1. Field of the Invention
This invention relates to the injection of idle current into a current switch and, more particularly, to the use of fast response diodes to cause the injection of the idle current in the current switch to reduce the propagation delay in said switch.
2. Description of the Prior Art
In the prior art, both Emitter-Coupled Logic (ECL) gate circuits and cascode circuits have been used at current switches. FIG. 1 illustrates the basic current-switch emitter-follower ECL gate circuit. The propagation time delay between the midpoint of a signal V.sub.IN, input to the basic circuit and the midpoint of (either of) its output voltage signal(s) may be expressed as:
t.sub.pd = t.sub.pd1 + t.sub.pd2 + t.sub.pd3 wherein (for a positive input signal transition), PA1 t.sub.pd1 is the time required to charge the emitter-base junction (capacitance) of the input transistor from its initial value to a value at which the transistor can begin to conduct (e.g. from 0.4 to about 0.7 volts), PA1 t.sub.pd2 is the time required for current transfer given that both switching transistors are fully activated, PA1 t.sub.pd3 is the time required to charge (or discharge) the equivalent capacitance at either collector node through the load resistor connected thereto.
To improve the speed of such basic ECL circuits, the possibility of incorporating idle current injection features and "keep alive" (KA) diodes in a manner similar to that suggested in FIG. 2 and discussed below was studied by Rigby with the intent of reduction of the t.sub.pd1 component of delay and addition instead of another smaller delay component, the diode switching time. Rigby's work, reported in 1963 in an article entitled High Speed Emitter Current Switching, published in the (Australian) technical journal, Proceedings of IREE, dealt only with the use of discrete component circuits and in general found that the response time of conventional p-n junction diodes available in 1963 and considered by him was too great to afford any meaningful overall circuit delay reduction. One aspect of the invention at hand, however, concerns the use of Schottky or other fast response KA diodes in the basic current switch, their incorporation in integrated circuits being accomplished in a manner similar to that used in the manufacture of "Schottky Transistor Logic" so as to entail the expense of relatively little extra monolithic circuit silicon area, and utilizing only currently "standard" manufacturing processes. The switching time of such Schottky diodes, being much shorter than t.sub.pd1 in a circuit not incorporating them, meaningful delay reduction can actually be attained.
Another aspect of the invention pertains to the use of Schottky or any other KA diodes and idle current injection circuitry in cascode switching circuits, in a manner suggested in FIG. 3 and discussed below. In this application, idle current injection permits improvements of cascode circuit performance in a manner not described in the literature or any known prior art. Neither cascode circuits or their improvements were discussed by Rigby. Such cascode circuits have greater logical flexibility and computational capability than basic ECL gates because of their series gating structure. One of the problems though with these switches is their slightly increased delay in switching relative to conventional current switch emitter follower circuits and their generation of spurious output signals or glytches under certain conditions, as described later below. The present invention though nearly eliminates these glytches and speeds up the switching time of, both lower and upper switching sections, said circuits by injecting idle current through the lower current switch transistors of said circuits and to the cascode nodes, i.e. those at the emitters of the upper switching transistors. The injection of said idle current is controlled by the use of Schottky diodes.
In still another aspect of the present invention, no KA diodes are used and idle currents are injected only at the upper current switch cascode nodes. This injection and the means for its accomplishment as well as the particular improvements obtained thereby, not described in any known literature or prior art, are also disclosed in the following.