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Injection locked oscillators (ILO) used as amplifying stages in a solid-state FM microwave radio repeater are presented. Circuit performances needed for such an application and design criteria used to obtain these performances are given. Two ILO modelling possibilities are analyzed and compared. Similarity between positive feedback model and negative conductance model is shown for circuits with a single resonator. Then the equivalence is extended to a more complex arrangement useful to increase the performances of the practical realization. The first model is still used to study the ILO overall performance and the second model to design the actual circuit. Tunnel-diode ILO with second-order passive network is described giving circuit design criteria and experimental results. Transistor negative conductance linear amplifiers are presented with a procedure for computer aided design of the passive network imbedding the active device. The result of the entire active network is a one-port circuit exhibiting a negative conductance constant with frequency in the whole tuning band. Transistor ILOs are obtained by means of a matching network between circulator and the negative conductance amplifier. Mention is made to the active stages implementation into thin-film microstrip configuration referring alumina thin-film technology.