GB588292A - Improvements relating to wide frequency band radio amplifiers - Google Patents

Abstract

588,292. Electric impedance networks ; thermionic valves. STANDARD TELEPHONES & CABLES, Ltd., and MONTGOMERY, W. A. Dec. 28, 1943, No. 21765. [Classes 39 (i), 40 (iii) and 40 (v)] An impedance-transforming filter is designed for coupling, particularly at ultra - high - frequencies, a pair of valves having earthed grids and consequently low input impedances, the capacitances of the valves being taken into account and their impedances matched by the filter at the geometrical mid-band frequency. Coaxial resonators may be used as inductance elements in the filter, which may terminate in shunt trimming condensers. The valves may be of the kind having parallel plane electrodes and described in Specifications 585,448, 585,449, 585,450 and 585,456. The network is taken to include the valve capacitances as terminal shunt arms C2, C3, Fig. 5, but these arms may be adjustable by means of trimming condensers Qt, Qi, Fig. 7. The arms are calculated by means of known formulae so that the image admittances of the network match the real parts of the valve admittances at the geometrical mid-band frequency. In a modification in which the inductance L8 is infinite, an asymmetrical pi-section comprising the elements Ct, Qt, L5, L7, Qi, Ci, Fig. 6 (not shown), and Fig. 7, is designed to be equivalent to a symmetrical pi-section together with a step-down transformer. The delta L8, L5, L7, Fig. 5, may be replaced by star-derived meshes. Mutual inductance between the elements L5, L7 is prevented by forming the latter as coils at right angles to one another. The heater chokes Lh may be open-wound solenoids and each byepass condenser K may comprise a small plate fixed to but insulated from the screen of the amplifier. An alternative impedance-transformer comprises a simple transformer whose windings are shunted by the capacitances Ct, Qt, Ci, Qi, Fig. 9 (not shown). The inductance elements shown in Fig. 5 may be composed, Fig. 10, of short lengths of coaxial line as described in Specification 541,634, and the valves and condensers may be located inside the outer conductor 1. Each grid may comprise a perforated partition across the latter. The anode of valve V1 is connected to the cathode of valve V2 by the inner conductor 2 which is divided at the centre by a blocking- condenser H and constitutes the inductance L5. The anode supply for valve V1 passes through the inner conductor of a branch coaxial pair 3, 5 which forms the inductance L8, Fig. 5, while a plate 6 forms with the end of the outer conductor 3 a byepass conductor. A branch coaxial pair 4, 7 forms the output shunt inductance L7, and the heater supply enters through the inner conductor 7 and returns through a lead 8 inside the latter. Several pairs of valves may be similarly housed in the tube 1. A coaxial system analogous to the circuit shown in Fig. 7 is described with reference to Fig. 11 (not shown).