Transmission circuit for direct current data transmission

Apparatus for producing at different times positive and negative direct current signals responsive to binary signal levels from a data source, said current singals having substantially equal magnitudes and being produced from two output signal terminals, said apparatus having a substantially zero source impedance, comprising:

• a source of operating voltage having a pair of output terminals,
• first and second transistors, each having a conductive path established between the collector and emitter thereof and a base electrode, the application of voltages to which causes proportional amounts of current to flow through said conductive path, p1 first and second equal-valued emitter resistances connecting the emitters of said first and second transistors, respectively, to one of said output terminals of said operating voltage source,
• first and second equal-valued collector resistances connecting the collectors of said first and second transistors, respectively, to the other of said output terminals of said operating voltage source,
• first voltage divider means having input terminals connected, respectively, to the collector of said second transistor and to said one output terminal of said operating voltage source and having output terminals connected, respectively, to the base of said first transistor and to said one output terminal of said operating voltage source,
• first switching means coupled to said voltage divider means for varying, responsive to the presence of a first of said binary signal levels, the voltage output at said output terminals of said first voltage divider means,
• second voltage divider means having input teriminals connected, respectively, to the collector of said first transistor and to said one output terminal of said operating voltage source and having output terminals connected, respectively, to the base of said second transistor and to said one output terminal of said operating voltage source,
• second switching means coupled to said second voltage divider means for varying, responsive to the presence of a second of said binary signal levels, the voltage output at said output terminals of said second voltage divider means,
• said operting voltage source output, said first and second emitter resistances and the base currents of said first and second transistors being of values such that when said first and second switching means are unoperated, currents of substantially equal values flow through said conductive paths of said first and second transistors and
• connection means connecting said emitters of said first and second transistors, respectively, to said output signal terminals of said apparatus as to produce therefrom direct current signals which are a function of the difference between the magnitudes of the currents flowing, respectively, through said first and second transistors.