it is always fruitful to write nodal equations at the inverting input nodes of the op amps, as we have done in this example. However, one generally avoids writing nodal equations at output nodes of op amps because it is difficult to find the current out of an op amp. There is no current into the input terminals, but because there are other terminals not shown, the output terminal carries a current. Also, as noted in Chapter 3. when op amps are present the ground node is not arbitrary, and because of the unshown terminals there may be currents into the ground. Thus we should also avoid writing a node equation at the ground node.