Patent GB-574823-A (GB)
Title: Preventing distortion in amplifiers and modulators
IPC: H03F1/28
Filed: 1943-11-19
Abstract: 574,823. Valve amplifying and modulating circuits ; impedance networks. STANDARD TELEPHONES & CABLES, Ltd. Nov. 19, 1943, No. 19377. Convention date, Nov. 25, 1942. [Classes 40 (iii) and 40 (v)] In an amplifying system comprising a preamplifier followed by an output amplifier or carrier modulator, two negative feed-back paths are provided, one round the pre-amplifier and the other round the whole system, the feedback paths being frequency-selective in complementary senses, with the object of eliminating interference or hum originating in the output stage. The signal source 1 is coupled through push-pull valves 3, 4 and additional amplifier stages 5 to a choke 77 connected in the H.T. lead to a radio-frequency power amplifier 8 forming the output stage of a radio transmitter feeding an aerial 10. The amplifier 8 and other stages of the transmitter have the valve cathodes heated by A.C. from a source 13, and it is the primary object of the invention to eliminate the hum caused thereby. Part of the radio-frequency output is fed through an impedance 80, a coaxial line 83 and a transformer 16 to a rectifier 18 with a balanced load resistance 20. The rectified signal is fed back from tappings on resistance 20 through a balanced network B to the grid circuits of the push-pull valves 3, 4. The network B is designed to accentuate the hum frequencies H in the manner indicated by the curve B, Fig. 2, and as the feed-back is negative, these frequencies tend to be suppressed in the transmitter output. This, however, would involve a distortion of the desired signal originating at 1, and to overcome this distortion a second negative feed-back network A is interposed between the output potentiometer 44 of the stages 5 and the cathodes of the valves 3, 4. This network A has an amplitude characteristic shown dotted in Fig. 2, which is the complement of that of the network B, so that the two jointly produce an undistorted amplification of the signal from the source 1. The phase-frequency characteristics of the two networks should be substantially the same, as indicated by the single curve P in Fig. 2. A further negative feed-back from anode to grid circuit of the valves 3, 4 is provided by capacity 95 in series with resistance 96, the object being to reduce the gain and to prevent instability at and above the highest signal frequencies. Condenser-shunted resistors 38 prevent singing at very low frequencies. Networks.-The network A is balanced about earth, and each side comprises a double-T section, one section comprising series resistors 50, 51 and a shunt capacity 52, and the other series capacities 53, 54 and a shunt resistor 55. The second network B is also balanced about earth and comprises a bridge arrangement of four divided resistances 60 ... 67, with the input connected at 68, 69 across one diagonal, and the output taken at 70, 71 across the other diagonal. The junction points along one pair of opposite arms are connected to earth through capacities 99, 100; and the junction points along the other pair of opposite arms are each connected to earth through a series connection of resistor 72 and capacity 73, with a small capacity 93 in shunt. In the network B, the resistors adjacent the input terminals 68, 69 are of smaller values than those adjacent the output terminals 70, 71. The Specification states suitable values for the network components. An output or power amplifier may replace the radio transmitter stages.