Beam current compensation for vertical size in a digital TV

摘要

A PWM signal of substantially constant amplitude is supplied to a B+ compensation circuit, for developing a vertical ramp voltage in a digital TV. One transistor of a current switching differentially coupled pair is supplied with the PWM signal and the other is properly biased so as to amplify the PWM signal without inverting its phase. The negative peaks of the PWM signal are clamped to voltage which varies with B+ and the positive peak of the PWM signal are a fraction of B+. However, the positive peaks vary more with B+ than do the negative peaks. The amplitude of the PWM signal, which is used for developing the vertical ramp, is the difference of the positive peak and the negative peak and therefore it will vary with B+. The B+ in the television receiver varies with beam current and by properly varying the vertical ramp in accordance with the variation in B+, the vertical size of the raster can be maintained.