JPH02280555A - Dc characteristic variable circuit for telephone set - Google Patents

Abstract

PURPOSE:To prevent noise from being sent to a telephone line when the DC characteristic is changed over by selecting a resistance ratio of a resistor at both sides of a 2nd connecting point so that a terminal voltage of a capacitance is unchanged when a switch circuit is switched. CONSTITUTION:When a switch circuit SW is closed, a current of a constant current source I flows to a 2nd resistor R2 and a 3rd resistor R3 and a power supply is supplied to a dialer circuit or the like. In this case, even when the switch circuit SW is closed, the resistance ratio of the 3rd resistor R3 and the 4th resistor R4 is set so that a terminal voltage V1 of the capacitor C is unchanged, no charge/discharge of the capacitor C is caused and a DC voltage V2 between telephone line connection terminal pair 1a, 1b is changed stably, and noise transmission to the telephone lines La, Lb is not caused and no mis- dialing due to noise is caused.

Description

[Detailed Description of the Invention] [iJr! , Object of the Invention] (Industrial Application Field) The present invention relates to a variable DC characteristic circuit for a telephone for instantaneously switching the DC resistance of a telephone communication circuit of the telephone between a communication state and a dial state.

(Prior Art) Normally, telephone lines are non-polar, so in electronic telephones, as shown in FIG.
, lb and the speech network integrated circuit IC to stabilize the input voltage direction of the integrated circuit IC.

Further, on the 'Ri speech lines La and Lb, a communication signal (a transmission signal and a voice signal) is superimposed on the DC voltage. Therefore, in order to ensure a stable power supply voltage for the integrated circuit IC, the voltage is quite low.

That is, as a standard for the direct current resistance of the telephone communication circuit of a telephone, the telephone line connection terminal pair 1a is used in the telephone communication state.

For example, when a current of 20 mA or more is passed between lb.
6. If the resistance is determined to be 300Ω, between the telephone line connection terminal pair 1a and lb. Ov(=20mAX300
Even if a DC voltage of Ω) is applied, the forward voltage drop of each diode D of the diode bridge circuit 70 is 0.7V, and the telephone line &! aL a.

The dynamic range of the collocation signal superimposed on Lb is 2.
Ov, the power supply voltage of the integrated circuit IC is 2.
6V (-6.0V-2X0.7V-2.0V).

On the other hand, in the dial state, etc., other circuits other than the call circuit, such as DTMF (Dual Tone
Since it is necessary to operate a dialer circuit etc. for generating a dial tone consisting of a Maltl Frequency) signal, the telephone line connection terminal pair la
, lb (for example, the DC resistance when a current of 20 mA or more is passed is 50 to 550 Ω).

As described above, since it is necessary to switch the DC resistance of the communication circuit, conventionally, in the integrated circuit IC, as shown in FIG.
a) Alternatively, a DC characteristic variable circuit as shown in FIG. 5(a) is used.

That is, in the circuit shown in FIG. 4(a), between the collector and emitter of the NPN transistor Q1 and the first resistor R1 (this resistor R1
may be omitted) are connected in series, and the second
A resistor R2 to a fourth resistor R4 are connected in series. The connection point between the third resistor R3 and the fourth resistor R4 is connected to the base of the NPN transistor Q1, and connected to one telephone line connection terminal 1b via the switch circuit SW and the constant current source I in series. and a second resistor R
2 and the third resistor R3 is connected to one telephone line connection terminal 1b via a capacitor C.

In addition, in the circuit shown in FIG. 5(a), an NPN transistor is connected between the telephone line connecting terminal pair 1a and lb, and a first resistor R1 is connected between the collector and emitter of the transistor Q1 (this resistor R1 may be omitted). ) are connected in series, and second to fourth resistors R2 to R4 are connected in series.

The connection point between the third resistor R3 and the fourth resistor R4 is connected to the base of the NPN transistor Q1, and the connection point between the second resistor R2 and the third resistor R3 is connected to the switch circuit SW and the constant It is connected to one telephone line connection terminal 1b via a current source I in series, and is also connected to one telephone line connection terminal 1b via a capacitor C.

In the circuit shown in FIG. 4(a) or FIG. 5(a), when a predetermined current is passed between the pair of telephone line connection terminals 1a and lb, a DC characteristic as shown in FIG. 6 is obtained. Here, by controlling the opening and closing of the switch circuit SW, its DC characteristic changes as shown in the figure. That is, when the switch circuit SW is closed, the base current of the NPN transistor Q1 decreases, its collector-emitter resistance increases, and the DC resistance between the telephone line connection terminal pair 1a and lb increases, ? The voltage between the pair of communication line connection terminals 1a and lb increases.

However, in the circuit shown in FIG. 4(a), the series circuit of the switch circuit SW and the constant current source is connected to a different point from the capacitor C, so when the switch circuit SW is closed, the There is a problem in that a transient phenomenon as shown in FIG. 4(b) occurs. That is, when the switch circuit SW is open, the NPN transistor Q1 is connected to the second resistor R2.
The divided voltage by the fourth resistor R4 is applied to the base,
The collector-emitter resistance is small, and the DC voltage between the telephone line connection terminal pair 1a and lb at this time is v
Represented by 2. At this time, the capacitor ilC is charged, and the terminal voltage across it is expressed as Vl (here, Vl<V2).

If the switch circuit SW suddenly closes in this state, a constant current source is applied to the second resistor R2 and the third resistor R3! A current flows, and the voltage v2 between the pair of telephone line connection terminals 1a and lb immediately rises, supplying power to, for example, the dialer circuit as described above.

However, as this voltage rises, the capacitor C is further charged and the terminal voltage v1 rises until it becomes stable.
Since an extra current flows through the resistor R2 of the telephone line La, the voltage v1 between the telephone line connecting terminal pair 1a and Lb instantly jumps up.

This may cause noise to be sent to Lb, causing erroneous dialing.

In addition, in the circuit shown in FIG. 5(a), the series circuit of the switch circuit SW and the constant current source I is connected to the same point as the capacitor C, so the switch circuit SW as described above is closed. There is no possibility that the voltage 2 between the telephone line connecting terminal pair 1a and Lb will instantly jump and send out noise to the telephone line La and Lb.

However, as shown in FIG. 5(b), the switch circuit SW
Telephone line connection terminal pair 1a when is idle.

The rise in DC resistance between Lb is delayed and the telephone line connection terminal pair 1
There is a problem in that the rising speed of the DC voltage v2 between a and Lb becomes slow, and the supply of power to, for example, the dialer circuit as described above is delayed.

That is, when the switch circuit SW is closed, if the DC resistance between the telephone line connecting terminal pair 1a and Lb increases, the current flowing between the telephone line connecting terminal pair 1a and Lb decreases, so that the terminal voltage V1 of the capacitor C is It becomes slightly lower than when the circuit SW was open, and the charge in the capacitor C is discharged through the third resistor R3.

As a result, during this discharge, the NPN) transistor Q1
The base current decreases slowly, the collector-emitter resistance increases slowly, and the DC resistance between the telephone line connecting terminal pair 1a and Lb increases slowly.

The rising speed of the DC voltage v2 between Lb becomes slower.

(Problems to be Solved by the Invention) As mentioned above, in the conventional variable DC characteristic circuit for telephones, when the switch circuit is closed, the voltage between the pair of telephone line connection terminals jumps instantaneously due to charging of the capacitor. There is a problem in that it sends noise onto the line, causing incorrect dialing.

Another conventional variable DC characteristic circuit for telephones is that when the switch circuit is closed, the capacitance is discharged, causing a delay in the rise in DC resistance between the pair of telephone line connection terminals, which delays power supply to dialer circuits, etc. There's a problem.

The present invention has been made to solve the above problems, and its purpose is to prevent noise from being sent to the telephone line when closing the switch circuit and switching the DC characteristics, and to prevent noise from being transmitted to the telephone line connection terminal. To provide a variable DC characteristic circuit for a telephone set, which can prevent incorrect dialing and delays in power supply to dialer circuits, etc., without delaying the rise in DC resistance between pairs.

[Structure of the Invention] (Means for Solving the Problems) A variable DC characteristic circuit for a telephone according to the present invention includes a plurality of resistors connected in series between a pair of telephone line connection terminals, and a plurality of resistors connected to each other in series. a capacitor connected between a first connection point of each connection point and one telephone line connection terminal of the pair of telephone line connection terminals; a constant current source connected between the connection point of No. 2 and the one telephone line connection terminal;
The constant current source and jf! A switch circuit for switching and controlling connection with the second connection point or operation of the constant current source, and a third connection point among the connection points of the plurality of resistors. The present invention is characterized by comprising a current circuit that causes a current to flow between the pair of telephone line connection terminals in accordance with a bias.

(Function) If the resistance value ratio of the resistors on both sides of the second connection point is set so that the terminal voltage of the capacitor does not change even when the switch circuit is opened and closed, charging and discharging of the capacitor will not occur, and the telephone The DC voltage between the pair of line connection terminals changes stably. Therefore, the voltage between the pair of telephone line connection terminals will not jump up instantaneously and send out noise to the telephone line (this will prevent incorrect dialing from occurring due to this noise. Also, the DC resistance between the pair of telephone line connection terminals will increase). There is no longer a delay in the power supply to the dialer circuit, etc.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

FIG. 1 shows the basic configuration of a variable DC characteristic circuit for a telephone, and compared to the conventional variable DC characteristic circuit for a telephone described above with reference to FIG. Since the connection point of the series circuit with the circuit is different and the other parts are the same, the same parts as in FIG. 4(a) are given the same reference numerals. That is, N between the pair of telephone line connection terminals 1a and lb.
PN) Between the collector and emitter of transistor Q1 and the first
are connected in series with the resistor R1 (this resistor R1 may be omitted), and the second resistor R2 to the fifth resistor R
5 are connected in series. The connection point (first connection point Nl) between the second resistor R2 and the third resistor R3 is connected to one telephone line connection terminal 1b via the capacitor C,
A connection point between the third resistor R3 and the fourth resistor R4 (second connection point N2) is connected to one telephone line connection terminal 1b via a switch circuit SW and a constant current source ■ in series. The connection point between the fourth resistor R4 and the fifth resistor R5 (third connection point N3) is connected to the base of the NPN transistor Q1.

Next, the operation of the variable direct current characteristic circuit for a telephone shown in FIG. 1 will be explained with reference to FIG. 2. When the switch circuit SW is open, the divided voltage of the third connection point N3 is applied to the base of the NPN transistor Q1, and its collector-emitter resistance becomes small, and the telephone line connection terminal at this time The DC voltage v2 between the pairs la and lb is small. At this time, the capacitor jlC is being charged, and the terminal voltage v1 across it is smaller than the voltage v2.

When the switch circuit SW is closed in this state, the second resistor R
Constant current source for the second and third resistors R3! A current of N
The base current of the PN transistor Q1 decreases, its collector-base resistance increases, and the DC resistance between the telephone line connection terminal pair 1a and Lb increases, and the telephone line connection terminal pair 1a increases.

The DC voltage v2 between Lb immediately rises, and power is supplied to, for example, the dialer circuit as described above.

In this case, even if the switch circuit SW is closed, the terminal voltage V1 of the capacitor C does not change (that is, the switch circuit SW
If the resistance value ratio of the third resistor R3 and the resistor R4 of m4 is set so that the potential v1 is always the same regardless of whether the The DC voltage v2 between 1a and Lb changes stably.

Therefore, the DC voltage v between the telephone line connection terminal pair 1a and Lb
2 instantly jumped up and the phone line La.

Noise is no longer sent to Lb, and erroneous dialing due to this noise no longer occurs. Furthermore, there is no longer any delay in the rise in DC resistance between the pair of telephone line connection terminals 1a and Lb, and there is no delay in the supply of power to the dialer circuit or the like.

FIG. 3 shows another embodiment of the variable DC characteristic circuit for telephones, and the differences from the variable DC characteristics circuit for telephones described above with reference to FIG. 1 are mainly (a) NPN
In place of the transistor Q1, a Darlington-connected first NPN transistor Qll, a PNP transistor QIO, and a second NPN) transistor Q12 are used, and their current capacity is high; The connection relationship with the current source I is otherwise the same, so the same parts as in FIG. 1 are given the same reference numerals.

That is, the emitter-collector of a PNP transistor QIO is connected between the collector base of the first NPN transistor Qll, and the second NPN transistor QIO is connected between the base of the PNP transistor QIO and the emitter of the first NPN transistor Qll. Q12 collector/
The emitters are connected. Also, the second NPN)
Base of transistor Q12 and one telephone line connection terminal 1
A first diode D1 is inserted in series with the fifth resistor R5.

Also, the second connection point N2 and one telephone line connection terminal 1b
Constant current source between! 3rd NPN) transistor Q
A second diode D2 and a second diode D2 are connected in series between the collector and emitter of the third NPN transistor 013 and between the collector and emitter of the third NPN transistor 013 and one telephone line connection end T-1b. A third diode D3 is inserted in series, a seventh resistor R7 is connected between the base of the third NPN transistor 01B and the first connection point N1, and a seventh resistor R7 is connected between the base of the third NPN transistor 013 and one of the telephones. A switch circuit SW is connected between the line connection terminal 1b and the line connection terminal 1b. Furthermore, an eighth resistor R is connected in parallel to the capacitor WLC.
8 are connected.

Next, the operation of the variable telephone DC characteristic circuit shown in FIG. 3 will be explained. When the switch circuit SW is idle, the third NPN transistor Q13 is turned off, and the second
The voltage divided by the resistors R2 through the fifth resistor R5 and the first diode D1 is applied to the base'! J2 NPN
The base current of transistor Q12 is small and the first NP
The collector-emitter resistance of the N transistor Qll is small, and the telephone line connection terminal pair 1a at this time.

The DC voltage v2 between lb and lb is small. At this time, the capacitor C is charged, and the terminal voltage V1 across it is smaller than the voltage V2.

When the switch circuit SW opens in this state, the third NPN
) transistor Q13 is turned on, current of the third NPN transistor Q13 for constant current source I flows through the second resistor R2 and third resistor R3, and the base current of the second NPN transistor Q12 decreases. , the collector-emitter resistance of the first NPN transistor Qll increases, the DC resistance between the telephone line connection terminal pair 1a and lb increases, and the DC voltage between the telephone line connection terminal pair 1a and lb increases.
immediately rises, and power is supplied to, for example, the dialer circuit as described above.

In this case, even if the switch circuit SW opens, the terminal voltage v1 of the capacitor C does not change (that is, the switch circuit SW
so that it always has the same potential v1 regardless of whether it is open or closed),
By setting the resistance value ratio of the resistor R3 of i3 and the fourth resistor R4, charging and discharging of the capacitor C will not occur, and the DC voltage v2 between the telephone line connecting terminal pair la and lb will stably change.

In the above embodiment, the 11th talk line connection terminal pair la, l
A current circuit is biased by the potential of the third connection point N3, and a current flows according to this bias, and is connected to a telephone line. It may be inserted between the connecting terminal pair 1a and lb.

[Effects of the Invention] As described above, according to the variable DC characteristic circuit for a telephone according to the present invention, when switching the DC characteristic by opening and closing the switch circuit, noise is not transmitted to the telephone line, and the telephone There is no delay in the rise in DC resistance between the pair of line connection terminals, and it is possible to prevent incorrect dialing and delays in power supply to dialer circuits and the like.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing the basic configuration of an embodiment of the variable DC characteristic circuit for a telephone according to the present invention, FIG. 2 is a waveform diagram showing the operation of the variable DC characteristic circuit for a telephone shown in FIG. 1, and FIG. Circuit diagrams showing other embodiments of the present invention, FIGS. 4(a) and 5(a)
4(b) and 5(b) are respectively corresponding circuit diagrams showing different conventional variable DC characteristic circuits for telephones. Figure 6 is a waveform diagram showing the operation of the circuit in Figures 4(a) and 5.
FIG. 7 is a diagram showing the direct current characteristics of the circuit shown in FIG. 7A, and FIG. 7 is a circuit diagram showing the connection relationship between a pair of telephone line connection terminals and a speech network integrated circuit in an electronic telephone. la, lb...telephone line connection terminal pair, Ql. QIO~Q13...Transistor, R1-R8...
Resistance, N1...first connection point, N2...second connection point, N3...third connection point, C...capacitance, SW...
・Switch circuit, 1...constant current source. Applicant's agent Patent attorney Takehiko Suzue 1a Figure Figure Figure Figure

Claims (1)

[Scope of Claims] A plurality of resistors connected in series between a pair of telephone line connection terminals, and a first connection point of each connection point of the plurality of resistors and the pair of telephone line connection terminals. A capacitor connected between one of the telephone line connection terminals and a second connection point of the plurality of connection points of the plurality of resistors and the one telephone line connection terminal. a constant current source; and a constant current source that is connected to the constant current source and switches whether or not the constant current source is connected to a second connection point or whether or not the constant current source operates depending on the talking state and dialing state of the telephone. and a current circuit that is biased by the potential of a third connection point among the connection points of the plurality of resistors and causes a current to flow between the pair of telephone line connection terminals in accordance with the bias. A variable DC characteristic circuit for telephones, which is characterized by: