Instability in Two-Port Active Networks

The general bilateral, two-port active network, defined by its , or parameters, is treated for arbitrary terminations and external feedback with and without transformers. Stability considerations necessary for amplifier design are first discussed. These considerations are then extended to the design of instability, and the concept of maximum potential instability with external feedback is introduced .This is shown to be related to Mason\´s invariant function. Simple small-signal oscillation conditions are obtained analytically in terms of the general active element parameters, from which practical oscillator circuits are easily designed by the substitution of transistor or vacuum-tube parameters. Circuits using feedback with and without transformers are considered. The use of these results is indicated for possible studies of large-signal oscillation, and for the stabilization of oscillation frequency and amplitude.