Negative differential resistance in two-transistor one-ports with no internal sources

The authors examine the occurrence of negative differential resistance (NDR) in transistor one-ports which contain resistors and only two transistors. Two recent conjectures of L.O. Chua et al. (1984) related to these issues are proved, namely that in such circuits similar types of transistors are required for N-type (voltage-controlled) NDR and complementary types for S-type (current-controlled) NDR behavior. The authors also give necessary topological conditions for achieving biasing of the transistor p-n junctions appropriate for the occurrence of NDR. These then lead to a specification of the minimum number of resistors that must be present in such circuits.