GB581887A - Circuit arrangements for obtaining a control or measuring voltage - Google Patents

Abstract

581,887. Automatic volume control. KOLB, O. K. (Electrical Fono-Films Co. Aktieselskab). Aug. 18, 1941, No. 10474. [Class 40 (v)] [Also in Group XXXVI] An arrangement for obtaining a control or measuring voltage logarithmically related to an applied signal voltage comprises a dry rectifier, which operates with a limited logarithmic range, connected in series and matched with one or more thermionic valves which correct and extend the logarithmic range of the device. The transformed voltage may be used for automatic gain control or other purposes. In Fig. 4, a portion of the signal voltage is fed across terminals A], A2 through transformer T1 to the control grid of correction valve K. The output is taken from across the anode load Ra and fed to two dry rectifiers D1, D2 connected in opposition through condenser C and resistance Rd and then to an amplifying valve U. Resistance Ra is adjusted to match valve K and the rectifiers so that the valve K does not affect (other than amplifying) voltages within the strictly logarithmic range of the rectifier. A high ohmic resistance Rg is included in the grid circuit of valve K to allow the flow nf grid current when the control grid is positively biassed, thus limiting the rise in anode current as the potential applied in the grid circuit rises. This has the effect, when the value of resistance Rg is correctly chosen, of changing gradually the characteristics of the valve K from a linear to a longarithmic relationship as those of the rectifier change from logarithmic to linear. Multi-grid valves may be used to increase this degree of correction by (1) using a pentode which also has its screen grid connected through a predetermined resistance Rg4 (Fig. 6, not shown), to the H.T. supply, or (2) using a heptode, Fig. 7, in which control grid G1 is connected in push-pull with G3 and grids G2 and G4 are connected to fixed potentials.