| 摘要 |
1,058,889. Logic circuits; bi-stable circuits- ENGLISH ELECTRIC-LEO-MARCONI CO. Ltd. Nov. 26, 1965 [Nov. 27, 1964], No. 48377/64. Heading H3T. A logic circuit has a number of inputs applied to the bases of transistors whose collectors are connected together and to the collector of a grounded-emitter output transistor and whose emitters are connected together and to the base of the output transistor, whereby the latter when conducting is prevented from becoming saturated by feedback through the input transistor(s). In Fig. 1 the outputs of a plurality of AND gates 19, 20, 21 are fed via optional diodes 16, 17, 18 to the bases of input transistors 13, 14, 15 which, together with the output transistor 5, are normally non-conducting. If one of the AND gates is opened, the corresponding input transistor (e.g. 13) is turned on and starts conduction in the output transistor 5. As current builds up in the latter, the collector potential falls and eventually transistor 13 saturates. Some of its emitter current is then diverted to its collector and a further rise in the base current of the output transistor is prevented. If two or more input transistors are turned on simultaneously the output transistor base current is shared between them and the same effect operates. Variations in load impedance connected to terminal 6 affect the collector potential of transistor 5 and adjust the feedback to regulate the base current accordingly, transistor 13 coming out of saturation if necessary. The effective output impedance of the circuit is therefore low and multiple fan-out is possible. Also, the current-carrying capacity of the circuit is not limited by permissible transistor dissipation. The diodes 7 and 10 have a very low forward resistance so that the voltage drop across them is largely independent of current, and this minimizes the dependence on resistor values and power supply voltage. Diodes 7 and 10 should have equal voltage drops so that their characteristics will not affect the feedback action. All the diodes (except 10) should have a large value of stored charge for the rapid transmission of transients. Resistor 9 may be returned to earth instead of to a negative supply. Two circuits of the type of Fig. 1 can be crosscoupled to form a bi-stable circuit (Fig. 2, not shown), the AND gates having only two inputs and one of them (e.g. 26) being connected to the output terminal 6 of the other circuit. The input terminals 26 of the other AND gates are used as inputs for an OR function and " set " pulses are applied to the input terminals 25 on one or both sides. These pulses are positive on the appropriate holding circuit transistor and negative on the other input transistors, the positive pulses overlapping the negative in time, at least at the commencement. This may be achieved by using the positive pulses to gate the negative pulses, the slight delay of the latter at the end being acceptable if it is less than the response time of an AND gate. The above arrangements may be constructed as integrated circuits, at least as regards the array of input and output transistors with their associated diodes and resistors. |
