Abstract:
A hybrid subscriber matching circuit for an electronic exchange which matches transmitted/received data so as to permit call communications between the exchange and subscribers. The subscriber matching circuit is constructed by individual parts without using a transformer or SLIC, and thus can simplify the peripheral parts thereof as well as strongly resist external impact.

Description:
This application is a continuation-in-part of Ser. No. 09/382,681 filed Aug. 25, 1999. 
    
    
     CROSS-REFERENCE TO CO-PENDING APPLICATION 
     This application makes reference to, incorporates the same herein and claims all benefits insuring under 35 U.S.C. §120 as a continuation-in-part of my application entitled  Subscriber Matching Circuit For Electronic Exchange  earlier filed in the United States Patent &amp; Trademark Office on Aug. 25, 1999 and there duly assigned Ser. No. 09/382,681. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a subscriber matching circuit for an electronic exchange. In particular, the present invention relates to a subscriber matching circuit for an electronic exchange which matches transmitted/received data so as to permit call communications between the exchange and subscribers. 
     2. Description of the Related Art 
     Generally, in an electronic exchange, a subscriber matching circuit for matching with subscribers requires the functions of line current supply, supervision of a subscriber&#39;s state, call signal supply (ring signal supply) and detection, 2-line/4-line conversion, etc. The line current supply function serves to supply current for operating a subscriber&#39;s telephone and to supervise the on-hook/off-hook state of the telephone by detecting the variation of the supplied current. The line current supply function, which is performed with a function of limiting a maximum line current, serves to prevent an unnecessary power consumption due to an oversupply of the line current to a short-distance subscriber. The 2-line/4-line conversion function serves to convert a 4-line signal transmitted from the telephone exchange or through transmission lines into a 2-line signal, while converting the 2-line signal transmitted from the subscriber&#39;s telephone into the 4-line signal. 
     Such an analog type subscriber matching circuit may employ a transformer. However, it cannot be adapted to the present-day trend of the high-density integration and miniaturization due to its large size and magnetic saturation caused by the line current. 
     In order to adapt the trend of the high-density integration and miniaturization, the subscriber matching circuit has been integrated into an SLIC integrated circuit (IC). A subscriber matching circuit using the SLIC IC is disclosed in Korean Patent Application No.1994-40809 filed by the applicant of the present application. According to the subscriber matching circuit disclosed in Korean Patent Application No.1994-40809, however, all circuit elements are integrated onto one chip, and thus it is relatively vulnerable to external impact such as lightning strikes in comparison to the circuit employing a transformer. As a result, it requires a protection device as well as many peripheral parts thereof for performing the subscriber matching function, thereby increasing the possibility of experiencing difficulties in operation. 
     The following patents each discloses features in common with the present invention: U.S. Pat. No. 4,887,293 to Molnar, entitled TRUNK CIRCUIT WITH LOOP LENGTH GAIN EQUALIZATION, issued on Dec. 12, 1989; U.S. Pat. No. 4,953,200 to Yamasaki, entitled PRIVATE BRANCH EXCHANGE CAPABLE OF DISCRIMINATING DIFFERENT TYPES OF TELEPHONE SETS CONNECTED THERETO, issued on Aug. 28, 1990; U.S. Pat. No. 4,358,643 to Levy, entitled TWO TO FOUR WIRE HYBRID CIRCUIT, issued on Nov. 9, 1982; U.S. Pat. No. 5,602,912 to Hershbarger, entitled TELEPHONE HYBRID CIRCUIT, issued on Feb. 11, 1997; U.S. Pat. No. 5,608,795 to Gay, entitled TELEPHONE LINE INTERFACE CIRCUIT, issued on Mar. 4,1997; U.S. Pat. No.5,710,811 to Tomasini et al., entitled SPEECH CIRCUIT FOR SUBSCRIBER TELEPHONE APPARATUS, issued on Jan. 20, 1998; U.S. Pat. No. 4,456,991 to Chea Jr. et al., entitled TELEPHONE LINE CIRCUIT AND SYSTEM, issued on Jun. 26, 1984; U.S. Pat. No. 4,381,561 to Treiber, entitled ALL DIGITAL LSI LINE CIRCUIT FOR ANALOG LINES, issued on Apr. 26, 1983; U.S. Pat. No. 4,759,059 to Christensen, entitled ANALOG TELEPHONE CIRCUIT FOR DIGITAL TELEPHONE SYSTEM, issued on Jul. 19,1988; U.S. Pat. No. 3,982,078 to Janssen et al., entitled LINE MATCHING CIRCUIT FOR USE IN A TONE PUSHBUTTON DIALING SUBSCRIBER&#39;S SET PROVIDED WITH A TONE GENERATOR, issued on Sep. 21, 1976; and U.S. Pat. No. 4,461,929 to Britt, entitled AMPLIFIER FOR ELECTRONIC AND ELECTRO-MECHANICAL TRANSMITTERS, issued on Jul. 24, 1984. 
     SUMMARY OF THE INVENTION 
     It is an object of the-present invention to solve the problems involved in the related art, and to provide a hybrid subscriber matching circuit for an electronic telephone exchange which can simplify the peripheral parts thereof and strongly resist external impact. 
     In order to achieve the above object, there is provided a subscriber matching circuit for a full electronic exchange comprising: transistors Q 1  and Q 2  for supplying a line current to a subscriber. through a tip terminal and a ring terminal; transistors Q 3  and Q 4 , having a Darlington structure and respectively connected to the transistors Q 1  and Q 2 , for limiting a maximum current; current supervising resistors R 1  and R 2 , respectively connected to emitters of the transistors Q 1  and Q 2 , for performing a current feedback operation to limit the maximum current and detecting in a voltage from a line current flowing through telephone lines; a resistor R 3 , connected between a collector of the transistor Q 1  and a collector of the transistor Q 3 , for preventing the transistor Q 1  from being saturated; a resistor R 4 , connected between a collector of the transistor Q 2  and a collector of the transistor Q 4 , for preventing the transistor Q 2  from being saturated; three bias resistors R 5 , R 6  and R 7  for determining a threshold value of the maximum current and causing the transistors Q 1  and Q 2  to always be in an active region; capacitors C 5  and C 6  for superimposing a received AC audio signal on the DC line current; composite impedances ZL 1  and ZL 2  for matching with a line characteristic impedance; a resistor R 11  for converting the line current flowing through the resistor R 1  into an input current for detecting an off-hook state; an operational amplifier AMP 3  for inversion-amplifying a signal inputted through the resistor R 1 , and a transistor Q 5  for converting a level of a signal inversion-amplified by the operational amplifier AMP 3 . 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention, and may of the attendant advantages, thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein: 
     FIG. 1 is a schematic circuit diagram of the hybrid subscriber matching circuit according to a preferred embodiment of the present invention. 
     FIG. 2 is a schematic circuit diagram of the DC bias equivalent circuit according to a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment of the present invention will be explained in detail with reference to the accompanying drawings. 
     FIG. 1 is a schematic circuit diagram of the hybrid subscriber matching circuit according to a preferred embodiment of the present invention. 
     Referring to FIG. 1, the hybrid subscriber matching circuit according to the preferred embodiment of the present invention includes transistors Q 1  and Q 2  for supplying current, transistors Q 3  and Q 4  for limiting a maximum current, current supervising resistors R 1  and R 2  for performing a current feedback operation to limit the maximum current and for detecting in a voltage from a line current flowing through telephone lines, resistors R 3  and R 4  for preventing the transistors Q 1  and Q 2  from being saturated, bias resistors R 5 , R 6  and R 7  for determining a threshold value of the maximum current and causing the transistors Q 1  and Q 2  to always be in an active region, a dummy load resistor R 8  for supplying a bias current to the transistors Q 1  and Q 2  to prevent the transistors Q 1  and Q 2  from being saturated when no fine load exists, temperature compensating diodes D 1  and D 2  for preventing the transistors from being overheated due to a variation of the threshold value of the maximum current which is caused by the heat generation of the transistors Q 1  and Q 2  due to the line current (I L ), bypass capacitors C 1 , C 2 , C 3  and C 4  for preventing a bad influence on call communications due to the generation or induction of noise in the DC line current supply, capacitors C 5  and C 6  for superimposing a received audio signal (i.e., an AC signal) on the DC line current, composite impedances ZL 1  and ZL 2  for matching with a line characteristic impedance, amplifiers AMP 1  and AMP 2  for receiving and amplifying the audio signal, protection elements CR 1  and CR 2  for protecting the amplifiers AMP 1  and AMP 2  from an overcurrent through the lines, a resistor R 11  for converting the line current flowing through the resistor R 1  into an input current for detecting an off-hook state, an operational amplifier AMP 3  for inversion-amplifying a signal inputted through the resistor R 11 , a resistor R 13  for determining an amplification factor of the signal inputted through the resistor R 11 , a transistor Q 5  for converting a level of a signal minversion-amplified by the operational amplifier AMP 3 , a resistor R 10  for detecting a ring trip voltage if a telephone handset is hooked off during supply of a call signal, a resistor R 12  for converting the voltage detected by the resistor R 10  into a ring trip current, a capacitor C 7  for making the operational amplifier AMP 3  serve as a low-pass filter so that an AC amplification factor is greatly lowered to remove AC ripple components included in the ring trip current, and a field effect transistor FET 1  for making the operational amplifier AMP 3  serve as a low-pass filter in a ring current supply state. 
     The resistor RL in FIG. 1 is an element which is not really a part of the subscriber matching circuit for an electronic exchange but it is introduced only to explain the operation of the subscriber matching circuit. Naturally, there exists resistance in the telephone line which is connected to the subscriber matching circuit and the resistance element RL indicates the existence of the resistance in the telephone line. 
     The ring relay K 1  in FIG. 1 is an element which is switched to the resistor R 10  in on-ring state and to the resistor R 3  in off-ring state under the control of an exchanger in order to provide ring signals generated from a ring generator of the exchange (not shown) to the telephone. 
     FIG. 2 is a schematic circuit diagram of the DC bias equivalent circuit according to a preferred embodiment of the present invention. 
     The operation of the preferred embodiment of the present invention will now be explained in detail with reference to FIGS. 1 and 2. 
     The transistors Q 1  and Q 2  are for supplying the current, and the transistors Q 3  and Q 4  are for limiting the maximum current. The equivalent circuit of FIG. 2 performs the line current supplying function. Since the circuit for supplying the line current has the construction in which a tip terminal and a ring terminal are symmetrically arranged, FIG. 2 illustrates only the equivalent circuit for the line current supply on the tip terminal side. Referring to FIG. 2, the line current (I L ) becomes Ib 1 *h FE . 
     The resistors R 5  and R 6  form a bias current Ib 2  of the transistor Q 2 , which is given by the following mathematical expression 1. 
     [Expression 1]       Ib2   =       (     V1   -     V   BE1     -     V   BE3       )       (     Rb   +   R1     )               V1   =       (       Vbat   2     -     2        V   D         )     ×     R5     (       R6   2     +   R5     )                 Rb   =     R5   //     R6   2                              
     The line current (I L ) increases if the resistance R L  of the speech line decreases, and as the line current (I L ) increases, the terminal voltage V R1  of the resistor R 1  also increases. If the voltage V RL  decreases due to the current feedback, which decreases the base current Ib 1  of the transistor Q 1 , and the decrease of the resistance R L  of the speech line, the collector voltage V CE1  of the transistor Q 1  increases, and this causes the increase of the current Ib 1  through the resistor R 3 . The bias stability is improved by the voltage feedback which decreases the collector voltage V CE1  of the transistor Q 1 . Another important role of the resistor R 3  is to prevent the saturation of the transistor Q 1 . Specifically, if the resistance R L  of the speech line increases, the line current (I L ) and the collector voltage V CE1  of the transistor Q 1  decrease. The decrease of the line current (I L ) causes the decrease of the amount of the current feedback through the resistor R 1 , and the continuous increase of the resistance R L  of the speech line causes the continuous decrease of the line current (I L ), resulting in the collector voltage V CE1  of the transistor Q 1  greatly decreasing so as to reach the saturation region of the transistor Q 1 . At this time, the base current Ib 1  of transistor Q 1  which is supplied through the resistor R 3  decreases due to the decrease of the collector voltage V C  of the transistor Q 1 , and thus the saturation of the transistor Q 1  is prevented. Actually, this prevents the distortion of the AC characteristic in a long loop in which the line resistance RL increases. 
     Also, the maximum current limitation is effected by the current feedback through the resistor R 1 . The increase of the line current I L  causes the increase of the terminal voltage V R1 , and the current limitation is performed when the line current (I L ) reaches a value corresponding to the state that the voltage of V R1 +V BE1 +V BE2  becomes equal to the voltage of V 1 . However, if the current limitation is actually generated due to the current increase, the V CE  of the transistor Q 1  increases, and this causes a power dissipation which may be as much as I L 1*V CE  to occur in the transistor Q 1 , resulting in heat generation in the transistor Q 1 . Since V BE  is in negative proportion to the temperature (that is, −2.4 mV/°C.), V BE  decreases as the temperature increases, and this causes the current limitation value increases. An undesirable repetition of such operations results in an error in the limited current. As a result, the maximum current, much higher than the limited current actually required, flows. In order to prevent this overheating phenomenon, the diode D 1  which has a forward voltage-temperature characteristic is connected between the base of the transistor Q 2  and the ground. In this case, since the forward voltage of the diode decreases at the same level as the V BE  of the transistors Q 1  and Q 2  due to the heat generated in the transistor Q 1 , the variation of the limited current to the temperature is offset, and thus the limited current can be more stably determined. Although the transistors Q 1  and Q 2  are connected in a Darlington structure, the base current Ib 1  of the transistor Q 1  is actually limited by the resistor R 3 . The purpose of this construction is not to improve the voltage gain by supplying an input signal to the base of the transistor Q 2 , but to improve the gain of the current feedback performed by the resistor R 1 . In other words, when the resistance R L  of the line varies within the range of the limited current, the limited current is kept constant with respect to the variation of R L  of the line by the transistors Q 1  and Q 2 . 
     Meanwhile, the line current supervising function serves to convert the line current into a logic signal by driving the transistor Q 6  with an inversion-amplified output of the voltage variation of the resistor R 1 . The line current supervising circuit of FIG. 1 also performs the ring trip function simultaneously. Specifically, if the line current (I L ) increases, the voltage drop is generated by the resistor R 1 , and this voltage drop is inversion-amplified with the gain determined by the resistors R 11  and R 13  to drive the transistor Q 6 . If the transistor Q 6  is driven, a logic “0” (low) signal is outputted from the collector of the transistor Q 6  to a detection terminal DET. At this time, the line current (I L ) corresponding to the condition that the detection terminal DET is in the logic “0” state is determined by the resistors R 11  and R 12 . The line current supervising circuit also supervises whether the handset is hooked off if the ring current is supplied during the supervision of the line current (I L ). Specifically, a ring relay K 1  is switched to the resistor R 10 , and the ring current supplied from the ring terminal is applied to the tip terminal via the line resistance R L  and the terminal telephone. In the off-hook state, the line resistance R L  including the DC resistance of the terminal telephone decreases abruptly, and this causes the voltage drop across the resistor R 10  to increase. This dropped voltage is applied to the inverting terminal of the operational amplifier AMP 3  through the resistor R 12  to be inversion-amplified. Accordingly, the output signal of the operational amplifier AMP 3  becomes high, and is supplied to the base of the transistor Q 6  to turn on the transistor Q 6 , so that a low level signal is outputted to the detection terminal DET to indicate the off-hook state of the telephone. When the voltage drop of the resistor R 10  increases, the signal inputted to the inverting terminal of the operational amplifier AMP 3  for inversion-amplifying the terminal voltage of the resistor R 10  becomes the DC signal superimposed with the AC ring signal, and thus it is required to greatly reduce the gain of the inversion amplifier AMP with respect to the AC signal in order to detect the DC signal only. The reduction of the AC gain in the operational amplifier AMP 3  invites the reduction of the AC gain by the capacitor C 7  connected in parallel to the resistor R 13 , and thus the DC signal with its ripple component greatly suppressed is outputted from the operational amplifier AMP 3 , enabling the ring trip operation to be performed accurately. The field effect transistor FET 1  is turned on only when the ring relay K 1  operates, and thus has no effect on the line current supervising function in a normal state. At this time, since the capacitance of the capacitor C 7  cannot be selected as an infinite value, the resistors R 11 , R 12  and R 13  should have a large resistance value in the range of several hundred kilo-ohms (KΩ) in order for the capacitor C 7  having a small capacitance value to act as a low-pass filter. 
     As described above, the subscriber matching circuit according to the present invention is designed to use general parts while it performs the same function as the conventional analog subscriber matching circuit using a transformer or SLIC, and thus the manufacturing cost thereof can be greatly reduced. 
     Further, the subscriber matching circuit according to the present invention has an on-hook transmission function, and thus can be applied to additional services such as remote charging, transmission of a calling subscriber&#39;s number, etc. 
     While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiment, but, on the contrary, it is intended to cover various modifications within the spirit and scope of the appended claims.