Theory of the Negative Impedance Converter

A N E W type of telephone repeater known as the El has been developed recently to meet the large demand in the exchange area for an economical means of providing transmission gains of about 10 db in two-wire telephone lines. This repeater costs less than the 22 type which has been the standard two-wire, two-way means of amplifying voice currents in the Bell System. The difference in cost is made possible by a difference in operating principle. The El repeater employs a type of feedback amplifier the action of which can be said to have the properties of a negative impedance converter. It is the purpose of this paper to describe the operation of the negative impedance converter, which is a device for transforming positive impedance into negative impedance. Negative impedance like positive impedance can have two components: reactance and resistance. The reactance component can be either positive or negative. However, for an impedance to be negative the resistance component should be negative at some frequency in the range from zero to infinity. The idea of negative resistance originated over 30 years ago, and in the beginning was associated with the concept of resistance neutralization. This concept grew from the observation that a two-terminal device could be found which had an unusual property when inserted in series with a single mesh circuit: it could produce the same flow of current as would flow otherwise at some frequency provided a resistance R were removed from this mesh.