JPH03217151A - Telephone set circuit - Google Patents

Abstract

PURPOSE:To prevent the reduction of impedance between line terminals to perform branch speaking by providing a current control circuit with a means which detects the DC voltage of the current supply circuit terminal to prevent this DC voltage from being reduced to a set voltage or lower. CONSTITUTION:If the DC voltage of a line voltage VL is reduced by branch speaking or the like, the DC voltage between terminals of a current supply circuit 3 is reduced also. When the DC voltage between terminals of the current supply circuit 3 is reduced down to the DC detection voltage set by a current control circuit 2, the current control circuit 2 limits the supply current of the current supply circuit 3 and reduces the DC voltage of a constant voltage circuit 4 to prevent the DC voltage between terminals of the current supply circuit 3 from being reduced to the DC detection voltage or lower. Consequently, the DC voltage between terminals of the current supply circuit 3 is not reduced to the set voltage or lower and the impedance between line terminals 1a and 1b is kept high though the DC voltage of the line voltage VL is reduced.

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention relates to a telephone circuit that operates by receiving current from a line, and in particular, when two or more telephones are connected to one line, and two or more telephones are connected to one line. This invention relates to a telephone circuit that enables so-called branch calls, where more than one telephone is in use.

[Conventional technology]

Conventionally, telephone circuits that draw current from the line and supply DC voltage to the telephone circuit or dial signal generation circuit, etc.
A circuit as shown in FIG. 6 has been used. In this diagram,
1a, ll) are line terminals, 5 is a power supply voltage terminal, 6 is a diode-connected transistor, 7 is a transistor whose emitter area is set to have a certain ratio with the transistor 6, 8 is a Zener diode, and 11 is a smoothing transistor. capacitor,
A current source 18 is composed of a constant current diode, FET, transistor, or the like.

Further, G represents ground.

In the circuit shown in FIG. 6, the Zener diode 8 and the capacitor 11 constitute a constant voltage circuit, and the collector current of the transistor 7 is set depending on the emitter area ratio with the transistor 6, and constitutes a current supply circuit that supplies current from the line. do. Generally, line voltage vL (voltage between ground G) has an audio signal superimposed on a DC voltage. therefore,
The current supply circuit is set to a high impedance state so as not to affect the audio signal. Current source 18
is for setting the supply current of the current supply circuit.

Note that DD is the power supply voltage, and DD is the line current.

[Problem to be solved by the invention]

Figure 7 shows the characteristics of the DC voltage in the line voltage VL and line current of the conventional circuit shown in Figure 6 above, and the voltage of the constant voltage circuit is 2.5V (a realistic value).
This is an example of a case where the Assuming that the saturation voltage of the transistor 7 of the current supply circuit is 0.2V, the direct current voltage of the line voltage (■) due to branch calls, etc. becomes equal to the power supply voltage Voo (2.5V) set by the constant voltage circuit (t)0 .2
When V drops to a high level of 2.7■, the transistor 7 enters the saturation region and its impedance drops significantly.

Here, saturation of the transistor 7 will be explained. 8th
The figure shows the static characteristics of a typical transistor, showing the collector current Ic and the collector-emitter voltage VC! This relationship is expressed using base current IIl as a parameter.

When a certain base current flows through a transistor,
Collector-emitter voltage VC. : decreases, 0.2
When it becomes less than V, it enters the saturation region (ON region). Therefore, the line voltage decreases and the collector voltage of the transistor decreases.
Emitter voltage■. When the voltage drops below 0.2V, the transistor becomes saturated.

Furthermore, the slope of the curve on the static characteristics represents the reciprocal of the impedance at that point. Therefore, in the saturation region, the slope is large and the impedance drops rapidly.

In a single call state, the line voltage ■, is 4.5 to IO
In the case of a branch, a voltage drop of about 1.5 to 3 .mu.V occurs, although it varies depending on the DC resistance of the branching telephone.

In this way, in the region where the DC voltage of the line voltage VL is 2.7 V or less, the power supply voltage is ■. Even if the communication circuit that operates by receiving DC voltage from 0 is operable, the line terminals 1a,
There was a problem in that the impedance between I and B was significantly reduced, making it impossible to communicate.

For this reason, one possible method is to set the power supply voltage vllllD low and widen the area where calls can be made, but in this case, the power supply voltage The disadvantage is that the maximum amplitude of the amplifier provided in the communication circuit that operates under the supply of .

The present invention was made to solve these drawbacks, and even if the power supply voltage V on is set high in a normal single telephone call state, the line voltage Vt. It is an object of the present invention to provide a telephone circuit that draws current from a line and supplies a DC voltage, which prevents a drop in impedance between line terminals and enables branch calls when the DC voltage of the line decreases.

[Means for Solving the Problems] A telephone circuit according to the present invention includes a constant voltage circuit that constantizes a supply voltage, a current supply circuit that supplies current from a line to this constant voltage circuit, and a current supply circuit that supplies current from a line to this constant voltage circuit. It is equipped with a current control circuit that controls the supply current, and the current control circuit is equipped with a means to detect the DC voltage at the end of the current supply circuit and prevent the DC voltage at the end of the current supply circuit from dropping below the set voltage. be.

[Effect]

In this invention, the current control circuit detects the DC voltage at the end of the current supply circuit, and supplies current by controlling the supply current of the current supply circuit to prevent this DC voltage from dropping below a set voltage. To prevent a decrease in circuit impedance and, furthermore, to prevent a decrease in impedance between line terminals.

〔Example〕 Hereinafter, this invention will be explained using the drawings.

FIG. 1 is a block diagram showing the configuration of an embodiment according to the present invention. la and 1b are line terminals, 5 is a power supply voltage terminal,
Reference numeral 4 designates a constant voltage circuit that makes the supply voltage constant; 3 a current supply circuit that supplies current from a line to the constant voltage circuit 4; and 2 a current control circuit that controls the supply current of the current supply circuit 3. The current control circuit 2 and the current supply circuit 3 are set to have high impedance so as not to affect the audio signal. Further, the current control circuit 2 includes means for detecting the DC voltage between the terminals of the current supply circuit 3.

When the DC voltage of the line voltage (2) decreases due to a branch call or the like, the DC voltage between the terminals of the current supply circuit 3 also decreases.

When the DC voltage between the terminals of the current supply circuit 3 decreases to the DC detection voltage set by the current control circuit 2, the current control circuit 2
limits the supply current of the current supply circuit 3 and reduces the DC voltage of the constant voltage circuit 4, thereby preventing the inter-terminal DC voltage of the current supply circuit 3 from dropping below the DC detection voltage. Therefore, even if the DC voltage of the line voltage (1) decreases, the DC voltage between the terminals of the current supply circuit 3 does not decrease below the set voltage, and the impedance between the line terminals 1a and 1b remains high.

FIG. 2 is a detailed circuit diagram of one embodiment of the present invention. 6 is a diode-connected transistor for detecting the DC voltage at the end of the current supply circuit, 9 is a resistor, and 10 is a capacitor, which constitute the current control circuit 2 shown in FIG. Reference numeral 7 denotes a transistor whose emitter area is set to a certain ratio with that of the transistor 6, and constitutes the current supply circuit 3 of FIG. 8 is a Zener diode, 11 is a smoothing capacitor, and these constitute the constant voltage circuit 4 shown in FIG.

In FIG. 2, transistor 7 constitutes a current source whose collector current is set depending on the emitter area ratio with transistor 6. That is, the current flowing through transistor 7 can be controlled by the current flowing through transistor 6. The resistor 9 sets the current flowing through the transistor 6, and its impedance is set to a large value so as not to affect the impedance between the line terminals 1a+1b.
The capacitor 10 bypasses the alternating current signal and allows the current supply circuit composed of the transistor 7 to operate stably.

Assuming that the forward voltage of the diode is 0.7V, since the transistor 6 is diode-connected, the applied voltage is maintained at about 0.7V. Therefore, the DC voltage of the line voltage VL is approximately 0.0. 7) When the voltage drops below a high voltage, the current flowing through the transistor 6 and the resistor 9 decreases rapidly, and the current flowing through the controlled transistor 7 also decreases, and the power supply voltage vIllllll decreases. Due to the drop in power supply voltage VDD, transistor 6 and resistor 9
When the DC voltage applied to the transistor becomes 0.7V or higher, the current flowing through it increases, and the current flowing through the controlled transistor 7 also increases. The applied DC voltage will drop to about 0.7V. At this time, the DC voltage between the emitter and collector of the transistor 7 is equal to the DC voltage applied to the transistor 6 and the resistor 9, so it is maintained at about 0.7cm, and saturation is prevented and the line terminal 1a , Ib remains high.

FIG. 3 is a circuit diagram showing another embodiment of the invention, in which a current amplification circuit is inserted between the transistors 6 and 7 of each circuit 2 and 3 in FIG. 2, and the current flowing through the transistor 6 and the resistor 9. FIG. 3 is a circuit diagram in the case where 12 is a transistor with the same polarity as the transistor 6; 13 is a diode-connected transistor with the opposite polarity to the transistor 6; 14 is a transistor whose emitter area is set to have a certain ratio with that of the transistor 13; 15 is the same as the transistor 6; It is a polar, diode connected transistor.

In FIG. 3, the current flowing through transistor 6 is reversed by transistor 12 and flows through transistor 13. In FIG. The current flowing through the transistor 13 is reversed by the transistor 14, amplified by the ratio of the emitter areas, and then flows to the transistor 15. Therefore, the current flowing through transistor 6 is
4.15, the current flowing through the transistor 7 can be controlled with a small amount of current.

FIG. 4 is a circuit diagram showing still another embodiment of the present invention,
FIG. 3 is a circuit diagram using transistors of opposite polarity to the circuit of FIG. 2; 16 is a diode-connected transistor with a polarity opposite to that of the transistor 6, and 17 is a transistor whose emitter area is set to have a certain ratio with that of the transistor 16.

In the circuits of FIGS. 3 and 4, the operation when the direct current voltage of the line voltage {circle around (2)} decreases is the same as that described for the circuit of FIG. 2.

FIG. 5 shows the line voltage (1) and power supply voltage (V) of each embodiment of the invention described above. D line current It. This shows the characteristics of the DC voltage at 2.5 when the voltage of the constant voltage circuit 4 is
This is an example when set to (2). The direct current voltage of the line voltage ■, is the power supply voltage VDD (2
．． 5V) J:'10.7■ High When it drops below 3.2V, the power supply voltage VDD also decreases, and the line voltage vL and power supply voltage V. The difference in DC voltage of Ill is maintained at about 0.7■. Therefore, the DC voltage at the end of the current supply circuit is maintained at approximately 0.7V, and the impedance between line terminals 1a and Ib remains high. If the minimum operating voltage of a communication circuit that operates by receiving DC voltage from the power supply voltage VDIll is 11 1 ■ (realistic value), branch communication is possible up to the DC voltage of line voltage VL of 1.7V, and branch communication is possible. The area will expand.

[Effects of the Invention] As explained above, the present invention includes a constant voltage circuit that constantizes the supply voltage, a current supply circuit that supplies current from a line to this constant voltage circuit, and a current supply circuit that controls the supply current of this current supply circuit. The current control circuit is equipped with a means for detecting the DC voltage at the end of the current supply circuit and preventing the DC voltage at the end of the current supply line from dropping below the set voltage. Even if the power supply voltage is set high during a single call, if the DC voltage at the line end drops due to a branch call, etc., the DC voltage at the end of the current supply circuit that supplies current from the line will be prevented from dropping below the set voltage. In order to be
It is possible to realize a telephone circuit that prevents a drop in impedance between line terminals and enables branch calls.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing details of an embodiment of the invention, and FIG. 3 is a circuit diagram showing details of an embodiment of the invention. A circuit diagram in which a current amplification circuit is added to the circuit in FIG. 2, a circuit diagram in which a current amplification circuit is added to the circuit in FIG. The figures are characteristic diagrams of DC voltage in each embodiment of the present invention, Figure 6 is a circuit diagram showing a conventional example, Figure 7 is a characteristic diagram of the conventional example shown in Figure 6, and Figure 8 explains the saturation phenomenon of a transistor. FIG. In the figure, 1a and Ib are line terminals, 2 is a current control circuit, 3 is a current supply circuit, 4 is a constant voltage circuit, 5 is a power supply voltage terminal, 6
, 7, 12, 13, 14, 15, 16.17 is a transistor, 8 is a Zener diode, 9 is a resistor, 10.11
is a capacitor. O Penance W Field Penance

Claims (1)

[Claims] In a telephone circuit that draws current from a line and supplies DC voltage, a constant voltage circuit that makes the supplied voltage a constant voltage, a current supply circuit that supplies current from the line to this constant voltage circuit,
A current control circuit is provided to control the supply current of the current supply circuit, and the current control circuit detects the DC voltage at the end of the current supply circuit to prevent the DC voltage at the end of the current supply circuit from dropping below a set voltage. A telephone circuit characterized in that it is provided with means for.