GB736899A - Cathode-ray tube apparatus - Google Patents

Abstract

736,899. Television. ILFORD, Ltd. April 15, 1954 [April 17, 1953], No. 10599/53. Class 40 (3). [Also in Group XL (c)] In cathode-ray tube apparatus, such as, for example, a flying spot scanner, a desired light output modulation control voltage characteristic of the tube 1 is obtained by means of an arrangement including a photo-cell 5 which feeds back a voltage ef proportional to the light output to the modulation control circuit. This circuit comprises a differential amplifier 15, 16 to one input grid 19 of which is applied a modulation voltage ec and to the other input grid of which is applied the feed-back voltage ef. A voltage ea proportional to the difference of the inputs appears in the anode circuit of amplifier 15 and is applied through D.C. amplifier 22 to the control grid 14 of the cathode-ray tube. The polarity of the feedback voltage ef is arranged such that a fall in light output causes a compensating increase in beam current. The circuit tends, therefore, to reduce any difference between ec and ef and by utilizing a high value of anode resistor 18, or a large gain in D.C. amplifier 22, the difference between ef and ec can be made negligibly small. Where the feed-back voltage ef is a linear function of the light flux incident on the photocell 5, the circuit operates to produce a linear relationship between the light output of the tube and modulation voltage ec. To produce a non- linear relationship there is inserted in the feed-back path between photo-cell 5 and differential amplifier 15, 16 a non-linear circuit element having a law corresponding to the inverse of the desired relationship. If it is desired to vary the electron beam current over a scanned raster to compensate for deficiencies in an optical system projecting image 3 this may be effected by placing a graduated filter or a replica of the optical system in the light path in front of the photo-cell.