Rollet factor and pole-zero identification are analysis methods used by circuit designers to prevent undesired oscillations in their microwave amplifiers. In some cases there is a bit of confusion on why or when use one method or the other. Rollet criterion (well-known as K Factor) is a useful and widely established stability criterion that tell us whether a linear two-port, which was intrinsically stable, will exhibit at its ports negative resistance for any value of passive source or load impedance. Pole-zero identification is an indirect method to get the eigenvalues of the linear (or linearized system) from the identification of a closed-loop frequency response. It is applied for fixed load and source conditions Zs(ω) and Zl(ω). In this paper we will try to explain the main differences between the two methods so as to better understand their scope and limitations. The goal is helping the designers to use them in the most effective way to design their Power amplifiers.
