Thevenin equivalent circuit of a circuit is expressed by parameters Uoc (open circuit voltage) and Req (equivalent internal resistance), which is equivalent to a constant voltage generator with internal resistance, so it is also called "generator principle". Under this structure, we use mathematical expressions to express their linear relationship, that is, U=Uoc+I×Req.
Therefore, finding out the VCR relationship of circuit ports is equivalent to finding out the Thevenin equivalent circuit in one step. Of course, without such a solution, it can be solved in two steps: the first step is to find the open circuit voltage of Uoc port; Secondly, because the circuit contains the controlled source, the equivalent resistance must be calculated by U/I method. This second step is actually equivalent to the simplification of "one-step" VCR, which is obtained by disabling the internal power supply (Uoc=0).