Resistance neutralization: An application of thermionic amplifier circuits (1922)

A power amplifier may be so connected to the circuit supplying the power to be amplified that it introduces terms into the equation for the current in the power circuit which subtract from the resistance terms. Upon this fact depends the operation of the device as a generator of sustained oscillations, as a regenerative amplifier, and as a resistance neutralizer. In the treatment of oscillating audion or triode circuits this fact has been pointed out by a number of writers but the more general significance of these subtractive terms has never been discussed. This paper points out that, by making use of the idea of resistance neutralization, circuits and systems having all the properties of low-resistance systems and also a number of other unique properties may be obtained. The first part of the paper derives and discusses the current and power relations which obtain in circuits having a resistance neutralizer associated with them. One of the things brought out is that not only does the neutralizer supply power to the circuit but it also causes the generator or source of driving e. m. f. to furnish more power to the circuit than it would were the neutralizer not present. Thus if the source of driving e. m. f. is the impinging of electromagnetic waves upon a wireless antenna the neutralizer not only amplifies the power received but it actually causes the impinging waves of the correspondent station to give up more power to the antenna circuit while it causes the waves from detuned station to give up less power to the receiving circuit. The power relations cited above while important are not the most important results obtained by resistance neutralization. At resonance, the power delivered to a wireless receiving system by impinging waves is inversely proportional to the net resistance, while the power received from detuned stations is practically independent of the net resistance. The neutralizer, by lowering the net resistance of the receiving system, thus causes the ratio of signal to interfered power to increase. In a simple series antenna circuit the ratio of signal to interferent power, where the interferent source may be either atmospheric strays or interferent station, is a function of (L/R) <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</sup> where n is a positive number. The neutralizer thus increases the selectivity of the receiving station against all types of disturbances. A general physical argument is given (paragraph 7) to show that a triode may be made to function as a resistance neutralizer. This physical argument is illustrated by a mathematical treatment of the steady and transient states for a particular method of associating the triode with the circuit in which neutralization is desired. This mathematical treatment shows that under all conditions both in the steady and transient state, the neutralizer circuit functions so as to reduce the net resistance of the circuit of interest to some predetermined value. A numerical example is given to illustrate the mathematical theory. In this particular circuit the ratio of the signal to interferent power in the steady state is increased 200 fold by the insertion of the neutralizer. Section (II) of the paper treats of the optimum conditions obtaining in receiving circuits containing a resistance neutralizer, and gives relations for the designing of an antenna and its circuits so as to obtain the maximum selectivity and maximum power abstraction from impinging waves. The last topic discussed (Section 12) deals with a circuit which neutralizes resistance for a narrow range of frequencies near a desired frequency and introduces resistance and reactance into the circuit for frequencies removed fom this band of frequencies. A numerical example is given to illustrate the theory. The performance curves of the neutralizer are given by Figs. 12 and 13. Fig. 14 shows the ratio of the power received at resonance to the power received from a detuned station by a given antenna circuit; first, without a neutralizer, second, with a pure resistance neutralizer, and third, with a selective neutralizer. For example, if the interferent source is detuned by 3 per cent the circuit without the neutralizer gives practically 1 for the ratio of signal to interferent power; with pure resistance neutralization the ratio is about 1300; with the selective neutralizer the ratio is increased to 3000.