Compensated transformer circuit
United States Patent 1 Kiko 1 1 COMPENSATED TRANSFORMER CIRCUIT [75] Inventor: Frederick J. Kiko, Sheffield Village.  
 Ohio  
 [73] Assignee: Lorain Products Corporation,  
 Lorain, Ohio 221 Filed: Aug. 23, 1973 211 App]. No.: 391,145  
 [52] US. Cl 323/6; 323/44 R; 323/60; 3 324/127; 324/158 R [51] Int. Cl. ..G01r 19/00 [58] Field of Search 323/44 R. 48, 60, 112, 323/6; 324/158 R, 127; 340/410; 307/89 [56] References Cited UNITED STATES PATENTS 2.891.214 6/1959 Rogers et a1. 323/44 R 2.944.207 7/1950 Rogers et a1. 323/44 R 1815.011 6/1974 Milkovic 323/6 3.815.012 6/1974 Milkovic... 323/6 3.815.013 6/1974 Milkovic 323/44 R X 1 Apr. 29, 1975 3.818338 6/1974 Chambers. Jr. et a1 323/6 X Primary [imminer-Gerald Goldberg Attorney. Agent, or Firm-Edward C. Jason [5 7] ABSTRACT A circuit for reducing or eliminating the effect of the magnetizing inductance of a transformer on voicefrequency signals coupled through that transformer.  
 Circuitry is provided to generate voltages and currents which simulate the presence of a negative inductance and to couple those voltages and currents to a transformer in cancelling relationship to the magnetizing inductance thereof. Circuitry is also provided to reduce or eliminate the effect of the equivalent shunt resistance of the transformer and thereby render the transformer virtually free of core losses. Finally. circuitry is provided to linearize the non-linear magnetization characteristics of non-linear cores and thereby substantially reduce the distortion of signals .couplcd through the transformer.  
 15 Claims, 7 Drawing Figures PATENIEBAPR29i7S SHEET 2 BF 2 COMPENSATED TRANSFORMER CIRCUIT BACKGROUND OF THE INVENTION I flow on power transmission through transformers.  
  Because of their usefulness in providing isolation, impedance transformation, and voltage and current transformations, transformers have become widely used in voice-frequency circuits. In a voice-frequency repeater circuit such as. for example. that described in the US. Pat. No. 3,706,862 of Charles W. Chambers, Jr., entitled Amplifier Circuit for Transmission Lines&#34;, transformers play a useful role in coupling the repeater circuitry is series with and across a transmission line. Because of the desirability of reducing the size of and stray flux produced by these transformers, it is desirable to utilize therein a core material such as iron or ferrite which has magnetic properties.  
  One serious problem with utilizing magnetic core transformers in voice-frequency circuits is that the magnetizing currents drawn thereby are a function of frequency. In particular, the magnetizing current drawn by a transformer is often negligible in the upper or middle regions of the voice-frequency band, due to the relatively large magnetizing impedance presented by the magnetizing inductance in such regions. At the lower end of the voice-frequency band, however, the magnetizing impedance of such transformers is much smaller. causing a substantial reduction in the amplitude of the low frequency components of the voltages and currents coupled through such transformers. This relatively large magnetizing current flow at low frequencies is manifested as a reduction in the quality of signal transmission.  
  Another problem with the utilization of magnetic core transformers in voice-frequency circuits is that, with the exception of magnetic core materials having special magnetization characteristics. the magnetizing inductance of such cores is a non-linear function of current. This non-linearity causes distortion of the signals coupled through such transformers and results in a reduction in signal quality. More frequently, voicefrequency transformers cause signal distortion both as a result of frequency dependent magnetizing current flow and as a result of non-linearity as a function of current.  
  In accordance with the present invention, there is provided compensating circuitry for transformers which linearizes the response of such transformers as a function of frequency, as a function of current or as a function of both frequency and current. More specifically, the circuit of the invention is adapted to allow a transformer which has a frequency dependent magnetizing current characteristic to transform voltages and currents having frequencies relatively near the low end of the voice-frequency band as accurately as it transforms voltages and currents having frequencies near the middle or upper end of the voice-frequency band. In addition, the circuit of the invention compensates for nonlinearities in the response of magnetic cores which exhibit non-linear current characteristics. Together these compensation characteristics allow the utilization of transformers made up of ordinary magnetic core materials to provide a .response which closely approaches that heretofore attributed to the ideal transformer so often discussed in electronics textbooks.  
 SUMMARY OF THE INVENTION It is an object of the invention to provide circuitry for reducing the signal distortion incident to the utilization of transformers in voice-frequency circuits.  
  Another object of the invention is to provide correction circuitry for reducing or eliminating the effect of frequency dependent magnetizing current flow in voice-frequency transformers.  
  Yet another object of the invention is to provide correction circuitry for reducing or eliminating signal distortion resulting from the utilization of a non-linear magnetic cores in voicefrequency transformers.  
  Still another object of the invention is to provide correction circuitry for reducing or eliminating signal distortion resulting from frequency dependent magnetizing current flow and from the utilization of non-linear core materials in voice-frequency transformers.  
  It is another object of the invention to provide correction circuitry of the above character which includes circuitry for substantially cancelling the effect of the equivalent shunt resistance of a voice-frequency transformer. 7  
  Another object of the invention is to provide circuitry of the above character which is usable in connection with existing voice-frequency transformers.  
 DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of one exemplary circuit embodying the invention,  
  FIG. 1a is a schematic diagram showing the equivalent circuit of a transformer,  
  FIG. 2 is a schematic diagram of a second embodiment of the circuit of the invention,  
  FIG. 3 is a schematic diagram ofa third embodiment of the circuit of the invention,  
  FIG. 4 is a schematic diagram of a fourth embodiment of the circuit ofthe invention,  
  FIG. 5 is a schematic diagram of a fifth embodiment of the circuit of the invention, and  
  FIG. 6 is a schematic diagram of a sixth embodiment of the circuit of the invention.  
 DESCRIPTION OF THE INVENTION Referring to FIG. 1 there is shown a source of voicefrequency voltage 10 for energizing a voice-frequency load 12 through a transformer 14. Source 10 may, for example, comprise a transmission line which is energized by a remote voice-frequency transmitter and load 12 may comprise the input or output of a repeater circuit which amplifies the transmission of signals through that transmission line. In transmission line-repeater systems of this type, a transformer such as transformer 14 is desirable to electrically isolate the repeater circuit from the transmission line and vice-versa.  
  In the present embodiment, transformer 14 comprises a suitable magnetic core 14a around which are disposed a primary winding 14b, a secondary winding 14c and a tertiary or auxiliary winding 14d. Because core has magnetic properties, transformer 14 exhibits a magnetizing impedance which is a function of frequency. This magnetizing impedance results from the presence of magnetizing inductance in transformer 14, the latter being a quantity which is best visualized in the context of the familiar transformer equivalent circuit shown in FIG. la. The effect of this magnetizing impedance is to cause the flow of a frequency dependent component of current in primary winding 14b which gives rise to no similar component of current in secondary winding 14c. The magnitude of this magnetizing impedance is ordinarily so large in the high and middle frequency regions of the voice-frequency band, that the effect thereof may be neglected. At the low end of the voice-frequency band, however, this magnetizing impedance is substantially lower, preventing an appreciable portion of low frequency components of the primary winding current from affecting load 12. This condition is manifested as an excessive attenuation of the low frequency components of the voicefrequency signal and results in a noticeable signal dis- 7 tortion.  
  To the end that the signal distorting effect of the magnetizing impedance of transformer 14 may be sub-&#39; stantially reduced or eliminated, particularly at the lower end of the voice-frequency band, there is provided a correction circuit 16 which, in the present embodiment, is coupled to core 14a through tertiary winding 14d. As will be described more fully presently, correction circuit 16 serves to establish at terminals 16a and 16b thereof, voltages and currents which alter the response of core 14a and thereby substantially cancel the effect of the magnetizing inductance thereof. As a result, correction circuit 16 allows voice-frequency transmission from source 10 to load 12 to proceed as if transformer 14 had no magnetizing inductance and thereby greatly improves the low frequency response of transformer 14.  
  In accordance with one feature of the present invention, the desired inductance cancellation is produced by simulating between terminals 16a and 16b the presence of a negative inductance having a magnitude determined by the magnetizing inductance of transformer 14 and the number of turns on winding 14d. The result of coupling this synthesized negative inductance to transformer 14 is that the negative inductance combines in parallel with the existing magnetizing inductance, according to the usual parallel combination rules, to generate an equivalent combined inductance having a value which approaches infinity. The latter condition effectively prevents transformer 14 from affecting signal transmission from source 10 to load 12.  
  To the end that correction network 16 may establish an effective negative inductance between terminals 16a and 16b, network 16 includes, in the present embodiment, an operational amplifier 18 having an inverting input 18a, a non-inverting input 18b and an output 180. Correction network 16 also includes feedback means which here takes the form of a negative feedback impedance comprising a capacitor 20 connected between amplifier output 18c and inverting input 18a, a positive feedback impedance comprising a resistor 22 connected between amplifier output 18 c and noninverting input 18b, and an input feedback impedance comprising a resistor 24 connected between network input terminal 16a and inverting input 18a. Therelationship governingthe magnitude of the impedance looking into terminals 16a and 16b is given by the formula Z Z2 Z3/Z, where Z is the impedance looking into terminals 16a and 16b, Z1 is the negative feedback impedance, Z2 is the input feedback impedance and Z3 is the positive feedback impedance. This relationship allows the magnitudes of capacitor 20 and resistors 22 and 24 to &#39;be&#39;selected so that the input impedance of network 16 establishes the desired cancelling value of negative inductanceI It will be understood that other correction networks utilizing different amplifier and feedback&#39;impedance connections may be utilized to simulate the required negative inductance at terminals and 16b. The particular amplifier and feedback impedance connections shown in FIG. 1, however, have the advantages of simplicity, economy and expandibility, that is, the ability to utilize additional feedback elements for the purpose of providing increasingly sophisticated forms of transformer correction.  
  For example, if the series resistance of winding 14d is sufficiently large that it prevents precise magnetizing inductance cancellation via terminals 16a and 1612, the series resistance of windings 14d may be effectively cancelled by connecting a suitable resistor 26 in parallel with feedback capacitor 20. This result follows because a positive resistance which is connected in parallel with feedback capacitor 20 causes a simulated negative resistance to appear in series with the existing simulated negative inductance between terminals 16a and 16b. This simulated negative resistance, in turn, cancels the effect of the series resistance of winding 14d so that the simulated negative inductance may be applied more directly in parallel with the magnetizing inductance and thereby afford more precise inductance cancellation.  
  A still further degree of transformer correction may be provided by effectively cancelling the equivalent shunt resistance of transformer 14, the latter being a quantity which is utilized to represent the core losses of a transformer and which is often represented as being in parallel with the magnetizing inductance of the transformer, as shown in FIG. la. This shunt resistance may be cancelled along with the magnetizing inductance by connecting a suitable resistor in series with capacitor 20. This result follows because the insertion of a resistor in series with capacitor 20 causes an effective negative resistance to appear in parallel with the existing effective negative inductance between terminals 16a and 16b. The effect of cancelling this equivalent shunt resistance is to render transformer 14 effectively free of core losses.  
  In view of the foregoing, it will be seen that the utilization of correction network 16 to simulate the negative inductance required for magnetizing inductance cancellation, to simulate the series negative resistance required to cancel the series resistance of winding 14d, and to simulate the shunt negative resistance to cancel the equivalent shunt resistance of transformer 14, causes transformer 14 to act as an ideal transformer, that is, a&#39;transformer which exhibits substantially no loss and substantially no magnetizing inductance.  
  In the event that it is desirable to utilize transformers which do not have a tertiary winding as, for example, in the case where it is desirable to add to the correction circuit of the invention to existing two-winding transformers, this may be accomplished by connecting cor-.  
 rection circuit 16 to an existing two-winding transformer in the manner shown in FIGS. 2 and 3. The circuits of FIGS. 2 and 3 are generally similar to that of FIG. 1 and like functioning parts are similarly numbered. It will be understood however, that the capacitance and resistance values-&#34;of the capacitors and resistors shown in FIGS.=2&#39; and 3 are not necessarily the same as those of the capacitors and resistors shown in FIG. 1.  
  As shown in FIG. 2, correction circuit 16 is connected across primary winding 14b of transformer 14. In this position, correction circuit 16 operates in generally the same manner as that described in connection with FIG. 1 to substantially cancel the magnetizing inductance of transformer 14. The correction provided by network 16 in the circuit of FIG. 2 is, however, not as complete as the correction provided in the circuit of FIG. 1. This lesser degree of correction results from the connection of the correction circuit across the primary winding and the resulting connection of the simulated negative inductance to the magnetizing inductance through the leakage inductance and series resistance of the primary winding, as shown in FIG. 1 a. However, because this series resistance and leakage inductance is generally small, particularly at the low frequencies of concern in connection with magnetizing inductance cancellation, the degree of improvement in transformer response provided by connecting correction circuit 16 in the manner shown in FIG. 2 is more than sufficient to justify the expense thereof.  
  Referring to FIG. 3, there is shown a correction circuit configuration in which correction network 16 is connected across the secondary winding of transformer 14. In this position circuit 16 operates in generally the same manner as that described in connection with FIG. 1 to substantially cancel the magnetizing inductance of transformer 14. As in the case of the circuit of FIG. 2, however, this correction is not as complete as that provided by the circuit of FIG. 1, due to the fact that in the circuit of FIG. 3, correction circuit 16 makes its effect felt through the series resistance and leakage inductance of secondary winding 14c. The degree of correction is, however, more than sufficient to justify the expense of correction network 16.  
  The magnetizing inductance compensating circuits shown in FIGS. 1, 2 and 3 are premised upon the utilization of a transformer in which the magnetizing impedance is substantially constant as a function of current. This constancy in the magnitude of the magnetizing inductance allows the utilization of correction networks having linear feedback impedances of fixed values. In the event that the material chosen for core 14a causes the magnetizing inductance of transformer 14 to be a non-linear function of current, the utilization of linear feedback impedances of fixed values in correction network 16 will produce a correction error as the transformer current varies from its maximum to its minimum value.  
  To the end that the desired magnetizing inductance correction may be provided in non-linear transformers, there are provided non-linear correction circuits 16&#39; which may be connected to the transformers to be corrected as shown in FIGS. 4, 5 and 6. In the embodiment of FIG. 4, correction network 16&#39; includes an operational amplifier 18 of the type described previously in connection with FIGS. 1, 2 and 3. Correction network 16 also includes a positive feedback impedance which here takes the form of a resistor 28, a negative feed back impedance which here takes the form of a resistor 30 and an input feedback impedance 32 which here takes the form of an inductor having a core made out of the same material as core 140. Inductor 32 is arranged so that the operating point of the flux density thereof is the same as the operating point of the flux density of core 14a. This flux density condition flows quite naturally from the configuration of the correction circuit since the low voltage across inputs 18a and 18b of amplifier 18 causes the voltage across and current through inductor 32 to be proportional to the primary voltage and current of transformer 14. The result of this flux density condition is to assure that as the magnetizing inductance of transformer 14 varies with the current through that transformer, similar variations occur in the inductance of inductor 32. Thus, the impedance of inductor 32 tracks variations in the magnetizing impedance of transformer 14.  
  When the impedances of resistors 28 and 30 and the nonlinear impedance of inductor 32 are substituted into the previously described equation for Z,-,,, it is apparent that the impedance looking into terminals 16a and 16b of correction network 16&#39; comprises an effective non-linear negative impedance which is proportional to the magnetizing impedance of transformer 14. Accordingly, it will be seen that however non-linear the magnetizing impedance of transformer 14 is, the correction provided by network 16 is correspondingly non-linear to provide a variable correction which, for suitable values of resistors 28 and 30, automatically and fully cancels the non-linear magnetizing impedance of transformer 14. Thus, the circuit of FIG. 4 not only eliminates the distortion which is due to the mere presence of magnetizing impedance but also the distortion which is due to the non-linear character of that magnetizing impedance.  
  It will be understood that inductor 32 need not be of the same size as transformer 14 so long as the flux density operating points of the core of inductor 32 and the core of transformer 14 are the same. Inductor 32 may, for example, consist of a small sample of the core material of transformer 14 around which a coil of wire of a suitable number of turns is wound. This smaller sized core will be satisfactory because inductor 32 serves merely as a non-linear reference inductance for imparting the desired non-linearity to the negative inductance simulated by correction network 16.  
  As in the case of FIGS. 2 and 3, correction network 16 need not be coupled to transformer 14 through tertiary winding 14d as shown in FIG. 4. Correction network 16 may, for example, be connected across primary winding 14b as shown in FIG. 5 or may be connected across secondary winding 14c, as shown in FIG. 6. The adoption of the correction network configurations shown in FIGS. 5 and 6 do, however, result in somewhat less accurate magnetizing inductance corrections than the adoption of the correction configuration shown in FIG. 4, due to the coupling of correction network 16&#39; to core 14a through the series resistance and leakage inductance of primary winding 14b or secondary winding 14c. Thus, correction network 16&#39; may be utilized with existing two-winding, non-linear transformers as well as with three-winding, non-linear transformers which have been designed with the presence of correction network 16 in mind.  
  In view of the foregoing, it will be seen that a transformer compensating circuit constructed in accordance with the invention is adapted to cancel the effect of magnetizing impedance on the transmission of a signal from a source to a load through a transformer, both in the presence of linear magnetic core characteristics and in the presence of non-linear magnetic core characteristics. In this manner the circuit of the invention reduces the distortion which has degraded the quality of voice-frequency voltage and current transformations,&#39;particularly at the low end of the voice-frequency band.  
  It will be understood that the above described embodiments are for illustrative purposes only and may be changed or modified without departing from the spirit and scope of the appended claims.  
 What is claimed is:  
  1. A transformer circuit which provides an uninterrupted path for the flow of primary and secondary current and which compensates for the frequency dependent attenuation of transformer signals that is introduced by the magnetizing inductance of the transformer comprising, in combination, a magnetic core,  
 &#39; primary, secondary and tertiary windings disposed on said core, an impedance simulating network including feedback circuitry for establishing voltages and currents which affect the circuitry to which said voltages and currents are applied as if a negative inductance were connected to said circuitry, said negative inductance having a magnitude proportional to the magnetizing inductance of said transformer, and means for connecting said impedance simulating network to said tertiary winding to effectively cancel the magnetizing inductance of said transformer.  
  2. A transformer circuit as set forth in claim 1 wherein the impedance simulating network includes means for establishing, in parallel with said negative inductance, a negative resistance having a magnitude that is proportional to the equivalent shunt resistance of said transformer.  
  3. A transformer circuit as set forth in claim 1 wherein the impedance simulating network includes means for establishing, in series with said negative inductance, a negative resistance having a magnitude substantially equal to the series resistance of said tertiary winding.  
  4. A transformer circuit as set forth in claim 3 wherein the impedance simulating network includes means for establishing, in parallel with said negative in-, ductance, a negative resistance having a magnitude that is substantially proportional to the equivalent shunt resistance of said transformer.  
  5. A transformer circuit as set forth in claim 1 wherein said magnetic core has a non-linear magnetizing characteristic which causes the magnetizing inductance of the transformer to be non-linear, and wherein said impedance simulating network establishes a negative inductance which is similarly nonlinear.  
  6. A transformer circuit as set forth in claim 5 wherein said impedance simulating network includes a second magnetic core made up of the same material as the magnetic core of said transformer, said second core being arranged to operate about the same flux density operating point as the magnetic core of said transformer.  
  7. A transformer circuit as set forth in claim 5 wherein the impedance simulating network includes means for establishing, in series with said negative inductance, a negative resistance having a magnitude substantially equal to the series resistance of said tertiary winding.  
  8. A transformer circuit as set forth in claim 7 wherein the impedance simulating network includes means for establishing, in parallel with said negative inductance, a negative resistance having a magnitude that is substantially proportional to the equivalent shunt resistance of said transformer.  
  9. A transformer circuit as set forth in claim 1 wherein said impedance simulating network includes: an operational amplifier having an inverting input, a non-inverting input and an output, a positive feedback impedance connected between the output and noninverting input of said amplifier, a negative feedback impedance connected between the output and inverting input of said amplifier, an input feedback impedance connected between said tertiary winding and one of the inputs of said amplifier.  
  10. A transformer circuit as set forth in claim 9 in which said negative feedback means includes a capacitor.  
  11. A transformer circuit as set forth in claim 10 including a resistor in parallel with said capacitor.  
  12. A transformer circuit as set forth in claim 10 including a resistor is series with said capacitor.  
  13. A transformer circuit as set forth in claim 10 including a first resistor in parallel with said capacitor and a second resistor in series with said capacitor.  
  14. A transformer circuit as set forth in claim 9 in which said input feedback impedance includes a second magnetic core, said second magnetic core being operated about the same flux density operating point as the magnetic core of said transformer.  
  15. A transformer circuit as set forth in claim 14 in which said positive and negative feedback impedances comprise resistors.