Equivalent Circuits of Linear Active Four-Terminal Networks

HE art of equivalent network representation has grown very considerably since its inception by Dr. G. A. Campbell. In his paper "Cissoidal Oscillations" which was published in 1911 he proved that any passive network made up of a finite number of invariable elements and having one pair of input terminals and one pair of output terminals is externally equivalent to an unsymmetrical T or II network. From this modest beginning the field of applications of the equivalent circuit concept has steadily expanded so that by now the whole field of linear passive circuit theory has been subjected to equivalent circuit interpretation. With the advent of the thermionic vacuum tube amplifier, linear active network theory had to be considered and almost immediately the attempt was made to obtain an equivalent circuit whose performance would depict the linear characteristic of the tube. The equivalent circuit art has also, in recent years, been used to describe the performance of certain classes of non-linear devices, such as mixers, and further applications in this field will no doubt be made. Equivalent circuit concepts have played an important part in electrical