Patent GB-845271-A (GB)
Title: Improvements in or relating to non-linear amplifiers
Inventor: HUTCHINSON GEORGE WILLIAM
IPC: H03F1/36
Filed: 1957-02-20
Abstract: 845,271. Non-linear amplifying and pulse discriminating circuits. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Feb. 8, 1957 [Feb. 20, 1956], No. 5152/56. Class 40(6). In a circuit of the type giving a constant output for input signals below a threshold value and output signals at terminal 5 of increasing amplitude for inputs exceeding the threshold, a linear amplifier 2 Fig. 2, is followed by non-linear elements 3, 4 with feed-back loops to the amplifier 2. By providing a positive feed-back connection 6 the circuit passes rapidly through the discontinuity which occurs around the threshold value and the negative feed-back 7 ensure linearity between input and output beyond that value. The circuit is described as applied to a pulse height analyser in which the elements 3, 4 are formed by discharge tube pairs 13, 14 and 17, 18 Fig. 3, with the tubes 14, 18 conducting and due to the common cathode loads the corresponding tubes 13, 17 are cut off. Tubes 11,12 operate as a cathode coupled pair to form the amplifier 2 so that a positive-going input pulse applied to tube 11 appears in amplified from at the anode of tube 12. Since tube 13 is cut-off no output appears until the signal fed to the grid of tube 13 is sufficiently positive to cause it to pass current. The increasing current through tube 13 causes the cathode potential to rise and feed-back occurs over capacitor 33 to the screen grid of tube 11 and the circuit 11, 12, 13 becomes unstable. This effect is enhanced by feeding the potential change occurring at the anodes of tubes 13, 14 to the grid of tube 16 which operates as the common cathode load. The rise in cathode potential of tubes 13, 14 is applied over coupling 27, 28 to the grid of tube 17 which rises in potential until it passes anode current causing the cathode potential of tubes 17, 18 to rise. This rise is fed over path 7 and attenuating networks of resistors and capacitors to the grid of tube 12 to provide a negative feed-back effect. With the tubes 14, 18 cutoff the circuit operates to provide a linear relation between output 5 and input 1. The rise in cathode potential of tubes 17, 18 is fed to the screen grids of tubes 12, 13 and 17 which is stated to provide a stabilizing effect and to allow larger output pulses. During the transition, when the input is such that the negative feed-back loop is established but tube 18 is not completely cut-off, some attenuation occurs between the grid and cathode of tube 17 for small increments of signals so that a step occurs in the response of the circuit. In order to provide a separate discriminator output the input to tube 17 is supplied to a double triode 20, the coupling 61, 62 providing an output with a rapidly rising leading edge. The step in the response may be obviated by coupling the grid of tube 18 to the anode of the first triode of tube 20. The circuit may be gated by connecting a pentode, which is normally cut-off, to the anode load 25 so that when the pentode is conducting the control grid of tube 13 cannot be driven by tubes 11, 12 sufficiently positive, for tube 13 to conduct. The gain of the circuit is controlled by the setting of ganged switches S1, S2 and the point at which the discontinuity in the response occurs is determined by the bias applied to the tubes 11, 12 and may be varied by adjusting the potential to which resistor 10 is connected or the cathode resistor 23. The resistors 22, 23, may be replaced by a pentode and positive feed-back may then be taken from an anode of tube 20 and fed to the control grid of tube 12 or the grid of the pentode. It is stated that the diodes 15, 21 which provide D.C. conditions for tubes 11, 12, 13 and 17 may be replaced by cathode followers.