Patent GB-1425739-A (GB)
Title: Circuit for switching transistors
IPC: G09G1/28
Filed: 1973-05-30
Abstract: 1425739 Transistor switching circuits; cathode-ray tube circuits MOTOROLA Inc 30 May 1973 [3 July 1972] 25670/73 Headings H3T and H4T The control electrode circuit for a transistor feeding a capacitive load comprises first means which provides an initially high drive while the capacitor charge is being set and subsequently a lower drive, and means for providing an off bias for the transistor in conjunction with the removal of the drive. Fig. 2 shows the control circuit for the base of one transistor 45 of a plurality of transistors connected in series in a circuit for supplying three different levels of high voltage to the capacitive load constituted by the anode of a penetration type colour cathode-ray tube. A square wave from 48 is rectified at 68 to provide across the smoothing capacitor 74 power required for biasing transistor 45 on. To switch the bias on the rectangular wave is also applied at 47 and rectified at 95. Current flows to the base of a transistor 71 via a capacitor 89 to provide from capacitor 74 and transistor 71 an initially high drive current to the base of 45. This bias terminates when capacitor 89 is charged but a holding bias is provided from capacitor 74 via a transistor 69 and a resistor 83. To switch off transistor 45 the rectangular "on" control voltage at 47 is replaced by a rectangular wave "off" control voltage at 49. This is rectified at 99 to provide a negative bias via transistor 101 to the base of transistor 45. It also switches on a transistor 107 which connects a load 110 across the capacitor 74 equal to the on drive bias provided by resistor 83 so as to stabilize the capacitor voltage. Each of the series transistors 44, 45 and 46 has a similar drive circuit and is provided with equalizing capacitorresistor circuits such as 124, 125 assisted by emitter resistors such as 79. The base resistor 112 in each circuit is made small to improve the voltage capacity of the transistor. Fig. 1 shows a plurality of these switch circuits (each labelled "switch section") for selectively connecting three different voltage levels to the anode of a penetration type cathode-ray tube 11. The lower voltage level is provided by supply 12 when sections 15 and 16 are conducting. The higher level is provided by supplies 13, 14 and 12 in series when switch sections 17 and 18 are conducting. The intermediate level is provided by making switch sections 16 and 17 conducting whereby supplies 13 and 12 are connected in series via diode 41, the section 16 serving to discharge the load when switching from a higher voltage level to a lower level. A control circuit, Fig. 3, prevents concurrent switching of sections which would short circuit the supply. A colour selection code signal at 21 is decoded at 164. One level of the resulting output of a gate 171 acts through inverter 181 to provide the "on" control at leads 147 and 148 and the other provides the "off" control at leads 154 and 155. The control path includes gates connected to the oscillator 158 to periodically interrupt the control signals so as to provide the necessary alternating square waveform of the control, the pairs of gates being in antiphase to provide the antiphase outputs required on the lines. The oscillator also provides a square wave for the lines 141, 142 providing the power supply for the bias circuit. The switch-on condition is delayed by a circuit comprising a differentiating transition detector 165 and a delay circuit 161 which inhibits at the gate 171 the switch-on voltages. This ensures that switch-off of the circuit occurs before switch-on so as to prevent short circuiting of the supplies through two transistors. In addition the turn-on is synchronized by connection to the oscillator via gate 168.