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 transfer 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 is the n×n identity matrix. it is shown that the Nyquist plots of the n transfer functions completely characterize the number of right-half complex-frequency-plane zeros of det M_n, and hence the amplifier stability. Insertion of an ideal circulator and isolators at the interface ports enables one to calculate the Nyquist plots and voltage distributions of possible instabilities using commercially available linear circuit simulators. Numerical simulations for two types of parallel-operated GaAs FET amplifiers are performed to verify the usefulness of the analysis in design-phase check of multidevice amplifier stability