Figure 4 Nyquist plots of open-loop transfer functions of the parallel-operated FET amplifier after the novel optimization procedure plications during the synthesis procedure with CAD tools is proposed. The method is based on evaluation of a set of stability factors and on verification of the Rollett proviso concerning the characteristic frequencies of the overall circuit. To the best of our knowledge, for the first time, expressions equivalent to the Rollett proviso have been determined to guide CAD optimizers to the design of a stable circuit. A case study of parallel-operated FET amplifier design is presented in which unconditional stability has been achieved by using the proposed method. REFERENCES 1. J.M. Rollet, Stability and power-gain invariants of linear twoports, Ž . IRE Trans Circuit Theory Mar. 1962 , 29᎐32. 2. R.W. Anderson, S parameter techniques for faster and more accurate network design, Hewlett Packard Application Note 95, p. 132. 3. K. Kurokawa, An introduction to the theory of microwave circuits, Academic Press, New York, 1969 chap. 7, p. 308. 4. G.E. Bodway, Two-port power flow analysis using generalized Ž . scattering parameters, Microwave J 10 1967 , 61᎐69. 5. D. Woods, Reappraisal of the unconditional stability criteria for active 2-port networks in terms of S parameters, IEEE Trans Ž .