This invention relates to serially connected logic units wherein each logic unit, such as an AND gate or an OR gate, is composed of a resistor connected serially to a combination of plural transistors connected in parallel and, more particularly, to the construction of one of the logic units of PNP transistors and another of the logic units of NPN transistors wherein the voltage drop across a transistor of a first of the logic units establishes an operating voltage level for feeding current to the transistor of a following logic unit.
There are increasing demands for higher performance of computers, data processing equipment, and control systems which employ logic circuits, the. higher performance being in terms of greater speed of operation Due to the wide use of logic circuits in the foregoing equipments, an increase in the speed of response of the logic circuit provides a significant improvement in the performance of the equipment.
A typical form of construction of a logic circuit, such as an AND gate or an OR gate has an interconnection of a plurality of identical transistors in a parallel circuit combination, the combination then being connected serially with a resistor. The ]unction of the terminals of the parallel combination with a terminal of the resistor serves an output terminal of the gate, the opposite terminal of the resistor being connected to one terminal of a power supply and the opposite terminal of the parallel combination being connected to the other terminal of the power supply. With such a circuit arrangement, the transistors force current through the resistor in one direction for a fast response to the waveform of a pulse signal, a change of current in the opposite direction being accomplished relatively slowly in the manner of a capacitive discharge via the resistor with the capacitance being provided by stray capacitance of the circuit With such construction, the speed of response can be increased by reducing the resistance of the resistor.
However, a problem arises in that the reduction of the resistance increases the dissipation of power within the logic unit. However, computers, data processing equipment, and control systems are limited in the amount of power which can be dissipated therein. This limitation, in turn, limits the amount of reduction in resistance which is permissible before exceeding power dissipation limits. As a result, there is a need for still further modification of the circuitry of a logic unit to produce the advantage of increased speed.