Patent GB-815611-A (GB)
Title: Improvements in or relating to electrical signal amplifying systems
IPC: H04B3/08
Filed: 1956-05-25
Abstract: 815,611. Valve amplifying circuits. WESTERN ELECTRIC CO. Inc. May 25, 1956, No. 16245/56. Class 40 (6). A valve amplifying circuit comprises two negative-feedback amplifying channels Á 1 # 1 , Á 2 # 2'  Fig. 1, connected in parallel between a common input and a common output together with a cross-connection # 12  from the output of one amplifier Á 1  to the input of the other amplifier Á 2  and effectively providing an increased negative feedback at the input of amplifier Á 2 . By this means an amplifier is provided having substantially the same gain as either amplifying channel but with considerably increased feedback. Preferably the two channels have substantially identical characteristics, so that, while under conditions of normal operation amplifier Á2 works at a low level only, in the event of failure in amplifier Á, amplifier Á 2  will operate at full level leaving the output of the amplifying circuit substantially unchanged. Fig. 2 illustrates one preferred modified form of Fig. 1 in which the element # 1  serves to provide feedback around the amplifier Á, and also provides the cross-coupling from the output of Á 1  to the input of Á 2 . In an alternative modification, Fig. 3 (not shown), the outputs from the two amplifiers are combined and then fed back to the input of Á 2  through the element # 2 . Fig. 6 illustrates an embodiment of the circuit of Fig. 3 (not shown) comprising two amplifiers V 1 , V 2  each provided with negative feedback by cathode impedances # 1'  # 2'  the impedances #1 '  # 2  being connected in series so that the input to valve V 2  comprises a component derived from the output current of the valve V 1 . Figs. 7 and 8 (not shown) illustrate modifications comprising two-stage and three-stage amplifiers respectively. Figs. 9, 10 and 11 (not shown) illustrate modifications of the circuits of Figs. 6, 7 and 8 in which the second or auxiliary amplifier is fed in anti-phase with the main amplifier and the output connections correspondingly reversed thus resulting in arrangements which are in push-pull apart from the symmetry in the feedback connections. In the arrangements so far described the large input and output impedances of the valve amplifiers are used to isolate the output and feedback signals in each amplifier from the input of the other channel, though the circuit of Fig. 1 shows directional coupling devices 18, 19, 22 and 23 for ensuring the absence of undesired couplings. Figs. 16, 17, 18 and 19 (not shown) illustrate embodiments in which hybrid transformers are used at the input and/or output of the complete amplifier. In further modifications further parallel amplifying channels are provided, each channel other than the first having cross connections to its inputs from the outputs of all the preceding channels. Thus Fig. 20 shows a plurality of parallel-connected three-stage amplifiers, each provided with negative feedback, the feedback impedances of the amplifiers being connected in series so that the signal applied to the input of the final amplifier comprises components derived from the outputs of all the other amplifiers.