GB395277A - Improvements in or relating to thermionic amplifiers - Google Patents

Abstract

395,277. Valve amplifying circuits. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., Marconi House, Strand, London.- (Assignees of Barton, L. E. ; 228, Burrwood Avenue, Collingswood,-New Jersey, U.S.A.) Jan. 26, 1933, No. 2572. Convention date, Jan. 30, 1932. [Classes 39 (i) and 40 (v).] Valves of the " class B " type with a high amplification factor and high internal impedance are used as audio-frequency or other power amplifiers, with low impedance in the input and output circuits, distortion being prevented by allowing grid current to flow, and the valves can be driven to the limits of their emission. A pair of valves 15, 15 of the type described is arranged as for push-pull working, the grid bias being preferably zero while the plate current in each valve is nearly zero at the working point so that each valve functions for only one half of the signal wave ; thus the grid-bias battery is eliminated and the plate voltage can be correspondingly reduced. To secure this result (which is illustrated in Fig. 3, where plate current is shown at 36, 45 and grid current at 43, 47, for zero and positive grid bias) valves with two control grids 33 at different levels are used, and the grids are connected together outside the tube as shown in Fig. 2, or inside the tube, Fig. 4 (not shown). In the latter case all the electrodes may be inside a single envelope, Fig. 5 (not shown), and Fig. 6. In the arrangement shown in Fig. 6 each pair of grids consists of the two layers 56, 57, of' a spiral wound on posts or pins which space the layers apart. A pair of coarse-meshed grids spaced apart in this way is preferred to a single fine-meshed grid, which gives somewhat similar results. In Fig. 2, pentode valves with screening grids 32 are shown. The signals are amplified at 24, 23 before being applied to the system, and the effective impedance of the input coil 16 is low in comparison with the internal gridcircuit impedance of the valve. The load on the transformer 20 is kept substantially constant by the flow of grid current. The transformer 19 steps down in the ratio 8 : 1 and the load impedance is small compared with the anodecircuit internal impedance of the valve.