GB610105A - Negative feedback amplifier - Google Patents

Abstract

610,105. Valve amplifying circuits. STANDARD TELEPHONES & CABLES, Ltd. March 23, 1946, No. 9021. Convention date, April 14, 1945. [Class 40 (vi)] Instability in a negative feed-back amplifier is prevented by combining with the normal feed-back voltage, an additional voltage such that the resultant is a net feed-back voltage substantially 180 degrees out of phase with the input voltage over the whole of the frequency range. In this way the overall amplification is directly proportional to the amplification of the forward path and the effective input voltage is in phase and in a constant ratio to the signal input over a range of frequencies. In Fig. 2, V1 is the input voltage without feed-back, V2 the normal feed-back voltage and V the net input voltage with feed-back, an additional voltage V3 being added so that the resultant feed-back voltage V4 is 180 degrees out of phase with V1 and the system is stable. In Fig. 1, the main and correlative amplifiers 9, 21 have identical gain and phase-frequency characteristics so that the voltages V1 and Vc across potentiometers 10 and 22 are in phase and equal in amplitude. The valves 26, 27 comprise a phase mixer so that when voltage Vc and V (Fig. 4) are applied to their grids, a resultant voltage V3 appears at their common anode circuit, the voltage being amplified and applied to the input circuit of the main amplifier as a voltage V3. By plotting a polar diagram of the magnitude and phase angle of ÁB over a given range of frequencies, the frequency at which # (the phase angle of ÁB) is 180 degrees can be found. A voltage at this frequency is applied to the input of the main amplifier and potentiometer 22 is adjusted until the output of the phase mixer is zero, i.e. Vc is in phase opposition to V. From the polar diagram of ÁB the ratio of the scalar value of ÁB at # = 0 degrees and # = 180 degrees can be determined. A voltage is fed into the input of the main amplifier at the frequencies for which # =0 degrees and # =180 degrees and the contact 43 is adjusted until the ratio of the output voltages of the main amplifier is equal to the above-mentioned ratio of the two scalar values of ÁB. In a modification, Fig. 7 (not shown), a multi-grid mixing valve is used in the input circuit of the main amplifier to combine the voltages V1, V2 and V3.