Application conditions of the closed-loop transfer function formula

Demonstrations have showed that the transfer function formula for closed-loop control system introduced in some books on automatic control has certain application limits. In order to improve the formula and use it correctly, it is required to limit its scope of application. Thus the two necessary conditions of the control system in using this formula have put forward and deduced theoretically based on the well- knows Mason Gain S J theorem. The conditions are that in studied control system there are no mutually independent feedback loops and no forward path independent on any a feedback loop.