Why doesn't ua741cn operate correctly with a single supply?

Question

I'm learning OpAmp and i wanted to amplify .5v dc voltage to 5v dc output so i decided to use opAmp in the non-inverting configuration with a gain of 10. So I connected it as shown :

I set the potentiometer output voltage at .5v but the OpAmp's output was at the saturation level (about 11.5v). So i thought it might be the single supply i'm using and I used another OpAmp to split the 12v source into +6,-6 and ground and it worked well.

My question is why does it need dual supply to work proberly even i don't need any negative output voltages at the output?

My knowledge is that i can't have negative swings at the output if it's connected to a single supply (and that's fair) and this is my case, then why do I need a dual supply?

Answer 1

It doesn't need a dual supply. It just needs to operate in a space that has some distance between the negative and positive rails, and for most common power supply schemes, that space is pretty small if the negative rail is at ground, and the arrangement isn't very useful.

As to "why" -- it's the configuration of the transistors in the device. I'm not sure you're interested in the gory details, and I haven't checked them, but often this has a lot to do with bias voltages used to keep transistor amplifiers in range for linear operation. When the chip was designed, just having access to a device as facile as an op amp was a pretty good thing. These days, we want less batteries with more life, less power, smaller voltages, and an op amp designed so long ago just doesn't fill the niche anymore.

If you look at the datasheet at http://www.ti.com/general/docs/lit/getliterature.tsp?genericPartNumber=UA741&fileType=pdf, you'll see in section 7.3, the \$V_{ICR}\$ line, that the common mode input needs to be about 3 volts above the negative rail and 3 volts below the positive rail. The \$V_{OM}\$ line shows a similar range for the output. Note that this means the chip is entirely unsuitable for power rails at 0V and 5V, but there is a bit of room if you're operating around 5V for rails at ground and 10V.

So, the "why" isn't there, but the "will my op amp work the way I want it to" is right there for anyone who knows how to find and read the instructions.

Answer 2

You have run across a problem which, to be fair, is not obviously mentioned in the data sheet, or at least not as you were thinking about it. That is the problem of input common mode voltage. First, keep in mind that the 741 was very much oriented toward the then-standard +/- 15 volt supplies which dominated at the time. Granted, the data sheet suggests that a 741 can operate down to +/- 5 volts, and your experiment with +/- 6 confirms that.

With that in mind, notice that the data sheet does not actually provide numbers for any power supply voltage other than +/- 15. Now look about halfway down and find "input voltage range", which will show a minimum of +/- 12. What this means is that, if your input voltage is less than 3 volts from either supply (for +/- 15), the IC is simply not guaranteed to work. Notice that this does not guarantee that the IC won't work, only that if it doesn't then it's not a concern of the manufacturer and you have no complaint. Is this 3 volt limit appropriate for a 12 volt or +/- 6 volt supply? Dunno. The data sheet doesn't say. You can play around with your +/- 6 volt setup and find out, but keep in mind that, just because an input closer than 3 volts works with the specific chip you have, there's no guarantee that it will work with another amp, particularly one from another manufacturer.

So, when your original setup provided a 0.5 volt input, that input was grossly out of bounds, and the result was as expected. When you went to +/- 6, the input was indeed within the proper range, and the circuit worked. You might want to play around and find out (just for your own amusement) just where the 741 stops working for different supply voltages. Just don't expect that another amp will behave exactly the same.