HIGH RESISTANCE CONDENSERS

Hawke's Bay Tribune, Volume XIX, Issue 300, 4 December 1929, Page 10

HIGH RESISTANCE CONDENSERS

Most modern receivers positively bristle with decoupling resistances and condensers that are intended to divert into harmless paths the various alternating currents that might cause trouble if allowed to stray about. When it is currents of low frequency that we are trying to by-pass through earthing condensers to the filament of the valve, it is noly necessary to choose condensers of large capacity, the type having paper di-electric serving perfectly well. The selection of a condenser for a high-frequency circuit, however, needs rather greater care. When dealing with Jiigh-frequency currents it is no longer quite safe to assume that of two condensers, one having 10 times the capacity of the other, the larger will offer one-tenth of the impedence to their flow. There now evters into the question of resistance iffered by the condenser plates themselves to the passage of highfrequency currents. Let us imagine that a foil condenser is constructed from a long, narrow strip of waxed paper, with a very thin metal foil on each side, and that connection is made to the two foils at the same end. If a high-frequency voltage is applied to thq the full voltage will be

operative across the waxed paper dielectric in their immediate neighbourhood, but owing to the very appreciable resistance of the foils, the volt-age-drop along them will be great enough to render the far end of the strip practically inoperative as a condenser. The tar ends of the foil are, in fact, protected from the applied voltage by a very thorough and elaborate decoupling scheme of series resistance and parallel capacity. A twomicrofarad condenser constructed on the lines suggested, in which the shortcomings of many paper condensers are illustrated in exaggerated form, might well offer no less impedence to currents of very high frequency than a condenser of nominal capacity one-tenth or one-hundredth of its value, but with foils of negligible resistance. In the usual mica condenser, for example, there are usually two sets of small paralleled foils so arranged that every individual foil in each set makes direct connection with the appropriate terminal on the ease. With such a mode of construction the effective resistance of the foils will be negligible. Several manufacturers of paper condensers have arranged to make connection to each foil at a very large number of places, so that with their products the full rated capacity will be effective even at very high frequencies. The choice of one of these, or of a mica condenser, is a measure of precaution that it would be wise to take when choosing condensers to by-pass highfrequency currents in a modern highgain amplifier.