A design theory for transistor frequency multipliers and power amplifiers is presented. The analytical approach is dependent upon two main assumptions, 1) that intrinsic base region behavior may be represented by a simple charge-control form model, and 2) that depletion layer charging current effects may be neglected initially and included when necessary as a perturbation of the analysis. The intrinsic base region is represented by a `partial sinusoid' model of base current flow during the conduction period of the emitter junction. This model is a simple representation of a complex process and is chosen on the basis of adequacy of characterization of the response of actual vices under practical operating conditions, together with a simple analytical form. The analysis of transistor frequency multiplier and power amplifier operation is developed in terms of the model and design relationships are presented. Results showing good agreement between the theory and measured data are given.