In this paper, an analytical expression comprising linear instability phenomena leading to microwave generation in a solid beam driven plasma-loaded backward wave oscillator has been developed. This analytical treatment is a process of three-wave interaction and is based on the approximate linear theory of instability established for annular beam driven vacuum backward wave oscillator. The dispersion relation derived here is an approximate one and it can be successfully employed to study a backward wave oscillator system consisting of a sinusoidal corrugated structure having very smaller corrugation depth.