Self-referenced current switch logic circuit with a push-pull output

Abstract

The base circuit comprises a self-referenced preamplifier (31) of the differential type connected between first and second supply voltages and a push-pull output buffer stage connected between second and third supply voltages. The push-pull output buffer stage comprises a pull-up transistor and a pull-down transistor connected in series with the circuit output node coupled therebetween. These transistors are driven by complementary and substantially simultaneous signals S and S supplied by the preamplifier. Both branches of the preamplifier are tied at a first output node (M). The first branch comprises a logic block performing the desired logic function of the base circuit that is connected through a load rsistor to the second supply voltage. The logic block consists of three parallel-connected input NPN transistors, whose emitters are coupled together at the first output node for NOR operation. The second branch is comprised of a biasing/coupling block connected to the second supply voltage and coupled to the first output node and to the base (B) node of the pull-down transistor. This block ensures both the appropriate polarization of the nodes in DC without the need of external reference voltage generators and a low impedance path for fast signal transmission of the output signal from node M to node B in AC, when input transistors of the logic block are ON. and base nodes. An anti-saturation block (AB), consisting typically of a Schottky Barrier Diode (SBD), is useful to prevent saturation of the pull down transistor (TDN) to further speed up the circuit.