The History of Electric Measuring Instruments and Active Components Society of Historical Metrology , Japan
Precision electric meters are indispensable to the development of technology in this age. The first electric measuring instruments, however, were physically impossible to transport, and were functionally inadequate for use in a laboratory. The measuring instruments of today have evolved into sturdy, easy-to-use instruments with higher performance, adopting new active components which have appeared one after another. Expanding in functionality, measuring instruments that have adopted such new technology have enabled even more precise measurement. The users have also expanded from research organizations and educational facilities, to the manufacturers of electrical equipment and electronic parts. The measuring instrument manufacturers were able to supply more convenient measuring instruments to the user, and conversely, the user, the manufacturers of electrical equipment and electronic parts, needed more easy-to-use and highly precise measuring instruments in order to respond to a rapidly increasing demand. Consequently, cheap and high quality electronic parts were supplied to the market, and the measuring instrument manufacturers were also able to develop new measuring instruments using those parts. In fact, it can be said that they have been mutually complementary. It should not be forgotten that the demand for electronic components for consumer use has also driven the whole in the meantime.
Some Applications of Negative Feedback with Particular Reference to Laboratory Equipment
The application of feedback to an entire amplifier rather than just to the final stage makes it possible to realize the characteristics of a perfect amplifier over wide frequency ranges. The use of such amplifiers to give direct-reading audio-frequency voltmeters with permanent calibration and any desired sensitivity is described. Negative feedback can be used to reduce the distortion in the output of laboratory oscillators for all loads from open circuit to short circuit by the expedient of throwing away a part of the output power in a resistive network. Means are described for applying feedback to tuned radio-frequency amplifiers so that the amplification depends only upon the constants of the tuned circuit and is independent of the tubes and supply voltages. The use of negative feedback to develop a stabilized negative resistance substantially independent of tubes and supply voltages is considered, and various applications described. High selectivity can be obtained by deriving the feedback voltage from the neutral arm of a bridge, one leg of which involves a parallel resonant circuit. It is possible by this means to obtain an effective circuit Q of several thousand, using ordinary tuned circuits, and the selectivity can be varied without affecting the amplification at resonance. The use of these highly selective circuits in wave analyzers is considered. Feedback can be used to give improved laboratory oscillators.
A New Type of Selective Circuit and Some Applications
This paper describes the use of the inverse feed-back principle to obtain sharply selective circuits. Important advantages of such circuits are (1) inductances are not necessary, (2) "tuning" may be changed by merely varying resistances, (3) ready adaptability for use at very low frequencies, (4) "tuning" may be varied over wide ranges of frequency while maintaining a selectivity curve which is a constant percentage function of the tuned frequency, (5) the possibility of using a single set of frequency-determining elements for "tuning" several amplifying stages, and (6) the general simplicity of construction and operation when compared with many other types of equipment designed to produce equivalent results. Many uses for a circuit of this type immediately suggest themselves. Two important applications are described, including a novel type of analyzer and an oscillator having extremely good wave form.