I understand if you do not have negative feedback in any op amp configuration then the output will be unstable (will oscillate for example) and the feedback resistor and input resistor on the negative input set the gain of the op amp.
What is the purpose of the 'feedback' resistor on the positive input of a differential op amp?
In the below example, there is a differential pair at the input of the op amp and a 2V DC bias being added to the +ve input. The red and blue traces are measured at the +ve and -ve inputs to the op amp and green trace measured at the output. It can be seen that the DC bias has reduced a little from 2V to ~1.98V because VR20(V +ve input)=1k/(10+1k)*2
So it seems having R20 is actually a bad thing? But I am sure I am missing something because in all textbook examples of differential op amps I have seen, there is always R20 on the +ve input. So what is its purpose?