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A small-signal analytical model is developed for a bulk silicon device subjected to avalanche multiplication induced by space-charge effects. Results show that transit-time negative resistance may exist, provided the solid is biased near the onset of the dc negative resistance or above it; this ac negative resistance is into the microwave range for practical devices. The main features of this phenomenon and the differences with respect to IMPATT diodes are pointed out by using suitable approximate expressions which can help in finding the overall frequency behavior. A check of the preceding analysis by a computer solution of the small-signal impedance has also been performed.

Congenital pulmonic stenosis may be defined as consisting of structural changes in the subpulmonary tract, the pulmonic valve, or the pulmonary trunk that result in a gradient between the systolic pressure in the right ventricle and that in the pulmonary artery. From a developmental point of view, the valvular type of pulmonic stenosis may be the outcome of fetal endocarditis of the pulmonary cusps 1 or, as Gross has suggested, the result of maldevelopment of the arteries that supply the valves, with consequent infarction and fibrosis. 2 There is agreement on the embryology of infundibular stenosis; the idea of Keith is usually accepted that infundibular stenosis is due to an arrest of the normal involution of the bulbus cordis, which should form a part of the right ventricle by the end of the second month. 3 ANATOMIC AND PHYSIOLOGICAL FACTORS Anatomically, pulmonic stenosis usually is not an isolated malformation. As

This paper describes the design concepts, principles and details of a high tip speed transonic rotor having low aerodynamic loading. The purpose of the NASA sponsored investigation was to determine whether good efficiency and large stall margin could be obtained by designing a rotor to avoid flow separation associated with strong normal shocks. Fully supersonic flow through the outboard region of the rotor with compression accomplished by weak oblique shocks were major design concepts employed. Computer programs were written and used to derive blade sections consistent from the all-supersonic tip region to the all-subsonic hub region. Preliminary test results indicate attainment of design pressure ratio and design flow at design speed with about a 1.6 point decrement in efficiency and large stall margin.

This phase consisted of compressor component analyses and investigations conducted using an existing low-speed compressor model and a transonic cascade tunnel. Aerodynamic information for the design of the high- speed radial outflow supersonic compressor to be investigated in Phase II was obtained. A stress analysis and bench tests of several possible rotor designs were conducted as well as an investigation of methods of fabricating and joining blades, rotating shroud, and discs. Target performance values are 11:1 pressure ratio with 81% adiabatic efficiency at 4.5 lb/sec airflow.

Excerpt Approximately 12% of patients with symptomatic rheumatic mitral valvular disease present with pulmonary artery pressure or pulmonary vascular resistance values approximating those found at ...