The compensation for phase errors in wide-band video amplifiers

Video amplifiers, for use in modern high-definition and colour television systems, must have the best transient response possible. Theoretical considerations show that the neutralizing of the phase error produced by the ¿top-correction¿ networks normally employed, can improve the rate of rise of transients by more than 40%, and at the same time reduce ¿overshoot¿ from greater than 20% to less than 9%. In a typical example, the phase error of a 16-stage amplifier can be corrected by three sections of bridged-T network, and almost the whole of the theoretically available improvements can be obtained. A method is outlined for evaluating the phase characteristic of an amplifier with an arbitrary amplitude-characteristic, and a practical example of its application is given. Photographs of square-wave transients taken from a cathode-ray tube, may be compared with the theoretical response, and the conclusion is drawn that optimum performance in multi-stage amplifiers is to be obtained by the use of ¿maximally-flat¿ circuits with phase correction.