GB626191A - Improvements in negative-feedback amplifiers - Google Patents

Abstract

626,191. Valve amplifying circuits. WESTERN ELECTRIC CO., Inc. July 25, 1947, No. 20135. Convention date, Jan. 12, 1943. [Class 40 (vi)] A negative feedback amplifier, unconditionally stable for low gains, is provided with a network including a current dependent resistance so that the sing current developed at higher gains flowing in the resistance modifies the characteristics of the feedback loop to limit the sing current to an unobjectionably low value. As shown, the second and third valves 2, 3 of a three-stage amplifier are coupled by a bridged-T network 8 and negative feedback is provided by a network 6. The shunt arm 20 of the bridged-T network is formed by a thermistor or other current variable resistance, which is heated by the high-frequency sing current flowing in it, thus changing the characteristics of the interstage coupling 8. In a modification, part of the anode current of valve 1 may be tapped off and used as a bias to heat the thermistor either directly, Fig. 2 (not shown), or indirectly, Fig. 3 (not shown). In another modification, Fig. 4 (not shown), the thermistor is heated by current derived from the feedback network through a network, so as to attenuate frequencies near the upper end of the useful 'band and thus ensure against operation by any save the sing frequency. Specification 514,567 is referred to.