Patent GB-537076-A (GB)
Title: Improvements in or relating to apparatus for producing amplitude modulated waves in radio transmitters
IPC: H03F1/06
Filed: 1939-12-04
Abstract: 537,076. Valve amplifying circuits for wireless transmission. SOC. FRANCAISE RADIOELECTRIQUE. Dec. 4, 1939, Nos. 31458, 31459 and 31460. Convention dates, Dec. 2, 1938, Dec. 29, 1938 and Feb. 25, 1939. [Class 40 (v)] Modulated waves are supplied to an aerial through two or more amplifying paths in different phases which combine vectorially to produce the aerial current so long as the carrier is below its mean value ; but at carrier values higher than the mean the vector angle is reduced so that although the output from each valve is held substantially constant, their vectorial sum increases The system thus leads to greater efficiency in the output stages. As shown in Fig. 2, the waves from a carrier source O are modulated at M and amplified at C, whence components U 1 , U 2  in phase opposition are applied to the grids g 1 , g 2  of the power valves 1, 2, and a third component Ub in phase quadrature is applied to the grids in parallel. The plate circuits of both valves include the aerial load Ra, and the voltage Ua across this load is also fed back into the grid circuits in opposition to Ub. The resultant voltages in the grids g 1 , g 2  have a certain phase difference which may be about 140 degrees, and so long as the carrier voltage is below normal this angle remains constant and the aerial current is proportional to the grid voltage. The valves, however, begin to saturate at carrier voltages greater than normal, with the result that the feed-back voltage diminishes relatively, and this causes a reduction in the phase angle between the grid voltages, and a consequent increase in the vectorial sum of the plate currents. Slight distortion occurring at the peaks of the modulating voltage may be corrected by known devices such as negative reaction from an auxiliary amplifier connected in parallel with the main amplifier (Fig. 5 not shown). The valves 1, 2 may each be replaced by two valves in cascade, and the feed-back voltage Ua may be taken from the combined output of the second stage to the input of the first (Fig. 6 not shown). The quadrature voltage Ub may be fed to a valve separate from the valves 1, 2, and the feed-back voltage Ua also applied to the grid of this additional valve (Fig. 7 not shown). Fig. 9 shows an arrangement using two pairs of push-pull valves with tuned input circuits 2, 21 fed with modulated waves from a source 1. The tuned output circuits 3, 31 are equally coupled one magnetically and the other statically to the load Ra. In addition the source 1 of modulated waves is coupled to a tuned circuit 211, the sides of which are connected through condensers and inductances respectively to the valve grids, as indicated by the letters A, B, A<SP>1</SP>, B<SP>1</SP>. The feed-back voltage across the load Ra is also applied to the grids through inductances and condensers as shown. Fig. 11 shows a further modification using two push-pull stages in cascade. The valves L<SP>3</SP>, L<SP>4</SP> of the first stage are fed with the modulated wave at A, B but are differently biased. The valve L3 operates as a Class B amplifier with saturation in the plate circuit when the carrier rises above the mean value. The valve L4 is biased for Class C amplification, and gives no output until the carrier is above the mean value. The valve L4 feeds the grids of the'next valve pair L<SP>1</SP>, L<SP>2</SP> in parallel, while the valve L3 feeds the same grids in push-pull through an intermediate coupling circuit G, giving a quadrature phase relationship with respect to the first path. Two supplementary circuits P, Q are inserted in the grid leads of the valves L<SP>1</SP>, L<SP>2</SP> for giving a phase advance on one grid and a phase lag to the other. The tuned output circuits are respectively coupled magnetically and statically to the output transformer feeding the load. A coil V feeds back from the output transformer to the input terminals A, B.. The phase angle between the grid voltages of the valves L<SP>1</SP>, L<SP>2</SP> remains constant for subnormal values of the carrier, but when the carrier rises above normal the valve L3 begins to operate, and causes the phase angle to vary with the result that although the valve L4 is saturated, the ultimate voltage delivered to the load rises in correct relation. For setting up purposes a dummy load W may be substituted for the aerial by the operation of switches X, Y. Specifications 270,749, [Class 40 (v)], 372,090, 383,928 and 409,628 are referred to.