For this op amp configuration:

http://www.flickr.com/photos/35364846@N05/4919292186/

Why the 100K ohm resistor at the positive terminal?

Is it because due to unideal specs of the bipolar transistors that given a differential input there is still current flowing towards the non-inverting input base? but why 100k and not 500k or 50k?

http://www.flickr.com/photos/35364846@N05/4918694615/

 

The resistor is used in some configurations to null the input bias current. The bias current flows through the resistor compensating for the bias current of the inverting input (which is usually equal.)

 

The inverting input has R2 supplying the bias current. The non-inverting input needs to have the same value resistor (R4) supplying the same bias current for its input.

If you used a Fet-input opamp (like the TL071) then R4 can be a piece of wire.