Millimicrosecond Transistor Current Switching Circuits

If transistor switching circuits are to have response times limited by the bandwidths of the transistors operating as amplifiers and by diffusion or transit time delay, it becomes necessary to avoid operation in saturation. Since a low collector to base voltage, or operation near saturation, has a detrimental effect on transistor bandwidth, it is desirable to avoid this operating region as well. Where nonsaturating circuits are used with transistors having cutoff frequencies of several hundred megacycles, transistor and circuit capacitances become the primary limitation on speed. To minimize the delays due to capacitance, it is desirable to operate with voltage swings as small as reliability consideration will permit. A new mode of operation has been developed whereby well-specified currents can be switched reliably with small voltage swings. The complementary current switching technique described provides a means of constructing relatively simple logical circuits in which transistors are operated well out of saturation in a region of maximum bandwidth. This technique offers several advantages besides high-speed switching. The resulting logical circuits have complemented outputs, and reset their own levels. There is no requirement on the upper limit of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\alpha</tex> and dc stability factors are essentially unity. The circuits are simple and relatively noise free. A few basic circuits and working examples are presented in the paper. Application and extension of this current switching technique results in a large body of new and useful logical circuits.