Patent GB-870089-A (GB)
Title: Improvements in or relating to self-indicating two-state circuit arrangements
IPC: H03K3/12
Filed: 1958-05-06
Abstract: 870,089. Valve and transistor two-state circuits. PHILIPS ELECTRICAL INDUSTRIES Ltd. May 6, 1958 [May 9, 1957], No. 14440/58. Class 40(6) In a two-state circuit comprising a thermionic valve and a transistor arranged in a positive feedback loop with the collector circuit of the transistor connected to the control grid of the valve, the valve is of a type which includes a fluorescent layer deposited upon its anode so that it produces light when a current passes through it. Hence the state of the circuit is indicated by whether or not light is being produced by the thermionic valve. In a first bistable circuit, Fig. 2, the anode circuit of the valve 10 comprising a fluorescent layer 14 is arranged in series with the base electrode of the transistor 19 so that when no current flows through the valve the transistor is cut off and the control grid potential of the valve is maintained at a low value by means of the potential divider 24, 25 and the bias battery 26. The circuit is changed to its conducting state by the application of a positive pulse to the control grid of the valve 10: this causes base current to flow in the transistor 19 so that the control grid potential determined by the potentiometer 24, 25 rises and the circuit assumes its second stable state with both the valve and the transistor conducting and light being omitted from the fluorescent layer 14. Resistor 31 is provided to limit the maximum value of potential difference which may be applied between the emitter and collector electrodes of the transistor 19 while resistors 32, 33 provide negative feedback around the transistor in order to stabilize its effective collector-base current-amplification factor at a value small enough for the transistor not to be bottomed when the circuit is in its conducting condition. This reduces charge storage in the base of the transistor and hence increasing the switching speed of the circuit. In a modified form of circuit Fig. 3 (not shown) a diode is connected between the collector electrode of the transistor and the anode of the valve 10 in such a sense as to prevent the collector from going positive with respect to the anode, this again reducing the charge-storage effect. This circuit also shows a capacitor connected across the resistor 24 which increases the switching speed of the circuit. The circuit of Fig. 2 may be modified to be monostable by the provision of a resistance-capacitance an inductance-resistance coupling in place of the resistors 24, 25. By providing an inductancecapacitance feedback network between the transistor and the valve the circuit may be modified to function as a free-running oscillator. In a second embodiment Fig. 4 the valve 50 and transistor 60 are arranged in an Eccles-Jordan bistable circuit, the transistor in this case being of a NPN type. Fig. 5 (not shown) illustrates a circuit similar to that shown in Fig. 2 but wherein the transistor 19 is replaced by a photo transistor and the connection between the anode circuit of the valve 10 and the base of the transistor is replaced by an optical connection between the fluorescent layer and the base of the photo transistor. In this circuit, conduction in the valve 10 causes the fluorescent layer to glow so switching on the photo transistor, whereby the control grid potential of the valve is increased to maintain the circuit in its conducting condition.