In this article, small-signal parameters are defined, a simple unstable RLC circuit for which the criterion of Equations 1 and 2 fails is analyzed, conditions under which it can be applied are presented and the Nyquist stability criterion is discussed.
Second, the admittance/impedance description may be easier to use than the S-parameters description, leading some to the conclusion that the admittance/impedance description is correct.[12] As can be seen, both descriptions are valid when used with the Nyquist stability criterion. The only advantage of an admittance/impedance representation is that for high Q resonators it more often produces a simple loop.