For systems with a negative feedback loop we can have the situation
shown in Figure 2.24 where the output is fed back via a system with a
transfer function H(s) to subtract from the input to the system G(s). The
feedback system has an input of Y(s) and thus an output of H(s)Y(s).
Thus the feedback signal is H(s)Y(s). The error is the difference between
the system input signal X(s) and the feedback signal and is thus: