GB554214A - Electric wave amplifiers of the negative feedback type - Google Patents

Abstract

554,214. Repeater systems. STANDARD TELEPHONES & CABLES, Ltd. May 1, 1942, No. 5889. Convention date, Feb. 25, 1941. [Class 40 (iv)] [Also in Group XL] In a negative feedback amplifier for signal and pilot currents applied to the input and feeding an output circuit, selective means are provided in the amplifier to divert pilot frequency current to an auxiliary load and introduce greater loss at this frequency than at the signal frequency. Fig. 4 shows a series network tuned to the pilot frequency, from the inductance 16 of which pilot frequency energy is withdrawn by the coupled coil 14 to an output circuit p<SP>1</SP>. In a modification the pilot frequency is withdrawn through a tuned acceptor circuit arranged in shunt to the valve, Fig. 2 (not shown), and if matching of the output impedance is necessary a similar tuned acceptor circuit may be connected across the outputtransformer primary. Fig. 5 shows a threestage amplifier in which a network 26 provides positive feedback at the pilot frequency to maintain the gain at the pilot frequency if the overall amplification round the negative feedback loop is insufficient to prevent an abrupt variation in gain of the normal amplifier path at the pilot frequency. As shown, the pilot current is utilized to control flat gain and slope control circuits 32, 33 through thermistors 51, 52, and the output and feedback voltages are taken from an output bridge in which a T- section 54, 55, 56 is used to avoid an undesirably high value of resistance for the arm XW. Magnification of the current changes due to the pilot current is produced by the directly heated thermistors 57, 58 and the resistance 59. Supplemental heating may be provided for the thermistors to compensate for changes in ambient temperature. The pilot power may be diverted at a stage in the amplifier anterior to the output stage, Fig. 3 (not shown), to increase the power margin in the latter stage.