GB464231A - Improvements in or relating to thermionic amplifiers - Google Patents

Abstract

464,231. Valve amplifying circuits. STANDARD TELEPHONES & CABLES, Ltd. April 3, 1936, No. 9886. Convention date, May 1, 1935. [Class 40 (v)] In a valve amplifier for use in signalling and sound-reproducing systems, a part of the output circuit is designed to produce evenharmonic voltages which inter-modulate with the fundamental and the odd-harmonic voltages in such a way as to increase the output of fundamental and decrease that of the odd harmonics ; or alternatively to increase the odd harmonic output. When the amplifier is of the push-pull type the even harmonics appear across an impedance in the common portion of the plate circuits. When only a single carrier frequency or a narrow band is involved, a single amplifier may have a secondharmonic acceptor circuit connected across its plate and cathode. As shown in Fig. 2, a resistance R5, R6 is connected between the cathode and the mid-point of the output transformer 15 of a push-pull amplifier pair 10, 11. The bias voltage arising across R5 and R6 is applied to the grids through a resistance R2 and a smoothing-condenser C11. The part-resistance R5 is shunted by a large condenser C12. The Specification shows that as the un-shunted resistance R6 is varied, a point can be found with large output of fundamental, and a minimum of third harmonic. If the condenser C11 is removed the resistance R6 provides negative feed-back of the second harmonic and the Specification indicates that this gives minimum third-harmonic with a lower. value of the resistance R6, accompanied by an increase in power output. By providing a phase-reversing transformer coupling between the terminals of the resistance R6 and the grid circuit of the valves, the resistance R6 may be made effectively negative i.e. the feed-back becomes positive, and the output of fundamental is further increased, Fig. 6 (not shown). The plate voltage may be supplied from an alternating source through a rectifier, and the resistance R5 and condenser C12 may form part of the smoothing circuit, Figs. 1 and 4 (not shown). When a single frequency or a narrow band is to be amplified, the output circuit of a single valve 34, Fig. 5, may include a shunt 38, 39, 40 tuned to accept the second harmonic and with all the elements variable.