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Resonant oscillators using transistors provide a suitable means for the conversion of d.c. into a.c. power, in particular where a reasonably sinusoidal output voltage is required. For the design of such oscillators, the current-amplification factor, the fraction of the cycle during which collector current is permitted to flow and the feedback conditions have to be considered. The maintenance equation correlating these factors is derived, and an angle-of-flow chart is described, by means of which sets of values of the amplitude of oscillation, the current amplification of the transistor, the amount of feedback and the fraction of the cycle (flow angle) during which collector current is flowing, simultaneously fulfilling the maintenance equation, can be readily determined. Practical oscillator design must take into account the non-linearity of the transistor circuit which has to be incurred in order to ensure correct amplitude limiting of the oscillator. Deviations from sinusoidal operation resulting from this as well as from other causes are considered in some detail, and graphical methods are described for the quantitive assessment of oscillator performance. These methods are based on Lienard's construction which, suitably extended, provides a convenient means of predicting the amplitude, output waveform, frequency and other performance data of the oscillator. The design of a Class B oscillator is described in detail and illustrated by means of a practical numerical example. Calculated data agreed well with practical measurements.