JPH0327157B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to a subscriber circuit connected to a two-wire subscriber line in a telephone exchange, and particularly to a signal transmission circuit suitable for computerization.

[Prior art]

Conventionally, a circuit as shown in FIG. 1 has been considered as this type of signal transmission circuit. In FIG. 1, 1 is a subscriber terminal (telephone), 2 and 3 are a pair of terminals connected to the two-wire subscriber line L, and 4 is a differential signal (balanced signal) output from the telephone 1. ); 5 is a balanced/unbalanced conversion circuit that converts a balanced signal into an unbalanced signal;
Reference numeral 6 denotes an impedance circuit consisting of a capacitor 7 and a resistor 8, which has the function of removing a DC component contained in the output signal of the balanced/unbalanced conversion circuit 5. Further, 9 is an amplifier circuit, which includes an operational amplifier 10 whose inverting input terminal receives the output of the impedance circuit 6 and whose non-inverting input terminal is grounded, and an output terminal 11.
and an impedance element 12 connected between the output terminal 11 and the inverting input terminal of the operational amplifier 10.
and power supply terminals 13 and 14.
This type of signal transmission circuit converts a signal (balanced signal) output from a telephone into an unbalanced signal, and can send the signal to other devices with an arbitrary gain.

However, for example, if the telephone is a push-button type, when the button is pressed, a multi-frequency signal may be output and the DC resistance may change, and this change in resistance causes the DC current flowing through the telephone to also change, which is generated in the subscriber line L. The differential signal (balanced signal) thus generated has a waveform as shown in FIG. 2a. Since a signal generated by a resistance change has a large voltage change compared to a multi-frequency signal, in the circuit configuration shown in FIG. 1, when the power supply voltage of the operational amplifier 10 is low, the dynamic range of the operational amplifier 10 is insufficient. There is a point that momentary interruptions of multi-frequency signals occur. (Waveform in FIG. 2b) This instantaneous interruption time t is determined by the impedance of the impedance circuit 6, the impedance of the impedance element 12, and the dynamic range of the operational amplifier 10. Normally, impedance circuit 6
The impedance of the impedance and the impedance of the impedance element 12 are large, that is, the discharge time constant of the charge accumulated in the capacitor 7 is large, and the power supply voltage of the operational amplifier 10 is low (±5V).
The instantaneous interruption time t becomes longer.

[Purpose of the invention]

An object of the present invention is to eliminate the above-mentioned drawbacks, to provide a signal transmission circuit that has good characteristics as a subscriber circuit of a telephone exchange, has a simple circuit configuration, and can be implemented as an IC or LSI. be.

[Structure of the invention]

The signal transmission circuit of the present invention includes a detection circuit that detects a balanced signal generated in a two-wire subscriber line connected to a subscriber terminal, and a balanced/balanced signal that converts the output of the detection circuit into a balanced signal.
an unbalanced conversion circuit; an impedance circuit including a capacitor connected to the balanced/unbalanced conversion circuit as a series element; an output from the impedance circuit is applied to one input terminal, and a reference potential is applied to the other input terminal. an amplifier; and a clamp element provided between an arbitrary point between the output side of the capacitor and the input terminal of the amplifier and the reference potential, and having a clamp voltage equal to or higher than a maximum AC signal voltage at the arbitrary point. It is characterized by having the following.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings. The range is a circuit diagram showing one embodiment of the present invention. Note that the members having the same reference numerals as those shown in FIG. 1 have functions, and their explanations will be omitted. The difference from the conventional configuration is that the clamp circuit 15
This is the addition of . The clamp circuit 15 consists of two diodes 16 and 17 connected in antiparallel, and is interposed in parallel between the impedance circuit 6 and the amplifier circuit 9 with one side grounded.

According to the circuit configuration according to this embodiment, even if a signal as shown in FIG. Furthermore, the electric charge charged in the capacitor 7 is transferred to the diode 16 of the clamp circuit 15,
17, the instantaneous interruption time of the multi-frequency signal can be shortened. This is because the impedance of the diode is extremely small, so the discharge time constant is small.

That is, if the voltage between the input of the impedance circuit 6 and the reference potential (+ side terminal) of the operational amplifier 10 is V _IN , then the output voltage of the operational amplifier 10 is V _O
is, V _O =V _IN・Z/Z _i , where Z _i : impedance of impedance circuit 6, Z: impedance of impedance element 12,
becomes.

When V _O calculated from the above equation exceeds the power supply voltage of the operational amplifier 10, the output of the amplifier 10 is saturated, and the potential difference between the + and - terminals of the amplifier 10 is no longer zero. Therefore, the discharge time constant of the capacitor 7 of the impedance circuit 6 is determined by the impedances Z _i and Z.

Normally, since the impedance of Z is large, the discharge time is long, and the time it takes for the output of the operational amplifier 10 to go from saturation to a normal state becomes long. If there is a clamp circuit 15 as shown in FIG.
0 saturates, and the potential difference between the + and - terminals of this amplifier 10 is limited by the clamp circuit 15. Therefore, the discharge time constant of the capacitor 7 of the impedance circuit 6 is determined by Z _i and the impedance of the clamp circuit 15. Since the impedance of the clamp circuit 15 is smaller than the impedance Z of the element 12, the discharge time is shortened, and the time it takes for the output of the operational amplifier 10 to go from saturation to the normal state is shortened.

In this example, a column using an anti-parallel connection circuit of diodes is shown as a clamp element, but known clamp circuits such as a clamp circuit using a field effect transistor or a clamp circuit combining a diode and a voltage source can also be applied. It goes without saying that this is possible and can be applied to the present invention regardless of the configuration of the clamp element.

Note that the voltage drop in the clamp circuit (approximately 0.75 V in this embodiment) needs only to be equal to or higher than the maximum AC signal level at the connection point of the clamp circuit. In this example, a clamp circuit is connected to the inverting input terminal of the operational amplifier.
0.75V is sufficient. Furthermore, the connection position of the clamp circuit is not limited to the input point of the amplifier, but the same effect can be obtained at any position from the amplifier side of the capacitor 7 included as a series element in the impedance circuit 6 to the input point of the amplifier.

〔Effect of the invention〕

As explained above, the present invention reduces the instantaneous signal interruption time by adding a clamp element, and has good characteristics as a subscriber circuit of a telephone exchange.
It also has the effect of making IC and LSI implementation possible.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing a conventional signal transmission circuit, FIGS. 2a and b are schematic diagrams showing input and output waveforms of the circuit shown in FIG. 1, and FIG. 3 is a circuit diagram showing an embodiment of the present invention. It is. 1...Subscriber terminal (telephone), 4...Detection circuit,
5... Balanced/unbalanced conversion circuit, 6... Impedance circuit, 7... Capacitor, 8... Resistor, 9...
...Amplification circuit, 10...Operation amplifier, 15...Clamp circuit, 16, 17...Diode, L...2
Wired subscriber line.

Claims (1)

[Claims] 1. A detection circuit that detects a balanced signal generated in a two-wire subscriber line connected to a subscriber terminal, and a balanced/unbalanced conversion that converts the output of the detection circuit into an unbalanced signal. an impedance circuit including a capacitor connected to the balanced/unbalanced conversion circuit as a series element; an amplifier that applies an output from the impedance circuit to one input terminal and a reference potential to the other input terminal; a clamp element that is provided between an arbitrary point between the output side of the capacitor and the input terminal of the amplifier and the reference potential, and has a clamp voltage that is greater than or equal to the maximum AC signal voltage at the arbitrary point. A signal transmission circuit characterized by: