of an LM358 is from slightly below 0V in your circuit to 1.5V less that its supply voltage.

The "common-mode input voltage range" of an LM833 is from 3V above 0V to 3V less than the positive supply voltage. Some will work with an input that is 1V above 0V to 1V less than the positive supply voltage.

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