Abstract:
In a telephone interface circuit, a first NPN transistor switch-controls a connection between a speech circuit and a pair of subscriber lines. A second PNP transistor controls an on/off state of the first transistor. A positive feedback circuit connects a collector terminal of the first transistor to a base terminal of the second transistor. An internal power source supplies current for driving the second transistor. The first transistor operates in a saturated region when a voltage that is in a range of standard voltages delivered over a pair of subscriber lines for normal operating conditions of a subscriber line device that is not being subjected to an over-voltage or an over-current event is applied in between the pair of the subscriber lines. The first transistor operates in an unsaturated region when an overvoltage exceeding said range of standard voltages is applied in between the pair of the subscriber lines.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an interface circuit which is capable of protecting a telephone set from transient overvoltage such as surge voltage, and from continuous inflow of overcurrent due to a fault contact between a commercial power line and a pair of subscriber lines. 
     2. Description of the Related Art 
     Since the subscriber lines being air-suspended have the possibility of getting transient lightning-induced voltage propagation thereto due to a lightning strike, or receiving continuous inflow of overcurrent for a somewhat long period of time due to a fault contact (or short circuit in connection) with the commercial power line, a protection circuit is often provided in the interface between the telephone set and the subscriber line. As counter measures to possible lightning surges, for example, a structure with a varistor element connected between two subscriber lines, a structure with a varistor element connected between the subscriber line and the grounding wire, etc. are known. When a transient surge voltage exceeding a varistor voltage is applied to the subscriber line, the varistor element will function to protect a speech circuit inside the telephone set by shifting to a conduction mode to absorb the surge voltage. 
     Moreover, as a counter measure to possible heat generation and fires in the telephone set due to a fault contact between the commercial power line and the subscriber line, for example, a structure having a PTC (positive temperature coefficient) thermistor inserted to the interface between the subscriber line and the telephone set is known. When there is a continuous inflow of overcurrent at the PTC thermistor for a some period of time, an input impedance at the interface will increase along with a rise in the element temperature, whereby the inflow of overcurrent into the telephone set can be prevented. 
       FIG. 3  is a circuit diagram showing a conventional telephone interface circuit  30 . The telephone interface circuit  30  is to perform interface control between a speech circuit  20 , which is to process audio signals, and a pair of subscriber lines L 1  and L 2 . 
     The telephone interface circuit  30  mainly includes a diode bridge  40  which serves to rectify signals traveling inside the pair of the subscriber lines L 1  and L 2  to supply the signals to the speech circuit  20 , a transistor Tr 3  which functions as a hook switch for switch-controlling the connection between the pair of the subscriber lines L 1  and L 2  and the speech circuit  20 , a transistor Tr 4  which functions as a driver for switch-controlling the on/off state of the transistor Tr 3 , and a zener diode D 5  which serves to absorb possible overvoltage that could be applied to the subscriber lines L 1  and L 2 . 
     The diode bridge  40  is composed of four diodes D 1 , D 2 , D 3  and D 4 . 
     The transistor Tr 3  is to turn on at an off-the-hook state so as to connect the pair of the subscriber lines L 1  and L 2  to the speech circuit  20 , whereas it turns off at an on-the-hook state so as to disconnect the pair of the subscriber lines L 1  and L 2  from the speech circuit  20 . 
     An emitter terminal E 3  of the transistor Tr 3  is connected to the subscriber line L 1 . 
     A collector terminal C 3  of the transistor Tr 3  is connected to the speech circuit  20  through a resistor R 7 . 
     A base terminal B 3  of the transistor Tr 3  is connected to a collector terminal C 4  of the transistor Tr 4  through a resistor R 4 . 
     A resistor R 3  is connected between the emitter terminal E 3  of the transistor Tr 3  and the base terminal B 3  of the transistor Tr 3 . 
     A base terminal B 4  of the transistor Tr 4  is divided into two lines, one connected to a terminal HC via a resistor R 5  and the other connected to the subscriber line L 2  via a resistor R 6 . 
     The transistor Tr 3  is a PNP transistor whereas the transistor Tr 4  is an NPN transistor. 
     The terminal HC is connected to a microcomputer (not shown). At the time when off-the-hook operation, on-the-hook operation, dial pulse transmitting operation or the like is to be carried out, this microcomputer serves to control a voltage V 4  at the terminal HC in order to control a base potential of the transistor Tr 4 . 
     For instance, in the off-the-hook state, the microcomputer will control the voltage V 4  at the terminal HC such that the voltage V 4  will be at high voltage. Then the base potential of the transistor Tr 4  will rise as an electric potential of the terminal HC rises, whereby the transistor Tr 4  will turn on. Then, because a base potential of the transistor Tr 3  will drop, the transistor Tr 3  will turn on, and thus the pair of the subscriber lines L 1  and L 2  will be connected to the speech circuit  20 . 
     In the off-the-hook state, in response to a dial input, the microcomputer will control the voltage V 4  at the terminal HC. Thereby, the transistor Tr 4  will transmit a dial pulse signal. 
     In the on-the-hook state, the microcomputer will control the voltage V 4  at the terminal HC such that the voltage V 4  will be at low voltage. Then the base potential of the transistor Tr 4  will drop, whereby the transistor Tr 4  will be cut off. Then, because the base potential of the transistor Tr 3  will rise, the transistor Tr 3  will be cut off, and thus the pair of the subscriber lines L 1  and L 2  will be disconnected from the speech circuit  20 . 
     With respect to the above-described telephone interface circuit  30 , however, transistors with high pressure resistance, which are quite expensive, are required to be used as the transistors Tr 3  and Tr 4  to be connected between the pair of the subscriber lines L 1  and L 2 , and this leads to increase in manufacturing costs. 
     Moreover, as the above-described telephone interface circuit  30  has to have the resistor R 4  inserted in between the two transistors Tr 3  and Tr 4 , the telephone interface circuit  30  is left with little design flexibility. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide a telephone interface circuit which can adopt transistors with low pressure resistance, which are inexpensive, as a first transistor that switch-controls a connection between a pair of subscriber lines and a speech circuit, and a second transistor that drive-controls the first transistor. 
     Furthermore, another object of the present invention is to provide a telephone interface circuit which does not require a resistor to be inserted in between the first and the second transistors, and which therefore is capable of having more design flexibility. 
     For the purpose of achieving the above-mentioned objects, a telephone interface circuit according to the present invention comprises: a first transistor which switch-controls a connection between a speech circuit and a pair of subscriber lines; a second transistor which controls an on/off state of the first transistor; a positive feedback circuit which connects a collector terminal of the first transistor to a base terminal of the second transistor; and an internal power source which supplies current for driving the second transistor. A circuit constant is set such that the first transistor is to operate in a saturated region when a voltage in a range of voltage for normal use is applied in between the pair of the subscriber lines, and such that the first transistor is to operate in an unsaturated region when an overvoltage exceeding the range of voltage for normal use is applied in between the pair of the subscriber lines. 
     Since the second transistor can operate by the current supplied by the internal power source, the second transistor does not need to have current supplied by the pair of the subscriber lines. Therefore, the first and the second transistors should be sufficient as long as they have pressure resistance based on the output voltage of the internal power source, and thus transistors with low pressure resistance which are available at low price can be used as the first and the second transistors. 
     Moreover, with respect to the telephone interface circuit according to the present invention, since it is not necessary to have a resistor to be inserted in between the first and the second transistors, the telephone interface circuit is allowed to have more design flexibility. 
     Furthermore, with respect to the telephone interface circuit according to the present invention, as the first transistor operates in the unsaturated region when an overvoltage is being applied to the pair of the subscriber lines, the first transistor, the second transistor and the positive feedback circuit will function as a self-propelled pulse generator. Accordingly, the first transistor and the second transistor will start oscillating while alternating between an on state and an off state, whereby the overcurrent flowing into the first transistor will be able to be cut off intermittently. In addition to that, it is possible to reduce the average value of overcurrent to a considerable extent, whereby the first transistor can be protected from the overcurrent. 
     The circuit constant is supposed to determine the base current of the second transistor. A boundary between the saturated region and the unsaturated region of the second transistor can be set based on the value of the base current. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram illustrating a telephone interface circuit according to an embodiment of the present invention; 
         FIG. 2  is a graphic representation showing a static characteristic of a transistor; and 
         FIG. 3  is a circuit diagram illustrating a conventional telephone interface circuit. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a diagram illustrating a circuit structure of a telephone interface circuit  10  according to an embodiment of the present invention. 
     The telephone interface circuit  10  is to perform interface control in between a speech circuit  20 , which is to process audio signals, and a pair of subscriber lines L 1  and L 2 . The telephone interface circuit  10  mainly includes a transistor Tr 1  which functions as a hook switch for switch-controlling the connection between the pair of the subscriber lines L 1  and L 2  and the speech circuit  20 , and a transistor Tr 2  which functions as a driver for switch-controlling the on/off state of the transistor Tr 1 . 
     The transistor Tr 1  is to turn on at an off-the-hook state so as to connect the pair of the subscriber lines L 1  and L 2  to the speech circuit  20 , whereas it turns off at an on-the-hook state so as to disconnect the pair of the subscriber lines L 1  and L 2  from the speech circuit  20 . 
     A collector terminal C 1  of the transistor Tr 1  is connected to the subscriber line L 2  through a resistor R 2 , while an emitter terminal E 1  of the transistor Tr 1  is connected to the speech circuit  20  and a base terminal B 1  of the transistor Tr 1  is connected to a collector terminal C 2  of the transistor Tr 2 . Moreover, the emitter terminal E 1  of the transistor Tr 1  is grounded. 
     A base terminal B 2  of the transistor Tr 2  is connected to a terminal HC through a resistor R 1 , while an emitter terminal E 2  of the transistor Tr 2  is connected to an internal power source V 2 . The internal power source V 2  is a source of power that can be obtained, for instance, by converting an alternate-current power supplied by a commercial power source into a predetermined direct-current power using a power conversion module. The transistor Tr 2  is to operate relying on the current supplied by the internal power source V 2 . The collector terminal C 1  of the transistor Tr 1  is also connected to the base terminal B 2  of the transistor Tr 2  through a capacitor C 1 . 
     The transistor Tr 1  is an NPN transistor whereas the transistor Tr 2  is a PNP transistor. It is noted that in  FIG. 1 , a zener diode which serves to absorb possible overvoltage that could be applied to the subscriber lines L 1  and L 2 , and a diode bridge which serves to rectify signals traveling inside the subscriber lines L 1  and L 2  in order to supply the signals to the speech circuit  20  are being omitted. 
     The terminal HC is connected to a microcomputer  30 . At the time when off-the-hook operation, on-the-hook operation, dial pulse transmitting operation or the like is to be carried out, this microcomputer  30  serves to conduct negative logic-based control on a voltage V 1  at the terminal HC in order to control a base potential of the transistor Tr 2 . 
     For instance, in the off-the-hook state, the microcomputer  30  will control the voltage V 1  at the terminal HC such that the voltage V 1  will be at low voltage. Then the base potential of the transistor Tr 2  will drop as an electric potential of the terminal HC drops, whereby the transistor Tr 2  will turn on. Consequently, a collector current of the transistor Tr 2  will start flowing. Since the collector current of the transistor Tr 2  is equivalent to a base current of the transistor Tr 1 , the transistor Tr 1  will turn on. 
     The lowered base potential of the transistor Tr 2  will be positively fed back to the collector terminal C 1  of the transistor Tr 1  through the capacitor C 1  that functions as a positive feedback circuit. In a case when a line voltage V 3  in between the pair of the subscriber lines L 1  and L 2  stays within a range of voltage for normal use, the oscillation condition will not be met because a circuit constant, which is to determine the value of base current of the transistor Tr 1 , has been selected such that the transistor Tr 1  will operate in a saturated region. Therefore, under the off-the-hook state, as long as the line voltage V 3  in between the pair of the subscriber lines L 1  and L 2  is within the range of voltage for normal use, the transistor Tr 1  will keep itself at the state of being turned on. 
     On the other hand, in a case when the line voltage V 3  in between the pair of the subscriber lines L 1  and L 2  exceeds the range of voltage for normal use under the off-the-hook state, the oscillation condition will be met because a circuit constant, which is to determine the value of base current of the transistor Tr 1 , has been selected such that the transistor Tr 1  will operate in an unsaturated region. As the oscillation condition is met, the transistors Tr 1  and Tr 2  as a pair will start oscillating according to the same oscillation principle as that of a multi-vibrator. Consequently, as a loop current flowing at the transistor Tr 1  will be cut off intermittently, the transistor Tr 1  will be able to be protected from the overcurrent. 
     In the off-the-hook state, in response to a dial input, the microcomputer  30  will control the voltage V 1  at the terminal HC. Thereby, the transistor Tr 2  will transmit a dial pulse signal. 
     In the on-the-hook state, the microcomputer  30  will control the voltage V 1  at the terminal HC such that the voltage V 1  will be at high voltage. Then the base potential of the transistor Tr 2  will rise, whereby the transistor Tr 2  will be cut off. Consequently, since the collector current of the transistor Tr 2  will not flow, the transistor Tr 1  will be cut off. 
     Next, a principle on the basis of which the transistors Tr 1  and Tr 2  are to oscillate when an overvoltage is being applied in between the pair of the subscriber lines L 1  and L 2  will be described. 
     As can be understood from the fact that a part of the base current of the transistor Tr 2  is to be positively fed back to the collector terminal C 1  of the transistor Tr 1 , as shown in  FIG. 1 , the transistors Tr 1  and Tr 2  as a pair are composing an in-phase amplifier circuit. 
     As mentioned above, when an overvoltage is applied in between the pair of the subscriber lines L 1  and L 2 , transistors Tr 1  and Tr 2  as a pair will start oscillating according to the same oscillation principle as that of a multi-vibrator (or a self-propelled pulse generator), because the circuit constant has been selected such that the transistor Tr 1  will operate in the unsaturated region. As the oscillation starts, the pair of the transistors Tr 1  and Tr 2  will alternate between an on state and an off state with a cycle period proportional to a time constant CIR 1 . For instance, when the transistor Tr 2  is at an off state at a certain moment, the transistor Tr 1  is also at an off state. At this moment, since the voltage V 1  of the HC terminal has been set at low voltage, a first electrode, among first and second electrodes composing the capacitor C 1 , which is connected to the base terminal B 2  of the transistor Tr 2 , will have its electric potential become lower than that of the second electrode, whereby the transistor Tr 2  will shift to an on state in due course, while the transistor Tr 1  will also shift to an on state. Due to charging and discharging by the capacitor C 1 , the pair of the transistor Tr 1  and Tr 2  will simultaneously alternate between an on state and an off state. A cycle period of alternation Tm is about “0.69×C 1 ×R 1 ”. 
       FIG. 2  shows a static characteristic of the transistor Tr 1 . 
     A horizontal axis in a graph of  FIG. 2  indicates a voltage difference between the collector and emitter of the transistor Tr 1 , that is, the line voltage V 3 , whereas a vertical axis indicates a collector current I C1  of the transistor Tr 1 . Reference numeral  40  shows one example of load profile at the time when the line voltage V 3  is within the range of voltage for normal use, while reference numeral  50  shows one example of load profile at the time when the line voltage V 3  exceeds the range of voltage for normal use. 
     At this point, provided that a base current of the transistor Tr 1  is indicated by I B1 , a collector current of the transistor Tr 1  is indicted by I C1 , a direct current gain of the transistor Tr 1  is indicate by h FE1 , a base current of the transistor Tr 2  is indicated by I B2 , a collector current of the transistor Tr 2  is indicted by I C2 , a direct current gain of the transistor Tr 2  is indicate by h FE2 , and a voltage difference between the base and emitter of the transistor Tr 1  and a voltage difference between the collector and emitter of the transistor Tr 2  with respect to the line voltage V 3  are disregarded, the following expressions can be derived.
 
 I   B2 =( V 1−0.6)/ R 1  (1)
 
 I   B1   =I   C2   =I   B2   ×h   FE2   (2)
 
 I   C1   =V 3 /R 2  (3)
 
 I   C1   =I   B1   ×h   FE1   (4)
 
     Based on expressions (1) to (4), the following expression can be derived.
 
 V 3=( V 1−0.6)× h   FE1   ×h   FE2   ×R 2 /R 1  (5)
 
     From expression (5), it can be understood that a value of the voltage V 3  at the time when the pair of the transistor Tr 1  and Tr 2  start oscillating (i.e. an oscillation start voltage) is inversely proportional to a resistance value of the resistor R 1 , and the oscillation start voltage can be adjusted arbitrarily with the resistance value of the resistor R 1 . 
     For example, when the line voltage V 3  is 10 V (i.e. when the line voltage V 3  is within the range of voltage for normal use), the transistor Tr 1  will need a base current I B1  of 50 μA in order to enter the saturated region, as indicated by a point A of intersection between a static characteristic curve of the transistor Tr 1  and the load profile  40 . According to the present embodiment, the circuit constant, which is to determine the value of the base current I B1  of the transistor Tr 1 , has been selected such that the operating point of the transistor Tr 1  will enter the saturated region when the line voltage V 3  is within the range of voltage for normal use. Since the transistor Tr 1  will not have an amplifying function in the saturated region, the oscillation condition will not be met even when a part of the base current of the transistor Tr 2  is to be positively fed back to the collector terminal C 1  of the transistor Tr 1  through the capacitor C 1 . 
     Meanwhile, when the line voltage V 3  is 50 V (i.e. when the line voltage V 3  is an overvoltage that exceeds the range of voltage for normal use), the transistor Tr 1  will need a base current I B1  of 200 μA in order to enter the saturated region, as indicated by a point B of intersection between a static characteristic curve of the transistor Tr 1  and the load profile  50 . According to the present embodiment, the circuit constant, which is to determine the value of the base current I B1  of the transistor Tr 1 , has been selected such that the operating point of the transistor Tr 1  will enter the unsaturated region when the line voltage V 3  is an overvoltage that exceeds the range of voltage for normal use. Since the transistor Tr 1  will have an amplifying function in the unsaturated region, the oscillation condition will be met, whereby the pair of the transistors Tr 1  and Tr 2  will start oscillating. 
     Since such oscillation will start at the very instant when an overvoltage is applied in between the pair of the subscriber lines L 1  and L 2 , an overcurrent passing through the transistor Tr 1  will be cut off at the instant when the overvoltage is applied in between the pair of the subscriber lines L 1  and L 2 . Although the overcurrent will start flowing into the transistor Tr 1  again when the cycle period of alternation Tm elapses from the time the overcurrent has been cut off, the overcurrent will be cut off again at the very instant when the overcurrent starts flowing. In this way, the overcurrent flowing into the transistor Tr 1  will be cut off intermittently. By adjusting the value of time constant C 1 R 1  to an appropriate value, it is possible to set a ratio of the period, during which the overvoltage is being applied in between the pair of the subscriber lines L 1  and L 2 , to the period, during which the overcurrent is flowing into the transistor Tr 1  during the period when the overvoltage is being applied in between the pair of the subscriber lines L 1  and L 2 , to about 10:1, for instance. Accordingly, it is possible to reduce the average value of overcurrent of the transistor Tr 1  to a considerable extent. 
     The oscillating behavior by the transistors Tr 1  and Tr 2  will continue during the time period when the overvoltage is being applied in between the pair of the subscriber lines L 1  and L 2  (i.e. during the time period when the oscillation condition is being met). After that, as the line voltage V 3  becomes lower to fall into the range of voltage for normal use, the operating point of the transistor Tr 1  will return to the saturated region again, whereby the oscillating behavior will stop, for the oscillating condition will no longer be met. In this way, since the protection function of the telephone interface circuit  10  for protecting the transistor Tr 1  has a self-recovery function, it will become available for normal use at the very moment when the overvoltage is stopped being applied to the pair of the subscriber lines L 1  and L 2 . 
     As for the circuit constant which is to determine the value of the base current I B1  of the transistor Tr 1 , for example, the voltage V 1  of the terminal HC, the resistance value of the resistor R 1 , etc. can be considered. However, the circuit constant is not to be limited to these examples. 
     With respect to the telephone interface circuit  10  according to the present embodiment of the invention, since the transistor Tr 2  can operate by the current supplied by the internal power source V 2 , the transistor Tr 2  does not need to have current supplied by the pair of the subscriber lines L 1  and L 2 . Therefore, the transistor Tr 2  should be sufficient as long as they have pressure resistance based on the output voltage of the internal power source V 2 , and thus transistors with low pressure resistance which are available at low price can be used as the transistor Tr 2 . 
     Furthermore, with respect to the telephone interface circuit  10  according to the present embodiment of the invention, since it is not necessary to have a resistor to be inserted in between the two transistors Tr 1  and Tr 2 , the telephone interface circuit  10  is allowed to have more design flexibility. 
     In case of power outage, the telephone interface circuit  10  should not be connected to the pair of the subscriber lines L 1  and L 2 . In the telephone interface circuit  10  according to the present embodiment of the invention, since the internal power source V 2  is to have power supplied by the commercial power source, the output voltage of the internal power source V 2  will become zero in the case of power outage. Therefore, the transistor Tr 2  will not turn on in the case of power outage. This means that it is guaranteed that the telephone interface circuit  10  will not be connected to the pair of the subscriber lines L 1  and L 2  in the case of power outage.