Can someone tell me why, when I connect a thermocouple to an op-amp, the comparator circuit I obtain can not work? I know that this is due to bias currents but I don't understand what really happens and what is the difference between this circuit and the circuit obtained from the previous one by grounding one of the input wires.
I found this at the following link: http://www.allaboutcircuits.com/vol_3/chpt_8/13.html (section "Bias currents")
Can somebody help me please?

 

In the article that you referenced, one of the lines refers to this exact question.

The problem is this: the wire loop formed by the thermocouple does not provide a path for both input bias currents, because both bias currents are trying to go the same way (either into the op-amp or out of it).

When you add a connection to ground to the circuit, the simple loop created by the thermocouple is given another connection to either source or sink current.

Let's back up a little here and go over it in a little more detail.

The article states that the bias currents of the op amp will go in the same direction for both the plus and minus terminals of the amplifier. That is, the bias currents will both go either in to the amplifier or out. The thermocouple, acting as a wire, would require that if one bias current in going out of one of the terminals, then the bias current of the other terminal must be in the other direction.

In the diagram above, where is the current coming from?


Adding a ground connection into the circuit frees up the input bias currents on the plus and minus terminals of the op amp to flow the way they naturally want. Currents throughout the thermocouple can also always flow in the same direction as well.

In this diagram, the current is being sourced from the ground. Notice as well that the supplies that feed the op amp are +V and -V.