GB1331265A - Electronic switching circuit arrangements - Google Patents

Abstract

1331265 Cathode-ray tube; circuits REDIFON Ltd 30 July 1971 [14 Aug 1970] 39373/70 Heading H4T The final anode voltage (EHT), for a colour display cathode-ray tube of the type in which the colour reproduced is determined by the energy of the beam, is switched between the (different) required levels by means including a transistor drive amplifier and an output stage including a thermionic valve operating as a cathode-follower. As shown in Figs. 1 and 2, the output at terminal 26 of the transistor drive amplifier 1 controls, via an intermediate valve 2, a cathode-follower stage 3 connected between the EHT supply line 4 and the zero voltage line 5 by means of a potentiometer from which via line 11 a feedback voltage is supplied to the inverting input terminal 22 of amplifier 1. The gain of the latter, which comprises a multi-stage operational amplifier formed as an integrated circuit (detailed in Fig. 3, not shown) is limited to 1000 times by resistors 20 and 70 and the EHT output at terminal 67 is determined by the level of the input at terminal 22. Thus to produce the EHT appropriate to production of the colours Orange (8À0 KV), Yellow (9À8 KV) or Green (12À5 KV) an "enable" voltage is supplied to input 17, 18 or 19 respectively which "turns-on" transistor 40, 45 or 50 to connect into the circuit the resistors pairs 41, 42; 46, 47 or 51, 52. The latter together with resistor 16 form potential dividers which cause the input level at terminal 22 to be progressively reduced so that the increasing level at the output 26 causes the anode of valve 2 and the grid of valve 3 to become more positive and thus produce the increasing levels of EHT at terminal 67. When no "enable" voltage is applied to terminals 17, 18 or 19 the increased input at terminal 22 (determined by resistor claims 13, 14 and 15) causes the output 26 to be reduced driving the anode of valve 2 negatively so that valve 3 is cut-off and diode 66 conducts to supply an EHT level at terminal 67 of 6 KV which is the required EHT for production of the colour Red. A focusing voltage appropriate to the various levels of EHT is derived at terminal 68.