JPH06101773B2 - High impedance circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high impedance circuit having an equivalent inductance as an electronic circuit, and more particularly to a high impedance circuit suitable for a current supply circuit of a subscriber circuit of a telephone exchange. .

[Background of the Invention]

The current supply circuit used in the subscriber circuit of the telephone exchange is
It is necessary to make a high impedance to an AC signal in order to receive a voice signal sent from the telephone without loss at the same time as supplying a large call current to the telephone which is a subscriber terminal. Therefore, conventionally, a letter coil or the like has been used. Electromagnetic components were used. However, since these electromagnetic parts are heavy and large, attempts have been made to digitize them for the purpose of miniaturization (see, for example, JP-A-53-41917). To this end, it is necessary to make a so-called equivalent inductance circuit that exhibits low resistance in terms of direct current and high impedance in terms of alternating current.
However, when an equivalent inductance is configured in an electronic circuit, when the output is turned on and off, the change state of the output current may be different from the actual inductance and may cause large vibration, which may cause a problem. It was

FIG. 1 shows a conventional high impedance circuit. In the figure, a current amplifier 1 including an operational amplifier OP1 and a transistor Q1 supplies a current I _L to a load R _L. Load voltage detection resistance R _i
And the capacitor CDC form a low-pass filter, whereby the current amplifier 1 is sufficiently negatively fed back in terms of direct current and its output resistance is sufficiently small, but it is a high impedance output because negative feedback is not applied in terms of alternating current. . The switch SW1 is a circuit breaker that cuts off the negative feedback in a direct current manner to set the output current I _L to 0. By constructing the circuit with an electronic circuit in this way, on / off control of the equivalent inductance circuit becomes easy. However, when the switch SW1 is closed and the output of the current amplifier 1 is cut off, the current I _L does not flow in the load R _L , and the power supply voltage is directly applied to the low-pass filter composed of the voltage detection resistor R _i and the capacitor C _DC , and therefore the capacitor C The voltage across _DC is significantly higher than when switch SW1 is open. Therefore, when the switch SW1 is opened next time to start supplying the load current I _L , the terminal voltage of the capacitor C _DC , which is the input of the current amplifier 1, becomes abnormally higher than the steady value, so that a large current transiently occurs. Flow to the load side,
If the load has a capacitance C _L , an oscillating waveform is generated in the load current.
FIG. 2 shows the waveform of the load current I _L corresponding to the opening / closing timing of the switch SW1. At time t ₁ , the switch SW1
Shows the state when is closed and then opened at time t ₂ .
Such an oscillating waveform is extremely harmful to a system that attempts to detect the state of the load side by monitoring the load current, for example, a loop detection function of a subscriber circuit of a telephone exchange.

[Object of the Invention]

An object of the present invention is to eliminate the above-mentioned drawbacks of the prior art and to provide a high impedance circuit configured so that the load current smoothly changes during on / off control.

[Outline of Invention]

The present invention provides a high-impedance circuit composed of a low-pass filter and a current amplifier, by providing a means for electrically disconnecting the low-pass filter circuit at the same time as cutting off the current amplifier that supplies the load current, thereby providing a low-pass filter when the load current is cut off. It is characterized in that it is configured to prevent an abnormal increase in the accumulated charge of.

Example of Invention

Hereinafter, the present invention will be described with reference to examples. FIG. 3 shows a first embodiment of the present invention, which is different from the conventional FIG. 1 in that a switch SW2 is provided in series with a capacitor C _DC , and other configurations are the same. Is. This switch
SW2 is provided to temporarily store the stored energy by disconnecting the capacitor C _DC that constitutes the low-pass filter from the circuit. In this embodiment, as in the case of FIG. 1, when the switch SW1 is opened and the switch SW2 is closed, a sufficient negative feedback is applied in terms of direct current to a low resistance, and in terms of alternating current, the feedback is stopped by the low-pass filter so that the impedance is high impedance. The output characteristics are as follows. FIG. 4 shows the response waveform of the load current I _L of the circuit of FIG. 3, and when t <t ₁ , the switch SW
With 1 open and switch SW2 closed, the high-impedance circuit in Fig. 3 constantly supplies current to the load resistance _RL . t = t ₁
Then, when the switch SW1 is closed, the output of the current amplifier 1 is cut off and only the current flowing through the voltage detection resistor R _i flows through the load R _L , but this is extremely small and can be regarded as almost zero. Therefore, most of the power supply voltage V _BB remains almost unchanged and is applied to the series circuit of R _i and the capacitor C _DC . As a result, the current of the voltage detection resistor increases and the charging charge of the capacitor C _DC also increases. A large current transiently flows at the start. Therefore, in the present invention, the switch SW2 is opened at the same time that the switch SW1 is closed, and the charge of the capacitor C _DC is held in the state immediately before the load current is cut off so that the voltage across the terminals of the capacitor C _DC does not increase. Therefore, at t = t ₂ , switch SW1 is opened,
When the switch SW2 is closed, the current amplifier 1 regains the load current I
While starting the supply of _L, its initial value, to the voltage of the capacitor C _DC as an input of the current amplifier 1 is held at the value immediately before the time of output shut-off, the most constant value equal to the current value . Actually the capacitor C when the switch SW2 is opened
_{Since the} potential drops slightly due to _DC self-discharge, as shown in FIG. 4, the load current I _L starts to be supplied from a value slightly lower than the steady value and reaches the steady value immediately. In this way, when the output is cut off, the accumulated charge of the low-pass filter is held in the state immediately before the output is cut off, and by controlling the on / off of the high impedance circuit so as to release the held state at the start of recharging the load current,
Smooth on / off control of load current is possible.

FIG. 5 shows a second embodiment of the present invention, in which the present invention is used in a current supply circuit for a subscriber circuit of a telephone exchange. In the figure, a current is supplied to a telephone 12 as a load by a sink type current amplifier 1 and a source type current amplifier 11. The equivalent circuit of the PNP type rent mirrors 4, 5 and 6 is shown in FIG. 6, and a current I _OUT equal to the input current I _IN is output. An equivalent circuit of the NPN type current mirrors 7 to 10 is shown in FIG. 7, and the operation is the same as that of the PNP type except that the current direction is opposite. R _DC and C _DC form a low pass filter. Transistors Q3 and Q4 are circuit breakers for the current amplifiers 1 and 11 and correspond to the switch SW1 in FIG. 3. Transistors Q5 and Q6 are current switches that disconnect the capacitor C _DC from the circuit together with the diode D. Equivalent to SW2. The circuit operation of FIG. 5 will be described below. Resistors R1, R2
As a result, a current proportional to the voltage on the B line side (2WB) flows in the current mirror 5 that detects the load voltage between 2WB and 2WA,
A current proportional to the voltage on the A line side (2WA) flows through the current mirror 7. The B line side detection current is inverted by the current mirror 8 and flows into the current mirror 6 together with the output current of the current mirror 7, so that a current proportional to the sum of the A line side voltage and the B line side voltage from the output of the current mirror 6. That is, a current proportional to the load voltage flows out. When the current is supplied to the load (steady state), the switch SW3 is open, the transistor Q5 is on, and the transistors Q3 and Q4 are off. Then, the load voltage detection current passes through a low-pass filter composed of C _DC and R _DC , where the AC signal component is removed, and the current mirrors 9 and 4 are driven to drive the current amplifiers 1 and 11. Therefore, when looking at the circuit side from the 2-wire load side (2WA, 2WB), it looks like low resistance due to the sufficient negative feedback for DC, and high impedance because the feedback is stopped for AC. . Now to outputs blocked by turning on drive transistor Q3, Q4 by bias current I _B closes switch SW3, the majority of the power supply voltage to the voltage detection resistors R1, R2 together with the load current as in the third figure is substantially zero Takes R1,
R2 of the current is increased which could result in an increase in the charging electric charge of the capacitor C _DC via the current mirror 5-8, but in the present embodiment through the current mirror 10 at the same time as on of the switch SW3 of the transistor Q6 The current switch is turned on and therefore transistor Q5 is turned off. Therefore, the load voltage detection current flowing out of the current mirror 6 flows into the resistor R _DC but does not flow into the capacitor C _DC due to the rectifying action of the diode D. Therefore, when the switch SW3 is turned on, the terminal voltage of the capacitor C _DC does not increase, and its potential is held by the steady method immediately before the output is cut off. Then switch SW
When 3 is turned off, the cut-off transistors Q3 and Q4 of the current amplifiers 11 and 1 are also turned off, and the load current starts flowing again.
Further, since the current switches Q5 and Q6 are also switched to the Q5 side, the load voltage detection current flowing from the output of the current mirror 6 flows into the current mirror 9 through the diode D, and the resistance R _DC and the capacitance The low-pass filter of C _DC starts to operate. Since the inputs of the current amplifiers 1 and 11 are determined by the input current of the current mirror 9, that is, the terminal voltage of the capacitor C _DC , the output current when the load current is turned on becomes equal to the value immediately before the output is cut off. A smooth rising current waveform without transient vibration can be obtained as in the case of. Generally, the current supply circuit turns on the telephone in addition to the function of supplying the call current to the telephone 12. Hook off. A function to detect the hook state is required. If there is no change in the state of the telephone side and ON / OFF control of the current supply circuit side is performed, if transient vibration occurs in the load current as in the conventional example, incorrect detection information is given to the state monitoring circuit (not shown) on the telephone side. . However, with the configuration of this embodiment, such malfunction can be prevented.

〔The invention's effect〕

According to the present invention, transient vibration of a current waveform at the time of on / off control of a high-impedance circuit configured as an electronic circuit is controlled, harmful noise can be prevented from being generated, and a state of a telephone side when used in a current supply circuit of a telephone exchange. The effect is that monitoring can be performed accurately.

[Brief description of drawings]

FIG. 1 is a diagram showing a conventional high impedance circuit, and FIG.
1 is a diagram showing a response waveform of a load current in the circuit of FIG. 1, FIG. 3 is a diagram showing a first embodiment of the present invention, and FIG. 4 is a response waveform of a load current in the circuit of FIG. FIG. 5 and FIG. 5 show the second embodiment of the present invention, FIG. 6 and FIG.
The figure is an equivalent circuit diagram of the current mirror. 1,11 …… Current amplifier, 4,5,6 …… PNP type current mirror, 7,8,9,10 …… NPN type current mirror, 12 …… Phone, C _DC …… Capacitor, D …… Diode, SW1, SW2, SW3 ... Switch.

 ─────────────────────────────────────────────────── ─── 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 low-pass filter for removing an AC component is provided in a feedback loop of a current amplifier circuit so that its output impedance is sufficiently low for direct current and sufficiently high for audio frequency, and the current is low. In a high impedance circuit having an output circuit breaker for cutting off the output of the amplifier circuit, a holding circuit for controlling to temporarily hold the electric energy of the low-pass filter when the output is cut off by the output circuit breaker is provided. Characteristic high impedance circuit.