STABILITY ANALYSIS AND NUMERICAL-SIMULATION OF MULTIDEVICE AMPLIFIERS

Stability analysis of multidevice amplifiers is made on a generalized circuit comprising two n-ports with S-matrices S (active devices) and S' (passive networks) connected at n interface ports. Open-loop transf er functions defined for a signal-flow graph and its (n - 1) subgraphs of incident and reflected waves at the interface ports are expressed in terms of det M(n) and its minors, where M(n) = S'S - I(n) and I(n) = n x n identity matrix. It is shown that the Nyquist plots of the n t ransfer functions completely characterize the number of right-half com plex-frequency-plane zeros of det M(n), and hence the amplifier stabil ity. Insertion of an ideal circulator and isolators at the interface p orts enables one to calculate the Nyquist plots and voltage distributi ons of possible instabilities using commercially available linear circ uit simulators. Numerical simulations for two types of parallel-operat ed GaAs FET amplifiers are performed to verify the usefulness of the a nalysis to design-phase check on multidevice amplifier stability.