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

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?

• Is there some accidental bias on the input signal? Try connecting it via a 1uF capacitor to remove that bias and see if that affects the behaviour. (or at 10Hz, 10 or 100 uF) This is not a solution, just a diagnostic experiment. – Brian Drummond Jul 11 '15 at 12:35
• 1. Or just probe the input using the scope, Check if it has a DC offset. 2. Probe the supply voltages, see if you have +/-5 equally or is there an offset there. 3. What is V_IN? An uncontrolled source? Or a lab style function generator? 4. Try desigining for slightly lower gain. Pergaps 3 or 5. You may be too close to the power rails, even through the op-amp claims to be rail-to-rail. – kabZX Jul 11 '15 at 13:08
• @BrianDrummond, I put capacitor next to the input and the problem seems solved with a compensation resistor with a proper resistance. i.imgur.com/4oOTvOE.jpg – Jeon Jul 11 '15 at 14:35
• @kabZX, V_IN is input voltage from a function generator. – Jeon Jul 11 '15 at 14:36