Hello. Could you explain to me what's the difference of connecting the non inverting input of an opamp : a) to a resistor and then to the ground b)immediately to the ground

At the following example(Qucs) , there is not any difference to the output.

 

Sure can. Temperature compensation. Why> Ib varies with temperature, right? So, the voltage aross R1 will vary with temperature and the voltage across R4 will vary inversely with temperature, so they theoretically cancel with temperature. There will still be an Ib offset, but it will vary less over temperature with R4.

 

The input current of the operating amplifier causes an additional harmful voltage offset at the input. MAX IIB Input bias current 800 nA. This is an additional offset of 800nA*0.5kOhm=400uV. If you put a resistor R4=R1||R2=500Ohm in the second input, you will get the same (approximate) offset. And since the difference between the signals at the input of the operator is increasing, compensation is provided. The effect is more noticeable at higher resistor ratings. Usually 10 kOhm resistors are used and the effect is 10 times greater. All this is well described in textbooks. Read them. You should know the basics!
This harmful effect does not affect the input amplitude! Influence (signal offset at the output) can be noticed only at small input signals.

 

Thank you very much for your answers!!