THEORY OF NEGATIVE IMPEDANCE CONVERTER 89 



In the early days of vacuum tube development the negative resistance 

 effect was considered to be an important one. Possibly the regenerative vac- 

 uum tube circuits associated with the early radio receivers stimulated interest 

 in the subject. One of the first text books on the theory of vacuum tube cir- 

 cuits^ devoted about as much space to regenerative means for producing 

 negative resistance as it devoted to the theory relating to any one of the 

 more conventional devices — namely: amplifiers, oscillators, modulators and 

 detectors. In spite of the interest in the subject, little practical use was made 

 of the negative resistance theory. 



Negative resistance cannot be completely disassociated from reactance. 

 A vacuum tube circuit arranged to develop negative resistance will present 

 reactance as well at some frequencies, and the effect on an external circuit 

 at these frequencies will be that of taking away resistance and adding or 

 subtracting reactance. At high and low frequencies the circuit may present 

 a positive impedance. Consequently, the term negative impedance is used 

 herein to designate the effect produced by a two-terminal device which has 

 the property of negative resistance at some frequency or frequencies, nega- 

 tive resistance plus reactance at other frequencies and positive impedance 

 at still other frequencies. 



The Negative Impedance Converter 



Heretofore, many vacuum tube circuits have been devised for converting 

 positive impedance into negative impedance. All known simple circuits 

 employing vacuum tubes for obtaining negative impedance, as distinguished 

 from the combination circuits which can be made up either of vacuum tubes 

 or of negative elements found in nature, have much in common and can be 

 treated as the same type of arrangement. Essentially, this type of arrange- 

 ment is a feedback amplifier and can be treated as such. Recently, a new 

 method of handling these devices has been developed which has the merit 

 of simplifying computations in many cases. This method is based upon the 

 fact that vacuum tube circuits devised for converting positive impedance 

 into negative impedance can be reduced to an electrically equivalent, four- 

 terminal network consisting of a combination of positive impedance ele- 

 ments together with an ideal negative impedance converter. 



This ideal converter. Fig. 1(a), resembles a form of transformer: it has a 

 ratio of transformation of —k:l, can have four terminals and is capable of 

 bilateral transmission. Assume that a positive impedance Zn is connected 

 to terminals 3 and 4 and —HZn is seen at terminals 1 and 2, Fig. 1(b). Then 

 it must follow from the theory described herein that if a positive impedance 



^L. J. Peters — Theory of Thermionic Vacuum Tube Circuits — McGraw-Hill Book 

 Company— 1927. 



