# Non-inverting op-amp output problem

I have built very basic non-inverting op amp circuit and for some reason it outputs just 4V DC line instead of amplified sine wave. It happens only on breadboard, simulation works ok. I'm pretty sure everything is connected in a right way.
I tried to replace the chip, but its the same for every chip. What's wrong?  • How did you arrive at the divider values at the non-inv. input? Because your amplifier has a gain of "2", the voltage divider should bias the opamp at app. (4.5/2)=2.25V. This gives an output quiescent value of app. 4.5 V.
– LvW
Feb 8, 2015 at 10:09
• My input voltage is going to be 2Vpk at maximum, so I decided to use such a divider so it will not be amplified to more than supply rail, i.e. max 4V value will be amplified only to 9V. Just trying not go into clipping. For the test I use 0.5Vpk sine wave, so I expect the sine wave amplified at the output. It works in simulation by the way, that's why I'm asking, nothing seems to be wrong :( Feb 8, 2015 at 10:19
• When a peak voltage of 2V is amplified with a factor of "2" we get 4Vpeak max. at the output. If the DC quiescent voltage is 4.5 V the output amplitude will be clipped . Hence, you need either dual supply of +- 9V or a larger single supply voltage of at lest 12V because the 741 is not a rail-to-rail amp.
– LvW
Feb 8, 2015 at 10:35
• the problem is the voltage is not clipped - it is not present at all. No matter what input voltage I use, the output is always straight 4V DC line. Feb 8, 2015 at 11:09
• You could check with a DMM what's the voltage between V+ (non-inverting input) and ground. It should be about 1,37V according to your voltage divider, but maybe the 741 is not allowing this input pin to be that close to the negative power rail (0V). Feb 8, 2015 at 11:18

V+ (non-inverting)=1.35 V and V-(inverting)=2 V, and therefore V- > V+. In this case an OPAMP would normally reduce Vout until V- = V+ or until Vout cannot be reduced anymore. It appears then that 741 cannot go lower than 4 V when negative power supply is 0 V.

You could see the reason looking at the datasheet. For example, http://www.ti.com/lit/ds/symlink/lm741.pdf

If you look at "Output voltage Swing parameter": You can see that when Power supply is $\pm 15 V$, with a load of $2 k\Omega$ (as the one you have in your circuit), output can only go to $\pm 10 V$ (minimum specifications) or $\pm 13 V$ (typical specs) . That's it, the output is only guaranteed to swing up to 5 V below the positive power rail and down to 5 V above the negative power. And this is what you see, your particular OPAMP cannot go lower than 4 V above negative power supply.

Yon can also see from datasheet that the voltage output swing depends at the load connected to the output. If you replace R1, R2 by a couple of 10 k resistors (or bigger), you should see an improvement in output voltage swing.

There is a second parameter than one should pay attention: "input voltage range": This data assumes a power supply $\pm 15 V$ You can see that the inputs (inverting and noninverting) can not approach power rails by more than 2V typically, for the device to operate. For an 9V unipolar power supply as yours this means that inputs must be between 2V and 7V.

I believe that this is the limiter factor in your circuit. V+ is at 1.35V, but this operation amplifier is not guaranteed to work for inputs below 2V. It indeed "stops" when V- is 2V.

You might increase the R4 (18k) resistor until V+ is about 2.5V. And see if it now produces an output.

• eh... replacing R1 and R2 for 22k resistors doesn't make any difference unfortunately :( Well, I'll try to research myself more about it later and post my results in separate answer, but thank you very much for your time anyway :) Feb 8, 2015 at 12:24
• You're welcome! Actually the voltage output swing in the datasheet is guaranteed for a bipolar $\pm 15 V$ power supply (equivalent to 30 V unipolar power supply). But you have a 9 V unipolar power supply, so the specs are probably worst. Feb 8, 2015 at 12:33
• yeah, I've read that 741 is kind of an op-amp that has a lot of troubles and very hard to make it work desirably. I'll read more about it, but I think it would be better to use other op-amp in some device. Feb 8, 2015 at 12:39
• 741 is quite old and cannot work close to the "edges" (power rails). So if your design needs an unipolar 9V power supply, you either choose an other OPAMP, or you create a -9V power supply from the 9V with a charge pump. Feb 8, 2015 at 12:45
• @ScienceSamovar, I've realized that there is still an other limiting factor and I've updated the answer. Feb 8, 2015 at 14:20