# Патент USA US2410077

код для вставкиOct- 29, 1946- 2,410,076 K. s. JOHNSON NEGATIVE IMPEDANCE CIRCUIT Filed March 5, 1943 AAAAAAAAAAAAAAA AAAAAAAAAAA "II I" IIYIIVIYVV'VYVVV VVY F163 riAr AlAAlAAAllA ""1""! //v VENTOR K S. JOHNSON BY, ATTORNEY 2,410,076 UNITED STATES PATENT OFFICE - Patented Oct. 29, 1946 2,410,076 NEGATIVE IMPEDANCE CIRCUIT Kenneth S. Johnson, South Orange, N. 1., aa signor to Bell Telephone Laboratories, Incorpo rated. New York, N. 'Y., a corporation 0! New York Application March 5, 1943, Serial No. 478,113. 8 Claims. (Cl. 178-44) The present invention relates to circuits using negative impedance and more especially to the provision of auxiliary impedance, which may comprise a network'of impedances, tor modify ing the characteristic of the negative impedance. One known‘ type of negative impedance device 2 Fig. 5 shows one method of coupling a ther mistor to a line according to the invention; Fig. 6 shows a circuit for utilizing a thermistor in accordance with the invention; and ' Fig. 7 shows curves to be referred to in the de .scription. Fig. 1 is the approximate circuit equivalent of consists or a substance having a negative tem a thermistor. The thermistor when suitably perature coeilicient of resistance connected in biased to its negative resistance region behaves circuit with a source or biasing current which like a series positive resistance R shunted by a raises the temperature of the substance to such 10 negative resistance —r and inductance L in series a point that its resistance changes in response to with each other. As stated, it is the temperature small impressed current ?uctuations in such time lag which imparts to the thermistor the re manner that the terminal voltage across the sub actance e?ect represented in the equivalent cir stance decreases as the current through the de cuit by the inductance L. As the frequency in- I vice increases. In other words, the device op creases, the negative resistance effect eventually erates on a falling ‘portion of its volt-ampere disappears and the thermistor behaves as an characteristic. This type of negative impedance ohmic resistance R. In terms of the thermistor device has been termed thermistor because its re this occurs when the frequency exceeds the sistance changes as a function of its temperature. highest frequency which the temperature varia 20 This and other types of devices that exhibit a tions can follow. In terms of the equivalent characteristic with a region in which the slope is circuit, this occurs at the frequency at which the downward are referred to as negative resistance impedance of the inductance L becomes so great devices since the derivative of voltage with re that substantially no current can traverse the spect to current variation is negative in sign. path through -r and all of the current must take It has been shown that the time lag of heating the path through R. inherent in a thermistor is equivalent to an in In accordancewith the invention, this time lag ductance in the equivalent electrical network. of the thermistor is reduced or substantially neu For some purposes this time lag is undesirable tralized by connecting in shunt across the ther and what is desired is a negative resistance with 30 mistor terminals a circuit comprising a positive substantially no reactance. resistance of numerical value nearly equal to r in It is an object of the present invention to com series with a capacity C of proper size, as indi pensate or neutralize to a material degree the cated in Figs. 2 and 3. It can be shown that the reactive component oi’ impedance of a negative impedance Z of the circuit of Fig. 2 (which is resistance device such as a thermistor. A further object of the invention is to provide a negative resistance whose magnitude varies with frequency. 35 identical to the circuit of Fig. 3 but uses a differ ent notation) is _ Thermistors that are suitable for carrying out the objects of this invention may comprise any of a number of known substances including par __L__ ticularly semiconductors, such as silver sulphide, / C—r(r+Ar) boron, oxides of uranium or other substances or, using the notation of Fig. 3, having negative temperature coe?icients of re sistance. Such thermistors have been con 45 Z_ .(r+ Ar):c+ry structed having a frequency response range cov ering the range of commercial speech and even extending to many kilocycles per second. One type of thermistor construction capable of re, sponding to frequencies of this order 01.’ band 50 width is disclosed in United States patent to G. L. Pearson 2,276,864, granted March 17, 1942, “7 Ar+i (av-21) where any = r(r + Ar) As Ar approaches 0, Z approaches the value T(Lw)’+r’ (Lay-r“ The nature and objects of the present inven which, since it contains no reactance term, rep tion will appear more fully from the following de tailed description together with the accompany 55 resents a pure resistance whose magnitude varies with frequency. Moreover, the resistance can be ing drawing in which: positive or negative depending upon the fre Figs. 1 to 4, inclusive, are impedance network diagrams to illustrate how the equivalent circuit ‘ quency. This effect is illustrated by the graphs in F18‘. 7. of a thermistor may be combined with a‘ com Curve '1‘ is the measured curve of a boron ther 60 pensating network according to the invention; 2,410,078 4 mistor without the compensating shunt circuit frequency variations into amplitude variations. of the invention. Its resistance is negative at all of the frequencies plotted but the value is de creasing with increasing frequency so that at some frequency in the neighborhood of '7 kilocy cles its resistance will turn positive. Solid line curve A is in two parts (A1, A1‘) and represents A source of variable frequency is shown at II which may in practice comprise a vacuum tube oscillator with a movable condenser plate for varying the frequency. The output of source II is sent over a circuit or line 2| to the point at which the variations in frequency are to be indi the ideal case in which A1‘ is zero and, therefore, cated. Resistance 22 (which could be replaced by transformer I l and condenser ll of Fig. 0, if desired) is for coupling the line H to the 1' is numerically exactly equal to —r. This curve passes from minus in?nity to plus in?nity in go-' ing through a frequency of about 5 kilocycles. indicator circuit comprising thermistor units II This curve represents an unstable case and can to I8 (Fig. 4) and a suitable current responsive not be actually realized in practice but can be approached. element shown for illustration as a moving coil Dotted curve B is a curve calcu~ lated for the case where Ar has a small positive 15 instrument 23 having a pen 24 movable across a paper 2! fed along by rollers 2| to make a value large enough to give stable operation. This curve is interesting in exhibiting increasing neg ative resistance eifect with rising frequency over a considerable range below 4 kilocycles and a steeply rising characteristic between 4 and 5 kilo~ cycles, this steep portion being entirely in the negative resistance range for several hundred cy cles. This offers the possibility of a circuit with net gain and with its gain varying rapidly as a function of frequency. The computations for 25 these curves assumed values as follows: record of the frequency variations. The frequency of source 20 may be varied by hand to transmit signs or signals or may be varied by a meter, ?oat, steering wheel or any other movable member whose position or move ment is to be indicated at a distance. The re sulting changes in frequency are translated by the thermistor circuit into amplitude variations which are recti?ed by any suitable type recti?er 21 shown as a solid element rectifier, such as copper oxide. The recti?er current flows through R=600 ohms the colic! the instrument 23 and the alternating current, component flows through shunting con denser 28. If operation is desired in accordance L=0.00577 henry C=0.096 microfarad —r=-240 ohms 30 with curve B of Fig. 7, the source 20 may have a mean frequency slightly above 4 kilocycles (e. g. The curve A was computed for r=240 ohms and 4,200 cycles) and the maximum range of varia curve B was computed for r=250 ohms. tion may be between 4,000 cycles and 4,400 cycles. In the case of curve B since the neutralization Over this entire of the inductive reactance L is incomplete, the impedance Z is not a pure resistance but has 35 and varying at mistor acts as a phase angle varying with frequency. The real time as a very or resistance component is plotted in curve B. changing frequency modulated waves to ampli tude modulated waves. By using two channels like the one in Fla. 6 and suitably coordinating them, a telautograph system may be provided. Other applications of the invention will occur to persons skilled in the art, from the illustrative examples that have been given showing the The invention provides what is believed to be a highly novel circuit combination or unit capable of general application. This unit is illustrated in Fig. 4 as comprising a thermistor l0 having a battery II and variable resistance I! for applying an adjustable bias to the thermistor to bias it to the desired point along its negative resistance The current. may be passed through the thermistor -by a conductive path or . characteristic. range the resistance is negative a steep rate so that the ther an ampli?er and at the same eil'ective conversion circuit for 45 principle of operation. ' ~ The invention is not to be construed as limited to the speci?c forms that have been dis closed but the scope is de?ned in the claims, applied to a heating winding surrounding the thermistor. Choke coil i3 is for preventing cur rent variations representing signals or other which follow. neutralizing circuit already described comprises current supply circuit for causing said thermistor waves to be ampli?ed from being shunted through 50 the bias current circuit. The compensating or ‘ What is claimed is: 1. A circuit comprising a thermistor and a bias to develop an impedance effect having a negative resistance component and an inductive compo ance and positive inductance e?’ects inherent in 55 nent, and a circuit connected in shunt relation to said thermistor and comprising positive resistance the thermistor operation as already set forth. and capacitive reactance proportioned to sub Fig. 5 shows a circuit for introducing into a stantially neutralize the inductive reactance of line or circuit represented by the fragment of said thermistor impedance. line II a resistance or a gain varying with fre a resistance l4 and capacity it proportioned in magnitude with relation to the negative resist quency over a definite range. The thermistor is . - 50 coupled to the line I8 through transformer I‘! inserted between the thermistor and a bridge across line l6, including condenser II which may 2. A circuit comprising a thermistor and a bias current supply circuit for causing said thermistor be a stopping condenser or a tuning condenser to develop an impedance effect having a negative resistance component and a reactive component, and a circuit connected in shunt relation to said i‘! may serve to step the impedance up or down and a reactive impedance proportioned to sub for tuning the shunt branch. The transformer 65 thermistor and comprising positive resistance stantially neutralize the reactive component of said thermistor impedance. resistance having a highly positive temperature 3. In combination a thermistor, means to bits coe?lcient of resistance to insure stability if 70 said thermistor to the negative resistance region of its volt-ampere characteristic whereby said high local circulatory currents should tend to as between the line and the thermistor circuit. The resistance I‘ may be partly or wholly a thermistor exhibits negative resistance over a given frequency range with an inductive re actance, and a passive network connected in sated themistor of the invention for converting 75 shunt across said thermistor of such character 6 shows a circuit for using the compen 2,410,076 istic that the combination thermistor and net work has a resultant impedance closely approach ing a pure negative resistance whose magnitude comprising a compensating network in shunt relation to said thermistor comprising positive resistance and capacitance. 7. In combination with a circuit carrying cur rents of di?erent frequency, a circuit unequally responsive to currents of di?erent frequencies comprising a thermistor shunted by resistance and capacitance of such value as to give a result ant impedance approaching a pure resistance shunt across said device, said positive resistance being nearly equal in magnitude to the negative 10 whose magnitude varies markedly at said dif ferent frequencies. resistance of said device and said capacitance 8. A circuit comprising a pair of terminals and being of such value that said inductive compo two parallel branches connected between said nent is substantially neutralized. terminals, the ?rst branch comprising a negative 5. A frequency-amplitude conversion circuit ' comprising a negative resistance device having 15 resistance device which is electrically equivalent in impedance to a pure resistance connected be inherent reactance and a compensating network tween said terminals, shunted by a negative re having resistance and reactance shunted across sistance of value --1' and an inductance L in said negative resistance device for giving a result series with each other, and the second branch ant impedance approaching a pure resistance consisting of a resistance r+Ar in series with a whose magnitude varies with frequency. ‘ 20 capacitance C of magnitude given by 6. A circuit for converting frequency variations to amplitude variations comprising a thermistor varies with frequency over said frequency range. 4. In combination, a negative resistance device whose impedance includes an inductive compo nent and means comprising a positive resistance and a capacitance in series with each other in of sumciently high response speed to permit its temperature to follow said frequency variations, said thermistor exhibiting an inductive reactive 25 effect, and means to reduce said inductive e?ect

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