GB858312A - Improvements in or relating to current controlling circuits - Google Patents

Abstract

858,312. Cathode-ray tube circuits. RANK CINTEL Ltd. Feb. 24, 1958 [Feb. 25, 1957], No. 6223/57. Class 40(7). In an arrangement for maintaining the beams of a television tube accurately in focus for all deflected positions the current flowing in the focusing coil 1 is controlled in accordance with a parabolic voltage derived from the tube deflection circuit and utilized to control the conduction of a valve 2 in the focusing current path of coil 1. The parabolic voltage which is effective at the control grid of valve 12, is derived from the sawtooth deflection voltage for the tube supplied to terminal "IN" by combining the first integral thereof, produced by means of resistor 16 and condenser 17, with the second integral produced by means of resistor 21 and condenser 20. Valve 12 drives a second valve 4 via the common cathode resistor 13 and the output of valve 4 is fed to the grid of valve 12 via the potential divider 9, 10 the valves of which are such that the reduction in the voltage fed back is equal to the gain of the two valves. Under this condition approximately infinite gain exists between the control grid and anode of valve 4 but oscillation is prevented by negative feed-back to the control grid of valve 4 via valve 2. A mathematical analysis of the operation of the circuit is given and it is shown that, when the gain of valves 12 and 4 is large the variation in the input (parabolic) waveform is accurately followed by the current in the load I and independent of changes in the resistance of the load due, e.g. to thermal changes and substantially independent of variations in both H.T. supply voltages although the latter, preferably, are stabilized. Fig. 5, shows a modification of the circuit in which the bias source 11 is omitted when H.T.2 is sufficiently stable and the resistor 9 connected to a suitable tapping on anode resistor 5. Other modifications shown include the provision of a potentiometer 22 to enable the current flowing in load 1 to be varied by varying the negative feed-back of valve 4; the provision of a resistor 23 shunting valve 2 so that the current handling requirement of the latter need be sufficient only for the variation of current produced in the load coil; the provision of a resistor 24 in series with the grid of valve 12 and condenser 17 to reduce distortions due to eddy currents generated conductive but non- magnetic material associated with the structure of the coil1. The invention is stated to be of general application and the thermionic valves may be replaced by transistors.