Adjusting output bias of op amp is different with what I calculated and expected

Question

Yesterday, I've posted attenuated and biased output from op-amp

And attenuation has been solved and bias remains.

After googling and with answers in stackexchange, I've built and improved the circuit:

^{simulate this circuit – Schematic created using CircuitLab}

Well, the output is somewhat better than what I have got yesterday (Yellow - input, Blue - output):

But somewhat seems wrong and different with calculations:

1. Still, output is negatively bias a little bit while input is almost unbiased.
2. A resistance of compensation resistor R_COMP is different from calculations
   • In allaboutcircuits and youtube video from 10:14, a resistance of R_COMP is calculated as R_G||R_F.
   • According to them, R_COMP should be less than 47 kOhm. But, when I've put a resistor with a such resistance value, output is negatively biased much more.
   • Sorry, forgot to take a picture of it. Picutre down below is taken with a different circuit configuration, but it is a similar one with what I've seen.

Is there something that I am missing? In the youtube video from 23:14, he reduces resistances of R_G and R_F by a tenth while keeping their ratio the same and also reduces R_COMP by a tenth. Then output bias has eliminated significatly. Is it somewhat related with my situation?

Answer 1

You are confusing a few things (as does your youtube). First, you need to distinguish between bias currents and offset current. The offset current is simply the difference between the two bias currents. If the offset current is significantly lower than the bias current, it says that the two bias currents are pretty much the same. If you look at the youtube, however, you'll see that the max bias current is listed as 100 pA, while the max offset current is 200 pA. In other words, there is no way to compensate since there is no way to tell exactly how the two bias currents will compare (the youtube guy got lucky).

In your case, though, you have an advantage. The independent bias currents which make offset current compensation difficult is characteristic of MOSFET op amps, while bipolar op amps do have correlated bias currents - and you are using a bipolar op amp. Please check the data sheet under Electrical Characteristics. For the TL972 the currents do correlate pretty well, so you can add a compensation resistor and expect some improvement. If you are going to do this, though, you need to calculate your parallel resistance correctly - in this case it is about 42k, not 140k. While you have done this and don't like the results you got, that does not change the fact that that's about as good as you're going to get. All this says is that the problem lies somewhere else, and fiddling with the compensation resistor will not help you understand it.

Once you've done that, though, it seems clear that you have missed the larger point of the youtube - offset voltage. Watch it again, and you will see him calculating the effect of Vos (input offset voltage) as Vos times the gain of the circuit. In your case, your gain is 10. And again, if you will read the data sheet, you will see that the max offset voltage is 6 mV. Multiply by 10 and you get a 60 mV output offset.

There is another possible error source which you need to check. Your scope offset controls may be contributing to the problem. Ground both inputs, and check to see what the scope is telling you the voltages are. Adjust the offsets to get exactly zero volts on both inputs, then recheck your measurements.