Patent GB-1126489-A (GB)
Title: Tristable circuit
Inventor: TAUB DANIEL MATTHEW
IPC: H03K3/14
Filed: 1966-01-07
Abstract: 1,126,489. Tri-stable circuits. INTERNATIONAL BUSINESS MACHINES CORP. 12 Aug., 1966 [7 Jan., 1966], No. 783/66. Heading H3T. A tri-stable circuit comprises two crosscoupled identical bi-stable circuits, two states having the bi-stable circuits in opposite states and the third state having the bi-stable circuits in the same state, the triggering threshold for switching from the third state being less than the threshold for switching from the first or second state. Such a circuit is used for reading the bipolar output of a computer store, as when switched into its first or second state the second pulse of the pair is unable to overcome the greater threshold for switching back and cannot therefore give a false output. First circuit (Fig. 2).-A pair of silicon transistors T1 is cross-coupled by silicon diodes D3, and each transistor has a negative feedback path through a pair of germanium tunnel diodes D1, D2 (or a single gallium arsenide tunnel diode). Initially both transistors T1 are conducting and the tunnel diodes are in their low-voltage state. The input is applied differentially to the bases of T2 and T2<SP>1</SP> so that the collector current of the former will rise and increase the current through the diodes D1, D2 to switch them to their high-voltage state. The collector voltage of T1 rises but is clamped by conduction of D3, and the base voltage therefore falls to cut off T1. At the end of the input pulse the currents in T2 and T2<SP>1</SP> revert to their former values, but D1, D2 remain in their high-voltage state and part of the current through D3 flows through D1<SP>1</SP>, D2<SP>1</SP> in the reverse direction, D3<SP>1</SP> remaining cut off. This ensures that D1<SP>1</SP>, D2<SP>1</SP> will not be switched by the subsequent positive pulse on T2<SP>1</SP>. The circuit is reset by injecting current into the collectors of T2 and T2<SP>1</SP> to switch the tunnel diodes to their low-voltage state. The output is taken from the collectors of T<SP>1</SP> and T1<SP>1</SP>. Second circuit (Fig. 4).-Tunnel diodes D4, D4<SP>1</SP> are held initially in the low-voltage state by currents through transistors T3, T3<SP>1</SP> which are determined by a Zener diode D5 associated with temperature-compensating transistor T7. Zener diodes D6, D6<SP>1</SP> are permanently conducting through resistors R8, R8<SP>1</SP> and allow T4, T4<SP>1</SP> to supply additional current to the tunnel diodes so that they are biased just below the peak current point. Differential input signals at A and B decrease the current through T4 and D4, and increase the current through T4<SP>1</SP> and D4<SP>1</SP>, switching the latter to its high-voltage state. Transistor T5<SP>1</SP> then cuts off by the fall in its base potential and turns on T10<SP>1</SP> through resistor R10<SP>1</SP>. Transistor T6<SP>1</SP> also turns on and sends a current through D4 in the reverse direction so that it cannot be switched by a subsequent pulse of the opposite polarity at A. If the initial signal at B is large enough to out off T4<SP>1</SP> the currents through the tunnel diodes are maintained by T3<SP>1</SP> and T6<SP>1</SP>. If the initial signal at B is very large in the positive direction, the supply current for T4<SP>1</SP> is augmented by a transient path through T8 and C1 to supplement the current through T8<SP>1</SP>. The circuit is reset by a positive pulse at C which turns on T13 and cuts off T12. The rise in T12 collector voltage is passed through T11 to turn on T9 and T9<SP>1</SP> and cut off those transistos T5, T5<SP>1</SP>, T10, T10<SP>1</SP> which are not already off. T6 and T6<SP>1</SP> then both conduct and send reverse current through both tunnel diodes; these revert to their low-voltage state when the reset pulse ceases. An output is taken from D. At least transistors T3, T7, T8, T3<SP>1</SP>, T8<SP>1</SP> should be in the form of an integrated circuit to ensure temperature compensation. Third circuit (Fig. 6).-Transistors T15, T16, T18 and T15<SP>1</SP>, T16<SP>1</SP>, T18<SP>1</SP> form the individual bi-stable circuits. Initially T15, T15<SP>1</SP>, T17, T17<SP>1</SP> are conducting and T16, T16<SP>1</SP>, T18, T18<SP>1</SP>, T19, T19<SP>1</SP> are cut off. Differential input signals at A1, B1 are applied (via optional emitter followers T17, T17<SP>1</SP>) to turn on either T16 or T16<SP>1</SP>. If T16 conducts it cuts off T15 and hence turns on T18 to allow current to flow through D10, R18 and R19. This latches T16 on and further increases the base potential of T15<SP>1</SP> so that the latter cannot be cut off by a subsequent pulse of opposite polarity which turns on T16<SP>1</SP>. The circuit is reset by a positive pulse at R which turns on T19 and T19<SP>1</SP> and cuts off T16 and T16<SP>1</SP>. The diodes D10, D10<SP>1</SP> may be omitted. An output is taken from OP.