Patent GB-866238-A (GB)
Title: Transistor two stable state apparatus
IPC: H03K3/012
Filed: 1957-04-26
Abstract: 866,238. Transistor bi-stable circuits. MINISTER OF NATIONAL DEFENCE OF HER MAJESTY'S CANADIAN GOVERNMENT. April 26, 1957 [Feb. 11, 1957], No. 13421/57. Class 40(6) The invention relates to bi-stable circuits comprising two complementary transistors having their base and collector electrodes cross coupled so that the circuit has one stable state in which both transistors conduct and another in which both are non-conducting. According to the invention, an output circuit is connected to the emitter of one transistor, and this transistor when conducting is saturated to a controlled extent by a constant base current composed of the algebraic sum of a constant current (which may be zero) and the constant collector current of the other transistor, this transistor being operated in an unsaturated condition. It is explained that by controlling the base current of the output transistor in this manner, delays due to hole storage effects are minimized. The invention also relates to circuits formed from an equivalent NPNP or PNPN structure. Fig. 1 shows a bistable circuit comprising complementary transistors 10 and 11 having their base and collector electrodes cross coupled and connected to an output circuit terminal 32, the circuit being such that in the "on" condition both are conducting and in the "off" condition both are non-conducting. In the "on" condition, a constant current is fed to the emitter of transistor 10 through a resistor 19 and the base electrode is clamped at - 10 volts by a diode 31. Accordingly, as the transistor is operated in an unsaturated manner a constant current flows into the collector circuit. A constant proportion of this current is supplied through a high value resistor 23 so that the base current of transistor 11 is also constant, its valve being sufficient to saturate the transistor. Transistor 10 is prevented from also saturating by the voltage drop across a resistor 13. When a negative "turn off" pulse is applied to terminal 29, transistor 34 becomes conductive and saturates. This causes the base of transistor 11 to become positive and rapidly clearing stored carriers through the low resistance paths of rectifiers 41 and 31. The "turn off" of transistor 34 may be speeded by following the "turn off" pulse at terminal 29 by a short positive pulse and by applying a constant current to the base of transistor 10 through a resistor 20. The circuit may be turned on again by applying a pulse to the terminal 30 so as to feed a charge of carriers into transistor 10. The input circuit of the "turn on" transistor 34 is arranged to prevent current through the collector capacitance of this transistor from returning the circuit to the "off" condition and diodes 39 and 41 clamp the electrode voltages during the "off" condition. The current in the output circuit connected to terminal 32 may be increased as a result of feedback provided by the forward resistance of diode 31 or by a resistor in series with it. A similar effect may be obtained by reducing the value of resistor 19. In order more quickly to recharge the capacitance of the output load when the circuit is turned off, a diode may be connected between the collector of transistor 34 and the output terminal so as to allow current to pass during the "turn off" period (Fig. 3, not shown). Any increase this diode may cause in a "turn off" time of the transistor circuit 10, 11 may be reduced by connecting the diode to the point 33 instead of to the output terminal (Fig. 4, not shown). When it is desired to maintain the emitter terminal of transistor 10 at a constant potential a further clamping diode may be provided in the emitter circuit and in addition it is stated that a diode may be connected to the base of transistor 11 to clamp the base potential during the conducting period while still maintaining the base current constant (Fig. 2, not shown). Fig. 5 shows an alternative input circuit connected to terminal 30 which permits the "turn on" charge to the transistor 10 to be reduced and thus the period before "turn off" may be effected decreased. In addition, in this arrangement, a diode 44 and a resistor 43 are provided so that at the end of the "turn off" period the diode becomes conductive to remove carriers from the base of transistor 34 and render the circuit more quickly available for re-triggering. Specification 854,910 is referred to.