Semiconductor Switching Circuits with Minimum Susceptibility

Abstract

In a complex digital electronic system the susceptibility, with respect to interference, of the individual switching circuits govern the integrated system susceptibility. Therefore, during the initial Switching circuit design the relationship between circuit susceptibility and the following three factors should be thoroughly examined: Switching speed; Switching pulse amplitude; and the location of the threshold region with respect to the switching pulse amplitude. A switching circuit should never be designed to switch any faster than necessary because of the proportionate increase in circuit susceptibility with respect to switching speed. It is obvious that a fast switching circuit will respond to a much wider bandwidth than a slow switching circuit, consequently the fast switching circuit becomes more of a potential problem area. Switching pulse amplitude, which affects switching speed and the amount of cross coupling, should be set at an optimum value determined by the expected level of ambient interference. A small amplitude switching pulse which is desirable for minimum cross coupling, normally does not allow sufficient margin for interference rejection. A large amplitude switching pulse which allows sufficient interference rejection margins also produces excessive cross coupling. Therefore, the optimum solution must be a compromise. The last, and perhaps the most important factor is the respective magnitudes of the interference rejection margins on either side of the threshold region. The bias controlled threshold shift technique presents an easy method for determining the respective magnitudes of the margins for interference rejection. This would be a valuable design technique for very complex system which result in many interference coupling paths that are not apparent until the system is completed. The scope of the paper will primarily deal with individual circuits and will not consider shielding or grounding techniques.