GB622830A - Improvements in or relating to electro/acoustic transducer arrangements - Google Patents

Abstract

622,830. Valve amplifying circuits. AIRMEC LABORATORIES, Ltd., (formerly P.R.T. LABORATORIES, Ltd.), SOLLEY, B. J., and ROBERTS, P. W. July 22, 1946, No. 21865. [Class 40 (vi)] [Also in Group XXXIX] An electro-vibratory transducer such as a capacity microphone is coupled to a cathode follower amplifier between the control grid and the common input and output terminal of the stage, the polarizing potential for the microphone being provided by a grid-leak resistor connected between the grid connected terminal of the microphone and a point at a suitable difference of potential from that of its other terminal. In this way increased sensitivity and an improved frequency response are obtained. The Figure shows a capacity microphone 16 coupled to a cathode-follower pentode amplifier stage, the grid-leak resistor 19 being connected to a tapping point on the cathode load resistor so that correct biassing is obtained for the valve and the voltage across the cathode resistor 15 supplies the necessary polarizing potential to the microphone. Under these conditions the effective input resistance of the stage is very high and the effective input capacity very low. In a modification, Fig. 2 (not shown) the terminal of the microphone which is not connected to grid is connected to the positive high-tension supply terminal. In a further modification, Fig. 3 (not shown), the microphone is connected between the control grid of the valve and the earthy end of the cathode load resistor and the grid-leak resistor is returned to the positive high-tension supply. The apparatus may be used for measuring the average roughness of a surface, one electrode of the transducer being fixed and the other moved by a feeler or stylus in engagement with the surface under examination. A high slope H.F. pentode may be used in the cathode follower stage.