DYNAMIC FOCUS CIRCUIT

PURPOSE:To always execute a normal focus even if a horizontal synchronizing frequency is different by using an inductance component of a resonance circuit for generating a parabolic voltage as a saturable reactor and providing a means for detecting the horizontal synchronizing frequency and controlling the saturable reactor by its detection output. CONSTITUTION:A dynamic focus circuit 1 detects a frequency of a flyback pulse, namely, a horizontal synchronizing frequency. Also, said circuit is provided with a control circuit 2 for varying an inductance of a saturable reactor L1 so that a resonance frequency of a resonance circuit becomes equal to the horizontal synchronizing frequency by controlling a current (i) in accordance with a result of said detection. Accordingly, even if a frequency of a flyback pulse is varied, the resonance frequency of the resonance circuit is varied so as to become equal to this frequency. In such a way, a parabolic voltage of a satisfactory waveform is always obtained, and a normal focus can always be executed.