Patent GB-1312943-A (GB)
Title: Current-switching amplifiers
IPC: H03F3/26
Filed: 1970-04-16
Abstract: 1312943 Transistor amplifying and switching circuits; cathode-ray tube displays BURROUGHS CORP 16 April 1970 [5 May 1969] 18180/70 Headings H3T and H4T An amplifier suitable for rapidly switching from one inductive load 161 to another 166 (e.g. deflection coils in an alpha-numeric C.R.T. display) comprises a pair of independently controlled current drivers (transistors 121, 126) connected to a first potential (-10 v.), a first pair of diode devices 151, 156 coupling the output of the drivers with a second potential ( - 50 v.), and a second pair of diode devices 131, 136 connecting the drivers to the loads, the other terminals of which are connected to a third potential (+ 10v.). The object is to provide for the stored energy when a load is switched off and to provide a higher than normal voltage during the change-over period. As shown, input signals at 101 via pre-amplifier 100 control common emitter transistors 121, 126, which in turn control common base transistors 131, 136. Further transistors 191, 196 are made conductive when the difference between the voltage across load resistors 171, 176 and the corresponding input voltages exceeds a predetermined amount, thus providing a high voltage across each inductor for rapid gross positioning of the C.R.T. beam. Since the energy stored (in the C.R.T. yoke) is common to both loads one, e.g. 161, can be switched off immediately; this causes current flow in the other, 166, in the opposite to normal direction (upwards as shown) and this is opposed by the 60 v. potential between + 10 v. and- 50 v. via 156, causing rapid decay. At current zero 196 is turned on to cause rapid build-up, after which normal conduction occurs through 126, 136. In one alternative embodiment (Fig. 2, not shown) each channel comprises two pairs of transistors. During transfer the lower (input) transistors only go off, the upper ones conducting the decaying inductor current to a potential of + 13 volts, maintained by a Zener diode (274). This potential source also supplies current to the transistors in the other channel when they are first turned on, before current flows through the corresponding inductance. In this case the inductors are normally connected to a +13 volt supply but for rapid positioning an external signal turns on transistors (293, 286, 284) to apply + 40 volts. In a third embodiment (Fig. 3, not shown) a two-transistor per channel arrangement is described in conjunction with input circuits in which signals representing alpha-numeric symbols, symbol size, or vectors are amplified and pass through networks (305, 325) which compensate for the effect on the amplifier characteristic of critical damping, which is applied to the inductors in this case. The input signals are then applied to a first differential amplifier, another input to which is an adjustable centralizing voltage, and the output of this is applied via a second amplifier to an emitter follower stage (Q12, Q13) which controls the inductor amplifiers. Negative feedback from both the emitter follower and the inductor amplifiers is combined with the inputs to the first differential amplifier.