GB481518A - Improvements in or relating to apparatus for measuring electric potentials - Google Patents

Abstract

481,518. Current and volt meters; valve amplifying circuits. STANDARD TELEPHONES & CABLES, Ltd., and JACOBSEN, B. B. Sept. 11, 1936, No. 24813. [Class 37] [Also in Group XL] A valve voltmeter comprises a negative feedback amplifier 3 in which the output is connected to a rectifier 12 through a condenser 8, the continuous - current instrument 11 and a series resistance 9 being in shunt to the condenser across which the voltage to be measured is built up. The instrument 11 can be calibrated in decibels to measure gain of the amplifier in respect of a standard voltage. In Fig. 2, a resistance 14 in the anode and grid circuits of the valve 3 gives a negative feed-back, the measuring instrument 11 and high resistance 9 being connected as before between the grid and filament of rectifier 12. The input and output transformers 1, 6 of Fig. 1 are dispensed with and the circuit is employed for measuring voltages at high-frequencies. In Fig. 3, a feed-back voltage is effected by the output circuit comprising condenser 16 and resistances 17, 18, variations of the proportion of resistances 17, 18 varying the amount of feed-back. If resistance 9 is high compared with the impedance of valve 12, peak-voltage measurements can be made, and the phase change due to transformer 1 kept constant or zero. If a resistance y, high compared with that of 9 is inserted, average volts are measured. Fig. 4 shows a bridge arrangement, the four arms comprising valve 3, and resistors 17 ... 19, the measuring circuit being connected across one diagonal and the feed-back across the other, the amount of feed-back being dependent upon the ratio of resistors 17, 18. Screen grid valves may be employed for the valve 3 to increase the sensitivity of the amplifier, or two separate valves may be employed for this purpose. When screen grid valves are employed the negative feed-back is applied to the screen grid, Fig. 8. In Fig. 7, a dry surface rectifier 12 is used in shunt to the instrument circuit, and where two valves such as 3, 3a are employed as the amplifier, these valves may be of the type described in Specification 478,272, [Group XL]. in this case, while the main feed-back is negative, the resistor 18 will provide a positive feed-back to that stage only in which the resistor is connected. The feed-back can be employed for range-changing if two or more fixed feed-back circuit conditions are incorporated, the sensitivity factor for example being adjusted in powers of 10.