Patent GB-722516-A (GB)
Title: Improvements in or relating to electronic trigger circuits
IPC: H03K21/00
Filed: 1951-07-25
Abstract: 722,516. Trigger circuits. INTERNATIONAL BUSINESS MACHINES CORPORATION. July 25, 1951 [Aug, 3, 1950], No. 17629/51. Class 40 (6). An electronic trigger circuit comprises a vacuum tube and a transistor, the emitter current flowing through the vacuum tube, and positive feedback being applied from the output of the transistor to the input of the tube, whereby variation of the emitter current causes the potential across the transistor output load to switch from one steady value to another. Fig. 1 shows the basic amplifier circuit in which the emitter e of a transistor 11 is connected to a positive voltage supply, and the base is connected to earth through the anodecathode path of vacuum tube 18. The collector c is connected to a negative voltage source through a load resistor 14. Input signals applied between the grid and cathode of valve 18 vary the impedance of the anodecathode path, and thus the emitter current, and result in amplified signals appearing across the load resistance 14. Fig. 4 shows a bi-stable trigger circuit adjusted to have a gain greater than unity by connecting the grid of valve 18 to a tapping on resistance 14. In the first stable state, the tube 18 is cut off, due to the low potential of the control grid, and the emitter and collector currents are at a minimum. In the second stable state, the control grid is at a higher potential and the tube, emitter and collector currents are at a maximum. Triggering from either state to the other is effected by applying negative pulses to terminal 41 which is connected to the base electrode and to the control grid. It is stated that if the circuit is originally in the first stable state, the pulse to the control grid has little effect, while the pulse to the base electrode increases the emitter current resulting in increased collector current which effects an overriding increase in the control-grid potential due to the feedback from resistance 14. The effect is cumulative. If the circuit is originally in the second stable state, the pulse to the base electrode has small effect (due to saturation conditions) while the pulse to the control grid increases the tube impedance to reduce the emitter current, resulting in a further negative potential effect on the grid from the resistor 14. To assist this operation, condenser 33 between the collector c and the control grid, and condenser 30 across cathode resistance 31 are provided. Rectifier 39 with an associated timeconstant network is provided to prevent rising portions of the input pulse affecting the trigger circuit. The output may be taken from theanode or cathode of the valve 18. Modifications are described in which separate connections are made to the base and control grid so that switching may be effected by applying trigger pulses to these two electrodes alternately. The base and emitter electrodes may also be connected to tappings on a resistance connected between the anode and the positive voltage supply, to facilitate adjustment of the emitter and base potentials. Separate rectifier and associated time-constant circuits may be associated with each of the base and grid circuits, and their values may be such that it is only possible to switch between the two stable states in one direction. In a further modification, Fig. 6 (not shown), the anode is connected to a new source of increased positive potential through a resistance, and the base is connected to a tapping on a resistance connected between the cathode and anode of the valve. Fig. 7 shows a gating circuit comprising a pentode tube. The screen, control grid and cathode are associated with transistor 11 to form a trigger circuit as previously described, switching being effected by applying negative trigger pulses to terminals 55 and 42 alternately. Signals are applied via terminal 73 to the suppressor grid, and only when the trigger circuit is in the conducting condition will these signals modify the anode current to provide an output at terminal 76. Points other than the base and grid may be used for the application of trigger pulses.