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An approach to the design of a microwave YIG-tuned transistor oscillator amplifier is discussed in this paper. A classic lumped-element model of the transistor is used to show that a simple but still accurate equivalent circuit can be useful in predicting the effect of transistor parameters on the tuning range of a microwave oscillator. On the other hand, this paper describes a design procedure `without model' based on the small-signal characterization of the transistor by its scattering matrix. This method is particularly suited to amplifier designing since scattering parameters are inherently power-flow parameters and well adapted to computer-aided design (CAD) and optimization techniques. A 3.0-6.5 GHz YIG-tuned transistor oscillator amplifier with 10 mW of output power illustrates the usefulness of these two methods.