514,567. Valve circuits ; impedance networks. STANDARD TELEPHONES & CABLES, Ltd. May 10, 1938, No. 13946. Convention date, June 22, 1937. [Classes 40 (iii) and 40 (v)] In a negative feedback amplifier, the passive networks in the feedback circuit and in the intervalve couplings are designed to give uniform and low attenuation in the working range, where the phase-shift changes with frequency, and rapidly increasing attenuation with constant phase-shift near the limits of the working range. Preferably the networks are of a type which gives minimum phase-shift for a given attenuation. Minimum phase-shift impedance networks.- The Specification shows that the expression for the voltage transfer ratio, or the voltage insertion loss of a four-pole network, can be written as the sum of two parts, P1 and P2, where P2 has a variable phase angle and constant modulus (i.e. it corresponds to an all-pass constant-resistance network) while P1 corresponds to that network which gives a minimum-phase-change characteristic for any given attenuation characteristic ; and that the former characteristic can be deduced from the latter without arbitrary choice. Equations (9) and (10) of the Specification enable this deduction to be made from any assigned attenuation characteristic, preferably by reference to the slope of the attenuation characteristic plotted on a logarithmic scale of frequency and measured in decibels per octave. Networks according to the invention have attenuation characteristics which conform as nearly as possible to an optimum characteristic which, for a band-pass network, is given by A=0 between f1 and f2 and A=k loge {# (q< 2> -1) + q (q<2>-1) + q) } below f1 and above f2 wherein A is the attenuation, f1 and f2 are the limits of the working range of the amplifier, k > 2, and q=(f<2>-f1, f2)/f(f2-f1) f being the frequency. For high-pass and lowpass networks the formula for q assumes limiting forms, and corresponds to networks of the type described in U.S.A. Specification 1,724,987. The phase-shift characteristic B, Fig. 4, deduced from this attenuation characteristic A reaches Œ180‹ (a value which, in a threevalve system, gives the maximum tendency to instability) at high and low frequencies if k=2 (the value of k assumed in Fig. 4) but allows a larger margin for stability if k < 2. Elsewhere B=k sin<-1>q. Networks having actual characteristics approximating to the foregoing basic or idealized characteristics are calculated by known methods. Two-terminal shunt arms Z1, Z2, ZB, Fig. 7,. are so designed, Figs. 8 ... 11 (not shown), that in conjunction with the transformers. T1, T2 they constitute a network of the kind described. The intervalve couplings Z1, Z2, Fig. 7, and feedback circuit ZB are designed conjointly with the transformers T1, T2 so as to be equivalent to a minimum-phase-shift network of the kind described above wherein the attenuation is uniform and low in the working range but rises rapidly according to a formula prescribed under the invention at the limits of the working range. For a narrowband amplifier a band-pass characteristic, Fig. 4, may be adopted, but for wide-band working a low-pass characteristic may be combined with a high-pass characteristic. Near the bandlimit, parasitic impedances become important, giving on a logarithmic scale a linear asymptote at high frequencies and the design of networks whose characteristics are combined with minimum-phase-shift characteristics of the kind described above is discussed in the Specification with reference to Fig. 6 (not shown) for a widerange low-pass amplifier. Fig. 7 shows an amplifier with a working range from 60 to 2000 kc./sec. in which the two terminal arms Z1, Z2, ZB have the forms given in Figs. 8 ... 11 (not shown), and the idealized characteristics with their realized approximations are given in Fig. 12 (not shown). The invention is also applied to a narrow-band amplifier, Figs. 13, 14 (not shown), in which the negative feedback circuit comprises shunt reactances and a high series resistance. The invention may be applied to systems including frequency-changing stages ; thus the negative-feedback path may, Figs. 15, 16 (not shown), include a subsidiary local aerial or other means for picking up the transmitted radiation, a demodulator, a wide-band amplifier with negative feedback, and a correcting network, designed as described above, whereby the feedback is applied in series with the speech input voltage fed to the transmitter. Specification 374,130 is referred to. The Specification as open to inspection under Sect. 91 also states that attentuation characteristics of the foregoing type may be combined with one another and with characteristics identical with the asymptotes of similar curves to yield further ideal characteristics such as those shown in Fig. 5 (Cancelled) where the attenuation A is given by A=k log [{ #(f<2>/f1<SP>2>-1) +f/f1}/{# (f2<SP>2>/f<2>-1) + f2/f}f/2f2] while the phase-shift B=k Sin<-1>(f2/f). This subject matter does not appear in the Specification as accepted.