Patent GB-665012-A (GB)
Title: Improvements relating to band-pass circuit arrangements
Inventor: PERCIVAL WILLIAM SPENCER
IPC: H03H7/01
Filed: 1949-01-29
Abstract: 665,012. Electric filters. ELECTRIC & MUSICAL INDUSTRIES, Ltd. Jan. 18, 1950 [Jan. 29, 1949], No. 2513/49. Class 40 (viii). [Also in Group XL (c)] A wide-band coup ling arrangement for coupling a source of signals to a load in cludes a series-resonant circuit disposed between the source and the load, the series resonant circuit being so coupled in the arrangement that the load as effectively transferred into the series-resonant circuit is reduced and the tuning inductance of the series tuned circuit is reduced in conformity with the reduced load, the reduction being such that the self-capacity of the said tuning inductance has no appreciable adverse effect within the pass-band of the arrangement. In Fig. 1 (not shown) a known coupling circuit for transferring signals having a wide band of frequencies from a high impedance source, e.g. a pair of push-pull output valves, to a low impedance source, e.g. an aerial feeder, comprises a parallel tuned circuit connected to the anodes of the valves, and transformer coupled to a series-tuned output circuit, but has the disadvantage that the inductance in the output circuit is partly parallel-tuned by its self-capacity, with undesirable effects on the response of the circuit. As shown in Fig. 2, the primary circuit comprises the inductance 1, connected to the anodes of valves 3 and 4, and tuned to parallel resonance by capacitor 6, which represents the anode-to-earth capacities of valves 3 and 4 in series plus stray capacities. The secondary series-resonant circuit is tunedto the same frequency as the primary, and comprises an inductance 2 which is divided into two equal parts with a capacitor 7 connected between them, the ends of inductance 2 being connected to taps on an inductance 12 which with capacitor 13 forms a tertiary parallel circuit tuned to the same frequency as the primary and secondary circuits. The output load 5 is connected across the circuit 12, 13. The impedance of the load 5 as effectively transferred into the secondary circuit is reduced by tapping the inductance 2 down the inductance 12, and the voltage step-down required to be introduced by the transformer 1, 2 for the purpose of matching the impedance of the source to the impedance of the load is therefore greater than if the actual impedance of the load 5 appeared in the secondary circuit. The value of inductance 2 is therefore reduced and that of capacitor 7 increased with consequent reduction of the effects of the self-capacity of the inductance. Also the self-capacity of inductance 2 is partly in parallel with capacitor 7 and partly in parallel with the part of inductance 12 between the taps, and compensation for this is achieved by suitable choice of capacitors 7 and 13, the residual self-capacity of the two parts of the inductance 2 having a greatly reduced effect. In a practical embodiment for use in a television transmitter, Fig. 3 (not shown), the coil 2 is separated into three parts divided by two capacitors; so that the central portion of the coil is substantially at earth potential. It is stated that the invention can also be applied to interstage coupling circuits in wide-band highfrequency amplifiers.