GB544175A - Improvements in negative feedback thermionic amplifiers - Google Patents

Abstract

544,175. Valve amplifying circuits. STANDARD TELEPHONES & CABLES, Ltd. Jan. 31, 1941, No. 1287. Convention date, Feb. 2, 1940. [Class40 (v)] In a negative feedback amplifier noise components arising from the power supply, which are normally increased due to the feedback connection, are reduced by means which impress the noise components on a grid of the amplifier in opposite phase. Fig. 3 shows a two-stage push-pull amplifier with negative feedback applied by resistances 39, 40 in which a condenser potentiometer 41, 43 applies. part of the noise voltage from the H.T. source to the grids of the output stage. In a modification, Fig. 4 (not shown) a positive feedback is applied through resistances connected between the output anodes and the centre tapping of the input transformer which is connected through a resistance to, earth. With this arrangement the voltages feedback cancel out in so. far as the push-pull stage is balanced and the residual noise voltage is in phase opposition to that introduced into the circuit by the negative feedback coupling. Fig. 5 shows an arrangement using two.. negative feedback paths from each output stage comprising resistances 116, 117 between plate and cathode and resistances 120, 121 cross connected between plate and grid. In a further modification, Fig. 6 (not shown) part of the noise voltage occurring In the anode supply is applied to the centre tap of the input transformer. With this arrangement low frequency voltages arising across the filter output condenser are also fed back thus preventing any tendency to low frequency oscillation due to these potentials. Fig. 7 shows an arrangement in which an inverter tube 244 is used to. feed one side of a push-pull output. In this arrangement, as shown, negative feedback is applied from each output anode to the cathode, and to the screen grid respectively of the input valve 241 through resistances 252, 255. Since the noise voltages in the two paths are in phase these may be arranged to balance one another in the output. Instability at high frequencies is avoided by providing condenser 271 between the anode of the inverter 244 and the anode of the input valve 241. The second feedback instead of being to the screen grid may be to the control grid. A resistance-capacity network 263, 264 may protect the output tubes from excessive transient voltages and the consequent loss at high frequencies may be compensated by reduced feedback at these frequencies by suitable choice of the condenser 259. The invention is also applicable to single side circuits. The Specification gives a mathematical analysis to show the conditions under which the hum voltage across the load is reduced to zero.