Abstract: A subscriber line interface circuit (SLIC) sets AC and DC output impedances as seen by first and second transmission signals. The common mode variation and differential variation of the first and second transmission signals is AC-sensed to provide a common mode feedback signal and a differential feedback signal which in turn controls the AC output impedance. The DC component of the first and second transmission signals is blocked allowing downstream amplifiers to operate at reduced power supply potentials. The DC variation of the first and second transmission signals is also sensed for providing a DC feedback signal to control the DC output impedance.

Abstract: A current-injection logic synchronous driver circuit is responsive to an input signal for providing a plurality of output signals from a plurality of logic gates. The logic gates are connected in parallel with their inputs sharing a common node. A current supply provides a predetermined current to a first one of the plurality of logic gates, while a driver circuit operating in response to the input signal provides drive levels to the common node at the inputs of the logic gates sufficient for each logic gate to sink the equivalent of the predetermined current from the current supply. Thus, the synchronous drive circuit provides a plurality of output signals having the same logic state as the input signal while increasing the fan-out to other logic circuits of the system. The propagation delay through the synchronous driver is small because of the parallel connection of the logic gates with common drive thereto.

Abstract: A negative feedback loop, including a two collector I.sup.2 L gate having one collector returned to the input of the gate and forcing a reference current into the other of the two collectors of the gate, is used to match the current at the collector of the injector of the gate to the reference current. The feedback loop further includes a current source for sourcing the reference current and a pair of cascoded emitter follower transistors coupled between the current source and the injector input of the gate such that the feedback loop will regulate the current through the injector to be equal to the reference current. The injector input of the gate of the feedback loop can be coupled to the injector inputs of similar I.sup.2 L gates to be biased therefrom such that the injector currents are well controlled.