Patent GB-472951-A (GB)
Title: Improvements in or relating to electrical measuring instrument arrangements
IPC: H03F1/36
Filed: 1937-04-07
Abstract: 472,951. Valve amplifying circuits. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. April 7, 1937, No. 9900. Convention date, April 7, 1936. [Class 40 (v)] [Also in Group XXXVI] An instrument which may be used for accurately measuring very small currents, such as micro-amperes, or for use as a high-impedance voltmeter comprises a D.C. amplifier, the voltage output of which is applied to the input circuit to provide negative regeneration, or degeneration. As shown diagrammatically in Fig. 1, a D.C. amplifier 5 has voltage output terminals 6, 7, input terminals 8, 9, and input leads 10, 11. The output terminals 6, 7 are connected through feed-back leads 12 to the input circuit so that the voltage impressed across the input circuit is opposed by the voltage fed back from the output circuit. Voltage changes, e.g. those due to changes of supply voltage or changes of valve characteristics, are thereby neutralized. The current gain is in the ratio of the input and output resistances or impedances R1, R0. In the construction shown in Fig. 2, a " high-mu " pentode valve 14 is connected to a " high-mu " triode output valve 15 through a coupling resistance 16. Between the input terminals 20, 21 is connected one of a series of input resistances 22 selected by a switch 23, these resistances being in multiples of 10 to give different ranges of measurement. The output resistance 28, 29 is connected in the cathode lead of the output valve 15. A tap 34 connects the portion 28 in series opposition in the input circuit. The output current is measured by means of a milliammeter 50. Fig. 3 shows an arrangement suitable for use with a rectified A.C. supply B. A pair of high-mu pentodes 55, 56 are connected in cascade as a D.C. amplifier between the input circuit 59 and a controllable oscillator 60, the output of which is rectified by a rectifier 53 and applied to the input circuit with degenerative coupling. Biassing potentials are obtained from resistances 74, 72, of which the latter is shunted by a voltage-regulating discharge tube 75. The output resistance 66 for the diode rectifier 53 is shunted by a condenser 80 for stabilizing and smoothing the output voltage. A milliammeter 81 is connected across the output of the rectifier and in series with a portion of the resistance 72. In order to obtain a zero reading a tap 71 is adjusted so that its positive potential balances the normal output potential from the oscillator through the rectifier 53. The rectifier output resistance 66 may have a tap 85, and the transformer 65 may have a higher voltage secondary terminal 86. With the anode connected to the point 86 and the lead 70 connected to the tap 85 instead of the terminal 90, a fraction, e.g. one hundredth, of the rectifier output may be fed back to the input circuit, so that an accurate voltage gain of 100 may be obtained. In the arrangement shown in Fig. 4, the input circuit is connected through a filter formed on a series resistance 94 and shunt condenser 95 with the control grid 96 of a first stage high gain pentode amplifier valve 97, the anode 102 of which is connected to the control grid 106 of a second pentode valve 107. This is followed by a triode output valve 110, the cathode 111 of which is connected through two series connected neon tubes or other glow discharge tubes 117 to the cathode 98 of the valve 97. The tubes 117 provide a sufficiently high potential drop to raise the cathode of the output valve sufficiently high to permit direct connection of its control grid 109 with the anode of the preceding valve 107. The output meter 121 is connected across the output resistance 99, which is in the cathode circuit of both the input and output valves. The milliammeter 121 is in series with a cell 127 and one of a number of resistances 122, 123 selected by a switch 124. The cell 127 opposes the potential across the output resistance 99, enabling a zero reading to be obtained. Exact balance is obtained by adjusting the screen grid potential of the valve 97 by a variable resistance 103, and by varying the anode current by a variable resistance 120.