In this case, introducing factor T oA into the third equation removes it from the first two. The optimal pickoff quantity is the one which minimizes common transmittances in these equations. However, it is not necessary to choose, or even identify, such a pickoff quantity. Let your intuition pick a quantity which seems most appropriate. Suppose you choose x f to be a voltage.
Voltages occur at nodes.
For a current, a pickoff loop must exist in which this current flows. A loop of G generates this current, and it flows through circuitry comprising the input of H . Now let's apply the same kind of reasoning to the summing circuit. If x E is chosen too close to the output, common factors occur in the two terms of x E . Let x E be the input to G B .