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A stability analysis technique of microwave amplifiers, valid for large- or small-signal regimes, is presented. The technique calculates the system poles and zeroes from a closed-loop frequency response of the circuit linearised around its steady state. The method has been applied to an S-band monolithic amplifier, detecting spurious oscillations for certain specific bias conditions and input power levels, in good agreement with measurements.
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Introduction: Rigorous methods for large-signal stability analysis of microwave circuits have been reported in the literature [1, 2]. In this Letter, a different technique is presented based on the computation of a closed-loop frequency response of the circuit linearisation around its steady state. The procedure is applicable to either small- or large-signal regimes and can be implemented in a simple manner in HB simulators. Unlike other stability analysis techniques [2], there is no need to analyse the open-loop transfer function associated with every nonlinearity in the circuit. Once the circuit closed-loop frequency response is obtained, system identification methods for linear models are applied in order to determine the system poles and zeroes and, therefore, the stability of the steady-state solution.
Analysis technique: From basic control theory, the existence of a pair of complex-conjugate poles with positive real part in the transfer function H(s) of a given system implies system instability, with increasing amplitude at a spurious autonomous frequency. The stability of a system can be determined by analysing either the stability of the system closed-loop transfer function Hcl(s) or, indirectly, the stability of the system openloop transfer function Hol(s) [3]. Since a similar computing cost is required to obtain Hcl and Hol of an electrical circuit, a closed-loop approach is adopted here because it allows a more straightforward analysis than the open-loop techniques.
The first step of the technique consists of obtaining the closed-loop frequency response Hcl(jw) of the circuit linearised around its steady state. Fig. 1a shows the block diagram of a general feedback system, with the corresponding closed-loop transfer Hcl(s) function being
... (1)
When a small-signal current perturbation ii is injected into a node n of an electrical circuit (Fig. 1b), there exists a direct equivalence between the circuit and the system in eqn. 1....