. The Bell System technical journal. Telecommunication; Electric engineering; Communication; Electronics; Science; Technology. TRANSFORMER COUPLING CIRCUITS 609 output terminals of the tube. Where the amplification factor and plate circuit terminating impedance are low, as in the case of tubes oper- ating as demodulators, the effective input impedance of the tube is comparatively high and depends mainly on the electrode capacities. The input impedance of a shielded-grid tube is likewise comparatively high and depends mainly on the capacity between the grid and filament terminals. The input imp
. The Bell System technical journal. Telecommunication; Electric engineering; Communication; Electronics; Science; Technology. TRANSFORMER COUPLING CIRCUITS 609 output terminals of the tube. Where the amplification factor and plate circuit terminating impedance are low, as in the case of tubes oper- ating as demodulators, the effective input impedance of the tube is comparatively high and depends mainly on the electrode capacities. The input impedance of a shielded-grid tube is likewise comparatively high and depends mainly on the capacity between the grid and filament terminals. The input impedance of a tube decreases with increase in frequency and at very high frequencies it becomes so low that difficulty is experienced in obtaining any increase in amplification in the coupling circuit. In the case of the single tuned transformer, previously referred to, the effective input capacity of the tube may act as part of the tuning condenser with the result that at the resonance frequency a substantial amplification can usually be obtained. How- ever, when it is desired to transmit a broad band of frequencies the magnitude of the effective input capacity becomes a very important controlling factor. For a definite band width the possible voltage amplification varies inversely as the value of the effective capacity across the secondary winding. A study of the various factors entering into the use of a suitable coupling circuit for successive stages of a high frequency amplifier has indicated that best results are obtained if the design of the trans- former is based upon the principles of the broad-band filter. Definite relations are obtained between the constants of the transformer windings, the tuning capacities, and the impedances of the circuits between which it operates. The structure is thus essentially a band- pass filter and has all of its elements properly proportioned to provide the desired band selectivity, but it still retains the form and the functions of a tran
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