I do not know how to compensate the DC-offset caused by the Input bias current.
simulate this circuit – Schematic created using CircuitLab
How to compensate the input current bias DC offset? I know the "trivial" method by adding the additional voltage (regulated by the potentiometer) to the OP-AMP input. But when I switch the SW1 the offset will change and it will require the additional calibration. Is there any "automatic" method to archive it?
Best Answer
Add series resistors to the input divider so the resistance looks the same looking into each switch position, then match that to R6||R8 (increasing the latter if necessary).
For example, in the top switch position (assuming DC coupling) add a resistor Rx1 such that Rx1+ R1||(R3+R4+R5) = R6||R8. And so on for the other two positions.
simulate this circuit – Schematic created using CircuitLab
Although this answers your question, it's probably not what you want to do unless this is a pointless homework exercise. I say this because normally amplifiers with relatively high bias current are used for either high speed or low noise and your high impedances mean that will likely not work out well- because of input current noise for low noise or parasitics in the case of high speed.