United States Patent 119] [111 3,881,149 Kiko [451 Apr. 29, 1975

1541 COMPENSATED TRANSFORMER CIRCUIT 3.818338 6/1974 Chambers. Jr. et a1 ...... 323/6 X [75] Inventor: Frederick J. Kiko, Sheffield Village. Ohio Primary [imminer-Gerald Goldberg [73] Assignee: Lorain Products Corporation, Attorney. Agent, or Firm-Edward C. Jason Lorain, Ohio [5 7] ABSTRACT [221 Filed: Aug. 23, 1973 A circuit for reducing or eliminating the effect of the [211 App]. No.: 391,145 magnetizing inductance of a transformer on voice frequency signals coupled through that transformer. ' [52] US. Cl ...... 323/6; 323/44 R; 323/60; Circuitry is provided to generate voltages and currents which simulate the presence of a negative inductance 3 324/127; 324/158 R [51] Int. Cl...... G01r 19/00 and to couple those voltages and currents to a trans [58] Field of Search ...... 323/44 R. 48, 60, 112, former in cancelling relationship to the magnetizing 323/6; 324/158 R, 127; 340/410; 307/89 inductance thereof. Circuitry is also provided to re duce or eliminate the effect of the equivalent shunt [56] References Cited resistance of the transformer and thereby render the transformer virtually free of core losses. Finally. cir UNITED STATES PATENTS cuitry is provided to linearize the non-linear magneti 2.891.214 6/1959 Rogers et a1...... 323/44 R zation characteristics of non-linear cores and thereby 2.944.207 7/1950 Rogers et a1. 323/44 R substantially reduce the distortion of signals .couplcd 1815.011 6/1974 Milkovic ...... 323/6 through the transformer. 3.815.012 6/1974 Milkovic...... 323/6 3.815.013 6/1974 Milkovic ...... 323/44 R X 15 Claims, 7 Drawing Figures

PATENIEBAPR29ié7S 3,881,149 SHEET 2 BF 2

. 1e

'2\ir______

L______.. 8 $2 3,881,149 1 2 COMPENSATED TRANSFORMER CIRCUIT “ideal‘” transformer so often discussed in electronics BACKGROUND OF THE INVENTION textbooks. The present invention relates to auxiliary circuits for SUMMARY OF THE INVENTION transformers and is directed more particularly to cir It is an object of the invention to provide circuitry for cuits for reducing the effect of magnetizing current reducing the signal distortion incident to the utilization I ?ow on power transmission through transformers. of transformers in voice-frequency circuits. Because of their usefulness in providing isolation, im Another object of the invention is to provide correc pedance transformation, and voltage and current trans tion circuitry for reducing or eliminating the effect of formations, transformers have become widely used in frequency dependent magnetizing current flow in voice-frequency circuits. In a voice-frequency repeater voice-frequency transformers. circuit such as. for example. that described in the US. Yet another object of the invention is to provide cor Pat. No. 3,706,862 of Charles W. Chambers, Jr., enti rection circuitry for reducing or eliminating signal dis tled “Amplifier Circuit for Transmission Lines", trans tortion resulting from the utilization of a non-linear formers play a useful role in coupling the repeater cir magnetic cores in voicefrequency transformers. cuitry is series with and across a transmission line. Be Still another object of the invention is to provide cor cause of the desirability of reducing the size of and rection circuitry for reducing or eliminating signal dis stray flux produced by these transformers, it is desir tortion resulting from frequency dependent magnetiz able to utilize therein a core material such as iron or ing current flow and from the utilization of non-linear ferrite which has magnetic properties. 20 core materials in voice-frequency transformers. One serious problem with utilizing magnetic core It is another object of the invention to provide cor transformers in voice-frequency circuits is that the rection circuitry of the above character which includes magnetizing currents drawn thereby are a function of circuitry for substantially cancelling the effect of the frequency. In particular, the magnetizing current equivalent shunt resistance of a voice-frequency trans drawn by a transformer is often negligible in the upper 25 former. 7 or middle regions of the voice-frequency band, due to Another object of the invention is to provide cir the relatively large magnetizing impedance presented cuitry of the above character which is usable in connec by the magnetizing inductance in such regions. At the tion with existing voice-frequency transformers. lower end of the voice-frequency band, however, the magnetizing impedance of such transformers is much 30 DESCRIPTION OF THE DRAWINGS smaller. causing a substantial reduction in the ampli FIG. 1 is a schematic diagram of one exemplary cir tude of the low frequency components of the voltages cuit embodying the invention, and currents coupled through such transformers. This FIG. 1a is a schematic diagram showing the equiva relatively large magnetizing current ?ow at low fre lent circuit of a transformer, quencies is manifested as a reduction in the quality of 35 FIG. 2 is a schematic diagram of a second embodi signal transmission. ment of the circuit of the invention, Another problem with the utilization of magnetic FIG. 3 is a schematic diagram ofa third embodiment core transformers in voice-frequency circuits is that, of the circuit of the invention, with the exception of magnetic core materials having FIG. 4 is a schematic diagram of a fourth embodi special magnetization characteristics. the magnetizing ment of the circuit ofthe invention, inductance of such cores is a non-linear function of FIG. 5 is a schematic diagram of a ?fth embodiment current. This non-linearity causes distortion, of the sig of the circuit of the invention, and nals coupled through such transformers and results in FIG. 6 is a schematic diagram of a sixth embodiment a reduction in signal quality. More frequently, voice of the circuit of the invention. frequency transformers cause signal distortion both as 45 a result of frequency dependent magnetizing current DESCRIPTION OF THE INVENTION flow and as a result of non-linearity as a function of cur Referring to FIG. 1 there is shown a source of voice rent. frequency voltage 10 for energizing a voice-frequency In accordance with the present invention, there is load 12 through a transformer 14. Source 10 may, for provided compensating circuitry for transformers example, comprise a transmission line which is ener which linearizes the response of such transformers as a gized by a remote voice-frequency transmitter and load function of frequency, as a function of current or as a 12 may comprise the input or output of a repeater cir function of both frequency and current. More specifi cuit which ampli?es the transmission of signals through cally, the circuit of the invention is adapted to allow a that transmission line. In transmission line-repeater sys transformer which has a frequency dependent magne tems of this type, a transformer such as transformer 14 tizing current characteristic to transform voltages and is desirable to electrically isolate the repeater circuit currents having frequencies relatively near the low end from the transmission line and vice-versa. of the voice-frequency band as accurately as it trans In the present embodiment, transformer 14 com forms voltages and currents having frequencies near 60 prises a suitable magnetic core 14a around which are the middle or upper end of the voice-frequency band. disposed a primary winding 14b, a secondary winding In addition, the circuit of the invention compensates 14c and a tertiary or auxiliary winding 14d. Because for nonlinearities in the response of magnetic cores core 140 has magnetic properties, transformer 14 ex which exhibit non-linear current characteristics. To hibits a magnetizing impedance which is a function of gether these compensation characteristics allow the 65 frequency. This magnetizing impedance results from utilization of transformers made up of ordinary mag the presence of magnetizing inductance in transformer netic core materials to provide ' a .response which 14, the latter being a quantity which is best visualized closely approaches that heretofore attributed to the in the context of the familiar transformer equivalent 3,881,149 3 4 circuit shown in FIG. la. The effect of this magnetizing resistors 22 and 24 to 'be'selectediso that the input im impedance is to cause the ?ow of a frequency depen pedance of network 16 establishes the desired cancel dent component of current in primary winding 14b ling value of negative inductanceI which gives rise to no similar component of current in It will be understood that other correction networks secondary winding 14c. The magnitude of this magne utilizing different ampli?er and feedback'impedance tizing impedance is ordinarily so large in the high and connections may be utilized to simulate the required middle frequency regions of the voice-frequency band, negative inductance at terminals 160 and 16b. The par that the effect thereof may be neglected. At the low ticular ampli?er and feedback impedance connections end of the voice-frequency band, however, this magne shown in FIG. 1, however, have the advantages of sim tizing impedance is substantially lower, preventing an plicity, economy and expandibility, that is, the ability appreciable portion of low frequency components of to utilize additional feedback elements for the purpose the primary winding current from affecting load 12. of providing increasingly sophisticated forms of trans This condition is manifested as an excessive attenua former correction. tion of the low frequency components of the voice For example, if the series resistance of winding 14d frequency signal and results in a noticeable signal dis is sufficiently large that it prevents precise magnetizing 7 tortion. inductance cancellation via terminals 16a and 1612, the To the end that the signal distorting effect of the series resistance of windings 14d may be effectively magnetizing impedance of transformer 14 may be sub-' cancelled by connecting a suitable resistor 26 in paral stantially reduced or eliminated, particularly at the lel with feedback capacitor 20. This result follows be lower end of the voice-frequency band, there is pro 20 cause a positive resistance which is connected in paral vided a correction circuit 16 which, in the present em lel with feedback capacitor 20 causes a simulated nega bodiment, is coupled to core 14a through tertiary wind tive resistance to appear in series with the existing sim ing 14d. As will be described more fully presently, cor ulated negative inductance between terminals 16a and rection circuit 16 serves to establish at terminals 16a 16b. This simulated negative resistance, in turn, cancels and 16b thereof, voltages and currents which alter the 25 the effect of the series resistance of winding 14d so that response of core 14a and thereby substantially cancel the simulated negative inductance may be applied the effect of the magnetizing inductance thereof. As a more directly in parallel with the magnetizing induc result, correction circuit 16 allows voice-frequency tance and thereby afford more precise inductance can transmission from source 10 to load 12 to proceed as cellation. if transformer 14 had no magnetizing inductance and 30 A still further degree of transformer correction may thereby greatly improves the low frequency response of be provided by effectively cancelling the equivalent transformer 14. shunt resistance of transformer 14, the latter being a In accordance with one feature of the present inven~ quantity which is utilized to represent the core losses of tion, the desired inductance cancellation is produced a transformer and which is often represented as being by simulating between terminals 16a and 16b the pres in parallel with the magnetizing inductance of the ence of a negative inductance having a magnitude de transformer, as shown in FIG. la. This shunt resistance termined by the magnetizing inductance of transformer may be cancelled along with the magnetizing induc 14 and the number of turns on winding 14d. The result tance by connecting a suitable resistor in series with ca of coupling this synthesized negative inductance to pacitor 20. This result follows because the insertion of transformer 14 is that the negative inductance com~ 40 a resistor in series with capacitor 20 causes an effective bines in parallel with the existing magnetizing induc negative resistance to appear in parallel with the exist tance, according to the usual parallel combination ing effective negative inductance between terminals rules, to generate an equivalent combined inductance 16a and 16b. The effect of cancelling this equivalent having a value which approaches infinity. The latter shunt resistance is to render transformer 14 effectively condition effectively prevents transformer 14 from af free of core losses. fecting signal transmission from source 10 to load 12. In view of the foregoing, it will be seen that the utili To the end that correction network 16 may establish zation of correction network 16 to simulate the nega an effective negative inductance between terminals 16a tive inductance required for magnetizing inductance and 16b, network 16 includes, in the present embodi cancellation, to simulate the series negative resistance ment, an operational ampli?er 18 having an inverting 50 required to cancel the series resistance of winding 14d, input 18a, a non-inverting input 18b and an output 180. and to simulate the shunt negative resistance to cancel Correction network 16 also includes feedback means the equivalent shunt resistance of transformer 14, which here takes the form of a negative feedback im causes transformer 14 to act as an ideal transformer, pedance comprising a capacitor 20 connected between that is, a'transformer which exhibits substantially no amplifier output 18c and inverting input 18a, a positive 55 loss and substantially no magnetizing inductance. feedback impedance comprising a resistor‘ 22 con In the event that it is desirable to utilize transformers nected between amplifier output 18 c and non which do not have a tertiary winding as, for example, inverting input 18b, and an input feedback impedance in the case where it is desirable to add to the correction comprising a resistor 24 connected between network circuit of the invention to existing two-winding trans input terminal 16a and inverting input 18a. Therela formers, this may be accomplished by connecting cor-. tionship governingthe magnitude of the impedance rection circuit 16 to an existing two-winding trans looking into terminals 16a and 16b is given by the for former in the manner shown in FIGS. 2 and 3. The cir mula Z", —Z2 ' Z3/Z, where Zm is the impedance cuits of FIGS. 2 and 3 are generally similar to that of looking into terminals 16a and 16b, Z1 is the negative 65 FIG. 1 and like functioning parts are similarly num feedback impedance, Z2 is the input feedback imped bered. It will be understood however,‘ that the capaci-_ ance and Z3 is the positive feedback impedance. This tance and resistance values-"of the capacitors and resis relationship allows the magnitudes of capacitor 20 and tors shown in FIGS.=2' and 3 ‘are not necessarily the 3,881,149 5 6 same as those of the capacitors and resistors shown in density of core 14a. This flux density condition ?ows FIG. 1. quite naturally from the con?guration of the correction As shown in FIG. 2, correction circuit 16 is con circuit since the low voltage across inputs 18a and 18b nected across primary winding 14b of transformer 14. of ampli?er 18 causes the voltage across and current In this position, correction circuit 16 operates in gener through inductor 32 to be proportional to the primary ally the same manner as that described in connection voltage and current of transformer 14. The result of with FIG. 1 to substantially cancel the magnetizing in this ?ux density condition is to assure that as the mag ductance of transformer 14. The correction provided netizing inductance of transformer 14 varies with the by network 16 in the circuit of FIG. 2 is, however, not current through that transformer, similar variations as complete as the correction provided in the circuit of occur in the inductance of inductor 32. Thus, the im FIG. 1. This lesser degree of correction results from the pedance of inductor 32 tracks variations in the magne connection of the correction circuit across the primary tizing impedance of transformer 14. winding and the resulting connection of the simulated When the impedances of resistors 28 and 30 and the negative inductance to the magnetizing inductance nonlinear impedance of inductor 32 are substituted through the leakage inductance and series resistance of into the previously described equation for Z,~,,, it is ap the primary winding, as shown in FIG. 1 a. However, parent that the impedance looking into terminals 16a’ because this series resistance and leakage inductance is and 16b’ of correction network 16' comprises an effec generally small, particularly at the low frequencies of tive non-linear negative impedance which is propor concern in connection with magnetizing inductance tional to the magnetizing impedance of transformer 14. cancellation, the degree of improvement in transformer 20 Accordingly, it will be seen that however non-linear the response provided by connecting correction circuit 16 magnetizing impedance of transformer 14 is, the cor in the manner shown in FIG. 2 is more than suf?cient rection provided by network 16’ is correspondingly to justify the expense thereof. non-linear to provide a variable correction which, for Referring to FIG. 3, there is shown a correction cir suitable values of resistors 28 and 30, automatically cuit configuration in which correction network 16 is 25 and fully cancels the non-linear magnetizing imped connected across the secondary winding of transformer ance of transformer 14. Thus, the circuit of FIG. 4 not 14. In this position circuit 16 operates in generally the only eliminates the distortion which is due to the mere same manner as that described in connection with FIG. presence of magnetizing impedance but also the distor 1 to substantially cancel the magnetizing inductance of tion which is due to the non-linear character of that transformer 14. As in the case of the circuit of FIG. 2, 30 magnetizing impedance. however, this correction is not as complete as that pro It will be understood that inductor 32 need not be of vided by the circuit of FIG. 1, due to the fact that in the the same size as transformer 14 so long as the flux den circuit of FIG. 3, correction circuit 16 makes its effect sity operating points of the core of inductor 32 and the felt through the series resistance and leakage induc core of transformer 14 are the same. Inductor 32 may, tance of secondary winding 14c. The degree of correc 35 for example, consist of a small sample of the core mate tion is, however, more than suf?cient to justify the ex rial of transformer 14 around which a coil of wire of a pense of correction network 16. suitable number of turns is wound. This smaller sized The magnetizing inductance compensating circuits core will be satisfactory because inductor 32 serves shown in FIGS. 1, 2 and 3 are premised upon the utili merely as a non-linear reference inductance for impart zation of a transformer in which the magnetizing im ing the desired non-linearity to the negative inductance pedance is substantially constant as a function of cur simulated by correction network 16’. rent. This constancy in the magnitude of the magnetiz As in the case of FIGS. 2 and 3, correction network ing inductance allows the utilization of correction net 16’ need not be coupled to transformer 14 through ter works having linear feedback impedances of ?xed val 45 tiary winding 14d as shown in FIG. 4. Correction net ues. In the event that the material chosen for core 14a work 16’ may, for example, be connected across pri causes the magnetizing inductance of transformer 14 to mary winding 14b as shown in FIG. 5 or may be con be a non-linear function of current, the utilization of nected across secondary winding 14c, as shown in FIG. linear feedback impedances of ?xed values in correc 6. The adoption of the correction network configura tion network 16 will produce a correction error as the tions shown in FIGS. 5 and 6 do, however, result in transformer current varies from its maximum to its somewhat less accurate magnetizing inductance cor~ minimum value. rections than the adoption of the correction con?gura To the end that the desired magnetizing inductance tion shown in FIG. 4, due to the coupling of correction correction may be provided in non-linear transformers, network 16' to core 14a through the series resistance there are provided non-linear correction circuits 16' 55 and leakage inductance of primary winding 14b or sec which may be connected to the transformers to be cor ondary winding 14c. Thus, correction network 16' may rected as shown in FIGS. 4, 5 and 6. In the embodiment be utilized with existing two-winding, non-linear trans of FIG. 4, correction network 16' includes an opera formers as well as with three-winding, non-linear trans tional ampli?er 18 of the type described previously in formers which have been designed with the presence of connection with FIGS. 1, 2 and 3. Correction network 60 correction network 16’ in mind. 16’ also includes a positive feedback impedance which In view of the foregoing, it will be seen that a trans here takes the form of a resistor 28, a negative feed former compensating circuit constructed in accor back impedance which here takes the form of a resistor dance with the invention is adapted to cancel the effect 30 and an input feedback impedance 32 which here of magnetizing impedance on the transmission of a sig takes the form of an inductor having a core made out 65 nal from a source to a load through a transformer, both of the same material as core 140. Inductor 32 is ar in the presence of linear magnetic core characteristics ranged so that the operating point of the flux density and in the presence of non-linear magnetic core char thereof is the same as the operating point of the ?ux acteristics. In this manner the circuit of the invention 3,881,149 7 8 reduces the distortion which has degraded the quality tive inductance which is similarly non‘linear. of ‘voice-frequency voltage and current transforma 6. A transformer circuit as set forth in claim 5_ tions,'particularly at the low end of the voice-frequency wherein said impedance simulating network includes a band. second magnetic core made up of the same material as It will be understood that the above described em the magnetic core of said transformer, said second core bodiments are for illustrative purposes only and may be being arranged to operate about the same ?ux density changed or modified without departing from the spirit operating point as the magnetic core of said trans and scope of the appended claims. former. What is claimed is: 7. A transformer circuit as set forth in claim 5 1. A transformer circuit which provides an uninter wherein the impedance simulating network includes rupted path for the flow of primary and secondary cur means for establishing, in series with said negative in rent and which compensates for the frequency depen ductance, a negative resistance having a magnitude dent attenuation of transformer signals that is intro substantially equal to the series resistance of said ter duced by the magnetizing inductance of the trans tiary winding. former comprising, in combination, a magnetic core, 8. A transformer circuit as set forth in claim 7 ' primary, secondary and tertiary windings disposed on wherein the impedance simulating network includes said core, an impedance simulating network including means for establishing, in parallel with said negative in feedback circuitry for establishing voltages and cur ductance, a negative resistance having a magnitude rents which affect the circuitry to which said voltages that is substantially proportional to the equivalent and currents are applied as if a negative inductance 20 shunt resistance of said transformer. were connected to said circuitry, said negative induc 9. A transformer circuit as set forth in claim 1 tance having a magnitude proportional to the magnetiz wherein said impedance simulating network includes: ing inductance of said transformer, and means for con an operational ampli?er having an inverting input, a necting said impedance simulating network to said ter non-inverting input and an output, a positive feedback tiary winding to effectively cancel the magnetizing in 25 impedance connected between the output and non ductance of said transformer. inverting input of said ampli?er, a negative feedback 2. A transformer circuit as set forth in claim 1 impedance connected between the output and invert wherein the impedance simulating network includes ing input of said ampli?er, an input feedback imped means for establishing, in parallel with said negative in ance connected between said tertiary winding and one ductance, a negative resistance having a magnitude 30 of the inputs of said ampli?er. that is proportional to the equivalent shunt resistance 10. A transformer circuit as set forth in claim 9 in of said transformer. _ which said negative feedback means includes a capaci 3. A transformer circuit as set forth in claim 1 tor. wherein the impedance simulating network includes 11. A transformer circuit as set forth in claim 10 in means for establishing, in series with said negative in 35 cluding a resistor in parallel with said capacitor. ductance, a negative resistance having a magnitude 12. A transformer circuit as set forth in claim 10 in substantially equal to the series resistance of said ter cluding a resistor is series with said capacitor. tiary winding. 13. A transformer circuit as set forth in claim 10 in 4. A transformer circuit as set forth in claim 3 cluding a ?rst resistor in parallel with said capacitor wherein the impedance simulating network includes 40 and a second resistor in series with said capacitor. means for establishing, in parallel with said negative in-, 14. A transformer circuit as set forth in claim 9 in ductance, a negative resistance having a magnitude which said input feedback impedance includes a sec that is substantially proportional to the equivalent ond magnetic core, said second magnetic core being shunt resistance of said transformer. operated about the same flux density operating point as 5. A transformer circuit as set forth in claim 1 45 the magnetic core of said transformer. wherein said magnetic core has a non-linear magnetiz 15. A transformer circuit as set forth in claim 14 in ing characteristic which causes the magnetizing induc which said positive and negative feedback impedances tance of the transformer to be non-linear, and wherein comprise resistors. said impedance simulating network establishes a nega * * * * *

55

60

65