Negative Impedances and the Twin 21-Type Repeater 



By GEORGE CRISSON 



This paper discusses negative resistances and impedances. It describes 

 their properties and some devices by which they may be produced physically. 

 Certain properties of negative impedances when used as series and shunt 

 boosters for amplifying speech waves in telephone circuits are discussed. 

 The paper concludes with a description of the circuit and properties of the 

 twin 21 -type repeater. 



WHEN an e.m.f. is applied to the terminals of an ordinary positive 

 resistance a current flows in at the terminal connected to the 

 positive pole of the source and out at the other terminal. This direc- 

 tion of current flow is considered positive and the value of the resistance 

 R, in ohms is given by Ohm's law as R = E/I where E is the applied 

 voltage and / is the current in amperes. Similarly a definite current / 

 may be passed through the resistance and a potential difference or drop 

 E = RI will appear across its terminals. With positive resistances it 

 makes no difference whether we "apply an e.m.f." or "pass a current". 

 The resistance may be a very simple device such as a coil of wire which 

 absorbs energy from the circuit at a rate W = EI — PR watts. 



It is possible, however, to construct assemblages of apparatus which 

 have the property of keeping the ratio of the voltage across a pair of 

 terminals to the current at the terminals constant, but with the relative 

 direction of the voltage and current opposite to that which a positive 

 resistance would give. In such devices the resistance is negative and 

 the apparatus contributes power to the circuit with which it is con- 

 nected. Each such device necessarily includes a source of energy such 

 as a battery and some means such as a vacuum tube for controlling the 

 delivery of this energy to the circuit. There are two varieties of such 

 devices. In one case, the internal arrangement of the mechanism is 

 such that, if a definite voltage is applied to the terminals, a current 

 flows in a direction opposite to the applied e.m.f. In the other, if a 

 definite current is passed through the system, the drop across the 

 terminals will be opposite in direction to that caused by a positive 

 resistance. These two arrangements are essentially different and 

 cannot be used interchangeably in a given circuit, though either one can 

 give any desired value of negative resistance. If the wrong arrange- 

 ment is used instability or singing will occur. To know whether a 

 given negative resistance will work satisfactorily in a given circuit it is 

 not sufficient to know its value in ohms. Something must be known 



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