Recently as a progress of electric technology, modern electric systems become more and more complicated. Thus, it is difficult to use a classical method to analyze responses of the systems in complicated electromagnetic environments because there are so many coupling phenomena. The electromagnetic topology (EMT) is a helpful solution to analyze electromagnetic interference / electromagnetic compatibility (EMI/EMC) of the large complex system. In this paper, EMT is applied to analyze an active element. And then to prove the validity of this analysis, a detector circuit including Schottky barrier diode is analyzed by EMT when input power is given directly.