JPH0666843B2 - Common mode noise reduction circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a common mode noise reduction circuit of a current supply circuit used in a subscriber circuit of a telephone exchange, and more particularly to a line fault suitable for a semiconductor integrated circuit of the current supply circuit. The present invention relates to a common-mode noise reduction circuit configured in consideration of circuit protection at the time.

[Background of the Invention]

The subscriber circuit of a conventional telephone exchange supplies current of about 20 to 120 mA to the subscriber's telephone from -48V of the station power supply. Therefore, since the current supply circuit directly connected to the telephone handles high voltage and large current, electromagnetic components such as a letter coil are used, and electronicization has been delayed. Further, in the case of electronization, it is necessary to sufficiently reduce the common mode impedance of the current supply circuit in order to sufficiently suppress the common mode noise voltage induced in the line.
For this reason, there is a problem that it is difficult to form a semiconductor integrated circuit because an excessive current flows in the current supply circuit at the time of line failure and the power loss of the circuit increases.

[Object of the Invention]

An object of the present invention is to provide a common mode noise reduction circuit for a current supply circuit that solves the above-mentioned problems of the prior art and limits the power loss at the time of line failure to facilitate the integration into a semiconductor integrated circuit.

[Outline of Invention]

In the present invention, a common-mode voltage detector for detecting a common-mode voltage of a balanced line, a common-mode amplifier for amplifying the common-mode voltage, and a circuit for a balanced output amplifier driven by the common-mode amplifier are used to perform a sufficient common-mode negative feedback to perform common-mode operation. The impedance is lowered to suppress common mode noise, and when a line fault occurs, an excessive common mode input signal is bypassed by the common mode input protection circuit to limit the input of the common mode amplifier to limit the output current of the balanced output amplifier. If a significant current limit is required, the common-mode cutoff circuit controlled by an external signal is used to stop the operation of the common-mode amplifier, and at least one of the common-mode input protection circuit and the common-mode cutoff circuit causes overcurrent at line fault. It is a common mode noise reduction circuit of a current supply circuit which is protected and suppresses power loss of the circuit and can be made into a semiconductor integrated circuit.

Example of Invention

An embodiment of the present invention will be described below with reference to FIGS. FIG. 1 is a circuit diagram showing an embodiment of a common mode noise reduction circuit of a current supply circuit according to the present invention.

In FIG. 1, reference numeral 1 is a current source type amplifier, which includes an operational amplifier OPB, a transistor Q1, and resistors R1 to R3, and 2 is a current sink type amplifier, which is an operational amplifier OPA and a transistor Q2.
And the resistors R6 to R8, the amplifiers 1 and 2 constitute a balanced output amplifier, and supply a signal current to the telephone TEL connected to the B line side B and the A line side A of the balanced line. A1 and A2 are current mirrors that have two outputs O1 and O2, respectively, and output a current output proportional to the input current.
As illustrated in the detailed circuit diagram showing the specific configuration of 2, A
1 is a current source type and transistors Q11 to Q13 and resistors R12 to R1
4 and A2 is a current sink type transistor Q14 to Q16
A current mirror similar to A1 and A2 except that B1 and B2 each have one output, and the voltage on the B line side B is applied to the resistor R4 and the current mirror A1.
The negative feedback circuit which detects the current at the current mirror B1 and inverts the current at the current mirror B1 is added to the amplifier 1 to determine the impedance on the B line side. Similarly, the voltage on the A line side A is detected by the resistor R5 and the current mirror A2. Moreover, a current is inverted by the current mirror B1 to form a negative feedback circuit which is added to the amplifier 2 to determine the impedance on the A line side.

In the current supply circuit as it is, since the common mode impedance is the same value as the differential impedance and the common mode noise is not reduced, the common mode signal is detected and a large amount of negative feedback is applied only to the common mode signal to obtain the common mode impedance. Add a common-mode noise reduction circuit that significantly reduces it. That is, assuming that the differential current id is the in-phase current ic generated in the balanced line, the differential current id is canceled by the output O1 of the current mirrors A1 and A2 due to the function of the current mirror as shown in FIG. Flow in the direction of addition and form an in-phase voltage detection circuit that detects an in-phase signal, and this in-phase signal is input to the in-phase amplifier 4. The common-mode amplifier 4 includes transistors Q5 and Q6 which supply complementary outputs to the current sink inputs to the balanced output amplifiers 1 and 2, and a transistor Q3 which supplies complementary outputs to the balanced output amplifiers 1 and 2 to the resistor R10 and the current source input. Q4, resistance
Diodes D2 to D5 and Zener diode that supply bias current for distortion prevention to R9 and these transistors Q3 to Q6
It is composed of D1, resistor R11, and constant current source I1. Reference numeral 3 is an in-phase input protection circuit, which is composed of transistors Q7 and Q8. Since the bases of the transistors Q7 and Q8 are connected to the middle point of the bias generating diodes D2 to D5 of the in-phase amplifier 4, they are cut off during normal operation. By operating only when the common mode input is too large, a large detection current is generated in the above common mode voltage detector at the time of line failure and further amplified by the common mode amplifier 4, and then the balanced output amplifiers 1 and 2 are driven excessively. The circuit protection is performed by limiting the current that an excessive current flows through the amplifiers 1 and 2 to increase the power loss abnormally. Further, reference numeral 5 is an in-phase cutoff circuit comprising transistors Q9 and Q10, current mirrors C1 and C2 for driving them, a current source I2 and a switch SW. The current mirrors C1 and C2 function in the same manner as the current mirrors B1 and B2. SW
Is turned on when the common-mode cutoff circuit 5 is operated by external control, and is stopped to stop the operation of the common-mode amplifier 4 when the line fault is more serious, thereby performing more thorough circuit protection.

With the above-mentioned configuration, in the normal operation, the common-mode signal of the balanced line detected by the common-mode voltage detector is sufficiently amplified by the common-mode amplifier 4 and a large amount of common-mode feedback is applied to the balanced output amplifiers 1 and 2, The common mode impedance is sufficiently reduced, common mode noise is sufficiently suppressed, and high quality signal transmission is performed. However, at the time of a line fault, for example, when the A line is grounded, the output O1 of the current mirror A2 becomes an excessive output, the collector currents of the transistors Q4 and Q3 of the common-mode amplifier 4 become excessive, and the resistor R9
When the voltage drop of V becomes larger than the forward base-emitter voltage V _BE (V _BE 0.7V) of the transistor Q7 of the common mode input protection circuit 3, the transistor Q7 operates and the transistors Q3, Q
Limit the collector current of 4 to V _BE / R9 (where R9 is the resistance of resistor R9) or less. Also, in the case of a line failure, for example, when the battery V _{BB of the} B line is contacted, the common mode input protection circuit 3 is similarly provided.
By the operation of the transistor Q8, the collector current of the transistors Q5 and Q6 is current limited to V _BE / R10 or less. In this way, the output of the common-mode amplifier 4 is limited to a certain value or less, so that the output currents of the balanced output amplifiers 1 and 2 are also current-limited to the corresponding certain value or less and the power loss is limited to a certain value or less to protect the circuit. To be done. However, in the case of a more significant line failure, for example, when the lines are in contact with the power line, the switch SW of the common mode cutoff circuit 5 is turned on by an external control to turn on the transistors Q9 and Q10. And Q4 to D5 are short-circuited, the transistors Q3 to Q6 are cut off, and the in-phase input signal that normally drives the transistors Q3 to Q6 flows through the transistor Q7 or Q8 of the in-phase input protection circuit 3. The output O1 of the mirrors A1 and A2 is not saturated, and the in-phase signal is cut off to perform more thorough circuit protection.

Various modifications can be made to the configuration of each circuit element in FIG. 1 of the above embodiment. For example, FIG. 3 is a circuit diagram showing another embodiment of the in-phase amplifier 4 and the in-phase cutoff circuit 5 of FIG. In FIG. 3, the same symbols or symbols as in FIG. 1 indicate the same or corresponding portions, and Q ′ _{3 to} Q ′.
₅ is a transistor pair connected in Darlington, D' ₂ ~
D' ₅ is also a Darlington-connected transistor pair, and I ₃ and I ₄ are current sources. With this configuration, the common-mode amplifier 4
Replaced with transistor pairs _Q '3 _{~Q' 6} that the transistor Q ₃ to Q ₆ of Figure 1 in Darlington connected in order to improve the relative accuracy of the complementary outputs of the effective, which together with the bias generator of Figure 1 The diodes D _{2 to} D ₅ are also equivalently diode-configured by the Darlington-connected transistor pair D ′ _{2 to} D ′ ₅ , and the transistors Q ₉ and Q ₁₀ of the common-mode cutoff circuit 5 are driven by one of the circuits shown in FIG. The two constant current sources I ₃ and I ₄ are separately driven instead of the constant current source I ₂ .

As described above, according to the above embodiment, the operations of the common mode input protection circuit and the common mode amplifier that form the input common mode signal bypass path in the common mode noise reduction circuit of the current supply circuit when the input of the common mode amplifier is excessive are stopped by the external control. By adding the common mode cutoff circuit, it is possible to easily and accurately prevent the balanced output amplifier of the current supply circuit from being excessively driven by at least one of the common mode input protection circuit and the common mode cutoff circuit at the time of a line failure.

〔The invention's effect〕

As described above, according to the present invention, it is possible to prevent an abnormal increase in power loss at the time of a line failure due to the addition of the common mode noise reduction circuit to the current supply circuit. This has the effect of enabling semiconductor integrated circuits.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a common mode noise reduction circuit of a current supply circuit according to the present invention, FIG. 2 is a detailed circuit diagram of the current mirrors A1 and A2 of FIG. 1, and FIG. 3 is of FIG. It is a circuit diagram which shows another Example of an in-phase amplifier and an in-phase circuit breaker. 1,2 …… Balanced output amplifier 3 …… In-phase input mixing circuit 4 …… In-phase amplifier 5 …… In-phase circuit breaker A1, A2, B1, B2, C1, C2 …… Current mirror TEL …… Telephone, A… … A line B …… B line, I1 to I4 …… Constant current source V _BB …… Battery, Q _{1 to} Q ₁₀ …… Transistor R _{1 to} R ₁₁ …… Resistance, D1 …… Zener diode D2 to D5 …… diode Q _{'3 ~Q' 6} ...... Darlington-connected transistor pair D _{'2 ~D' 5} Darlington-connected transistor pair ...... diode function

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toshio Hayashi 1839 Ono, Atsugi City, Kanagawa Pref., Atsugi Electro-Communications Research Laboratories, Nippon Telegraph and Telephone Corp. Atsugi Research Institute of Electrical Communication

Claims (1)

[Claims] 1. A current supply circuit for supplying a signal current to a balanced line, a common-mode voltage detector for detecting a common-mode voltage induced in the balanced line, a common-mode amplifier for inputting and amplifying the common-mode voltage, and the common-mode amplifier. A balanced output amplifier which is driven to drive the balanced line; an in-phase input protection circuit which forms a bypass path for bypassing the input in-phase signal when a predetermined value is generated when the input of the in-phase amplifier is excessive; and the in-phase amplifier. And a common-mode cutoff circuit that stops the operation of the above-mentioned by external control, and is configured to prevent the balanced output amplifier from being excessively driven by at least one of the common-mode input protection circuit and the common-mode cutoff circuit when the balanced line fails. A common-mode noise reduction circuit characterized by the above.