NEGATIVE IMMITTANCE CONVERTER

Abstract: A negative immittance converter, comprising a first transformer that includes a first coil (1) with a centre tap and a second transformer that includes a second coil (2) with a centre tap, a first contact of the first coil being connected to a first contact of the second coil through a first amplifier element (10) comprising a control input, a second contact of the first coil being connected to a second contact of the second coil through a second amplifier element (11) comprising a control input and the centre taps of the first and the second coil each being connected to a supply point of the converter and a 180° phase shifter being inserted between the first and the second contact of the second coil and the two control inputs of the two amplifier elements, characterized in that the 180° phase shifter is formed by a direct voltage differential amplifier (9), whose one input is connected to a contact of a voltage divider (16, 17) that is inserted between the first contacts of the first and the second coils, the other input of the differential amplifier being connected to a contact of a voltage divider (23, 28) that is inserted between the second contacts of the first and the second coil and the control inputs of the first and the second amplifier elements each being connected to an output of the differential amplifier (9).

Description: The invention relates to a negative immitance converter, comprising a first transformer that includes a first coil with centre tap and a second transformer that includes a second coil with centre tap, a first contact of the first coil being connected to a first contact of the second coil through a first amplifier element comprising a control input, a second contact of the first coil being connected to a second contact of the second coil through a second amplifier element comprising a control input, the centre taps of the first and the second coil each being connected to a supply point of the converter and the first and second transformers being coupled to each other by means of a 180° phase shifter. Negative immittance converters are used in so-called two-wire repeaters forming part of a telephone system. Such a two-wire repeater, also called negistor, comprises a negative impedance converter for a negative immittance converter and a negative immittance converter for a negative impedance converter. The negative impedance converter is connected in series with the line and the negative admittance converter in shunt with the line. The said negistors are used in local cables, if the subscriber is connected to the distant exchange by means of a large line length. These local lines are generally unpupinized lines. Negistors are also used between, for example, a trunk circuit and a junction circuit which usually has pupinized lines, and also if these circuits are connected by long lines having a high attenuation. A negistor primarily has to repeat and equalize in the voice-frequency band from 300 - 3400 Hz. Signals outside this band further have to be allowed to pass with no more than a slight attenuation. These signals, for example, are the DC power supply of the telephone set, count pulses at for example 50 Hz and 12 or 16 KHz, so that the negistor is to guarantee a reasonably reproducible signal transmission between 0 and 16 KHz. A negistor of the type mentioned hereinbefore is described in, for example, Philips Telecommunication Review, Volume 23, no. 1, October 1961, pp. 1-20. On page 10 of this article an example of a negative impedance converter is shown and on page 11 of this article an example of a negative admittance converter. The design of these two converters is substantially identical. In the negative admittance converter the first coil of the first transformer is coupled to the line and the second coil of the second transformer is terminated by a terminating impedance. In the negative impedance converter the second coil of the second transformer is coupled to the line and the first coil of the first transformer is coupled to an additional winding accomodated on the transformer, between whose contacts the terminating impedance is arranged. In the negative admittance converters described above the 180° phase shifter is formed by an additional winding which is accomodated on the first coil. Each of the contacts of this additional winding is connected to a control input of one of the repeater elements. The use of this additional winding is disadvantageous in that the desired 180° phase shift is restricted to a relatively narrow frequency range as a result of the sub-ideal behaviour of the first transformer. This sub-ideal behaviour is caused by the finite self inductance of the transformer, occurring parasitic capacitances, winding resistances and leakage inductances. The above deviations as a result of the sub-ideal behaviour are exhibited in the voice frequency band from 300-3400 Hz and over, by among other things the non-linear distortion, and outside this band dips in the transmission characteristic, i.e. attenuation peaks, occur below and above this band in connection with the terminating impedances of the converters. In addition, these attenuation peaks are irreproducible owing to the tolerances in the elements, which are even enhanced by the principle of the conversion (feedforward). This implies that the frequencies at which the said attenuation peaks occur are subjected to large tolerances, so that the count pulses at for example 50 Hz or 12 KHz are not always transmitted properly and, besides, lead to audible distortions during communication owing to their rather high level when compared to speech. The invention has for its object to provide a solution to the above disadvantages and is characterized in that the 180° phase shifter is formed by a direct voltage differential amplifier, whose one input is connected to a contact of a voltage divider that is inserted between the first contacts of the first and the second coil, the other input of the differential amplifier being connected to a contact of a voltage divider that is inserted between the second contacts of the first and the second coil and the control inputs of the first and the second amplifier element each being connected to an output of the differential smplifier. The invention will now be described with reference to the drawings, in which: Figure 1 shows an embodiment of a negative immittance converter according to the invention, Figure 2 shows a diagram to explain the operation of the converters as shown in Figure 1, Figure 3 shows an embodiment of a negative impedance converter according to the invention. In the embodiment as shown in Figure 1 a negative admittance converter is represented for a negative immittance converter. In this Figure, I denotes the first transformer comprising a first coil 1 and two primary windings 3 and 4. The contacts 51 and 52 are connected through a capacitive impedance 60. The contacts 50 and 53 are connected to the respective centre taps e and f of the coils 56 and 57 of the negative impedance converter as shown in Figure 3. The primary windings are thus effectively connected in shunt with the line. The capacitive impedance 60 is used for blocking the direct current. The first contact 5 of the first coil 1 is connected to the collector of the power transistor 10 operating as the first amplifier element. The first contact 5 of the first coil 1 is also connected to the first contact 7 of the second coil of the second transformer II via the voltage divider formed by the resistors 16 and 17. The emitter of the transistor 10 is connected to the first contact 7 of the second coil via the impedance 24. The second contact 6 of the first coil 1 is connected to the collector of the power transistor 11 operating as the second amplifier element. The second contact 6 of the first coil is also connected to the second contact 8 of the second coil 2 via a voltage divider formed by the resistors 23 and 28. The emitter of the transistor 11 is connected to the second contact 8 of the second coil 2 via the impedance 25. The centre tap 20 of the first coil 1 is connected to a point of positive constant potential and the centre tap 21 of the second coil is connected to a point of negative constant potential. The junction of the resistors 16 and 17 is connected to the input 29 of a DC differential amplifier 9. The junction of the resistors 23 and 28 is connected to the input 30 of the differential amplifier 9. Between the inputs 29 and 30 of the differential amplifier 9 a capacitor 19 is inserted. The input 29 of the differential amplifier 9 is connected to the centre tap 20 of the first coil 1 via an asjustable resistor 18. The input 30 of the differential amplifier 9 is connected to the centre tap 20 of the first coil 1 via an adjustable resistor 22. The output 31 of the differential amplifier 9 is connected to the control input of the transistor 10. The output 32 of the differential amplifier 9 is connected to the control input of the transistor 11. The second transformer II comprises the primary windings 56 and 57 which are connected in series and comprises between the contacts 33 and 34 the terminating impedance Z. For simplicity, the effects of the resistors 18 and 22, the capacitor 19, the impedances 24 and 25 and the base-emitter voltages of the transistors 10 and 11 are initially disregarded in an explanation of the operation of the negative impedance converter with reference to Figure 2. The resistances of the resistors 16, 17, 23 and 28 are selected to be equal to R. It is assumed that between the contacts 5 and 6 of the first coil 1 of the transformer I there is a voltage equal to U₁ and between the contacts 7 and 8 of the second coil of the second transformer II a voltage equal to U₂. The voltage between the outputs 31 and 32 of the differential amplifier 9 will be equal to U₂ when disregarding the effects stated above. If the gain of the differential amplifier 9 is equal to -A, the voltage between the inputs 29 and 30 of the differential amplifier 9 will be equal to -U₂/A. This voltage is substantially equal to 0 at a sufficiently large A. This implies that a current i equal to U₂/2R will pass through the resistors 17 and 28. Since the input current of the differential amplifier 9 is negligibly small, these currents i will also pass through the resistors 16 and 23. The voltage drop U(3) across resistor 16 will thus be equal to 1/2 U₂ and the voltage drop U(4) across resistor 23 will also be equal to 1/2 U₂. Since the voltage between the junctions 29 and 30 is substantially equal to 0, U₁ will be equal to -U₂. If it is further assumed that the winding ratio n of the two transformers I and II is equal to 1, and that the current i«I₁, the following relationships will hold: Z in =U₁/I₁ (1) I₁=U₂/Z (2) A combination of the relationships (1) and (2) provides, with U₂=-U₂ The use of the differential smplifier 9 in conjunction with the resistors 16, 17, 23 and 28 for realizing the 180° phaseshift between the voltages U₂ and U₁ is advantageous in that no additional winding is required on the second transformer II. Consequently, the two transformers I and II can be arranged completely identical. In addition to the advantage of better impedance conversion and better admittance conversion this also has advantages from a manufacturing point of view, because only one type of transformer is required. As a result of the feedback with the aid of the resistors 16, 17 and 23, 28 the converter operates virtually free of distortion up to the limits at which it is driven to full voltage swing, even for the agressive count pulses at for example 25 or 50 Hz. Because the frequency characteristic is less dependent on the sub-ideal behaviour of the transformers, it has strongly improved. Outside the 300-3400 Hz speech frequency band there is a uniform decrease of the gain without the marked attenuation peaks occurring with known converters. The resistors 18 and 22 are inserted to give the proper D.C. set values to the converter in view of the best possible drive to full voltage swing. The resistance of the resistors 18 and 22 is chosen to be substantially equal to the resistance of the resistors 16 and 17. With the insertion of the resistors 18 and 22 the effective gain of the differential amplifier 9 becomes approximately equal to -A/3. The two resistors 18 and 22 allow to be slightly varied with the aid of a potentiometer to correct the direct voltage off set at the outputs 31 and 32. This off set mainly arises as a result of the tolerances in the winding resistances of the transformers, because the largest currents pass through the power transistors 10 and 11. In order to reduce the mutual influence of the differential amplifier and the power transistor 10 and 11, these transistors are preferably arranged as a Darlington amplifier. The capacitor 19 between the inputs 29 and 30 of the differential amplifier 9 is inserted to cancel the negative admittance conversion for frequencies >10 KHz. The converter behaves as a positive admittance converter at these frequencies and neutralizes the behaviour of the transformers at these frequencies. In addition, at relatively high frequencies, 50-100 KHz, weak instabilities can be suppressed by inserting small capacitors 26 and 27 between the collector and the base of the transistors 10 and 11. In the embodiment as shown in Figure 3 a negative impedance converter is represented for the negative immittance converter. The structure of the section III, except for being a reflection, is the same as that of the negative admittance converter as shown in Figure 1 between the dashed lines. The terminating impedance Z is inserted between the contact 50 of winding 3 and the contact 53 of the winding 4. The contact 51 of coil 3 and the contact 52 of coil 4 are interconnected. The contact 5 and the contact 6 of the first coil of the first transformer I are connected to the section III in the way shown in Figure 1. The line b is connected to the contact 33 of the winding 56 having a centre tap on the transformer II. The line a is connected to the contact 34 of the winding 57 having a centre tap f on the transformer II. The centre taps e and f are connected to the points e and f of the negative admittance converter as shown in Figure 1. The line d is connected to the contact 54 of the winding 56. The line c is connected to the contact 55 of the winding 57. In this way the negative impedance converter is effectively arranged in series with the respective lines b and a , c and d . The operation of the section III is exactly the same as the operation of the section III of Figure 1, so that here too relationship 3 applies. The transformers I and II of the negative impedance converter as shown in Figure 3 are of the same type as the transformers I and II of the negative impedance converter as shown in Figure 1. If a negistor is constructed with the aid of these converters, a single type of transformer can be used which is tremendously advantageous from a manufacturing point of view.

Claims: A negative immittance converter, comprising a first transformer (I) that includes a first coil (1) with centre tap (20) and a second transformer (II) that includes a second coil (2) with centre tap (21), a first contact (5) of the first coil being connected to a first contact (7) of the second coil through a first amplifier element (10) comprising a control input, a second contact (6) of the first coil being connected to a second contact (8) of the second coil through a second amplifier element (11) comprising a control input, and the centre taps (20, 21) of the first and the second coil each being connected to a supply point of the converter and a 180° phase shifter (9) being inserted between the first and the second contact of the second coil and the two control inputs (31, 32) of the two amplifier elements, characterized in that the 180° phase shifter (9) is formed by a direct voltage differential amplifier, whose one input (29) is connected to a contact of a voltage divider (16, 17) that is inserted between the first contacts of the first and the second coils, the other input (30) of the differential amplifier being connected to a contact of a voltage divider (23, 28) that is inserted between the second contact of the first and the second coil and the control inputs of the first and the second amplifier element each being connected to an output of the differential amplifier. A negative immittance converter as claimed in Claim 1, characterized in that between the inputs of the differential amplifier and a point of constant potential, adjustable resistors are inserted. A negative immittance converter as claimed in Claim 1 or 2, characterized in that between the inputs of the differential amplifier a capacitor is inserted. A negative immittance converter as claimed in one of the preceding Claims, characterized in that the first and the second amplifier element is formed by a Darlington amplifier. A negative immittance converter as claimed in Claim 4, characterized in that a capacitor is inserted between the collectors of the Darlington amplifier and the base of the input transistor of the Darlington amplifier.

Inventors: VISSER, JACOB SIEBREN
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1988
Main classification: H04B3
IPC codes: H04B3, H04M19