Abstract:
The size and power consumption of a ring trip monitor circuit, which performs one of the BORSHT functions of a telephone line card, is reduced by utilizing an operational amplifier with feedback resistors, a resistor network, and a ring signal generator that is only connected to the line when a ring condition is present.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to telephone circuits and, more particularly, to a telephone circuit with a small, low-power ring trip monitor.  
           [0003]    2. Description of the Related Art  
           [0004]    A ring trip monitor is a circuit that detects when a ringing telephone has been answered. When a telephone call is placed, the central office places an oscillating voltage on the line that leads to the telephone. The ring trip monitor detects changes in the oscillating voltage that occur when the telephone has been answered so that the oscillating voltage can be removed from the line. A ring trip monitor performs one aspect of the Battery feed, Over voltage protection, Ringing, Supervision, Hybrid, and Test (BORSHT) function used in telephone line interfaces.  
           [0005]    [0005]FIG. 1A shows a circuit diagram that illustrates a prior-art telephone circuit  100 . As shown in FIG. 1A, circuit  100  includes a battery  110  that has a first terminal  112  and a second terminal  114 , a ring relay  116  that is connected to first terminal  112 , and a ring relay  118  that is connected to second terminal  114 .  
           [0006]    In addition, telephone circuit  100  has a line feed resistor LFR 1  that is connected between terminal  112  and a ring node N 1 , and a line feed resistor LFR 2  that is connected between terminal  114  and a tip PATENT node N 2 . Circuit  100  outputs a ring voltage RING on ring node N 1 , and a tip voltage TIP on tip node N 2 . Ring node N 1  and Tip node N 2  are connected to a telephone  115  via a twisted pair TP.  
           [0007]    Circuit  100  also includes a ring generator  120  that outputs a ring signal RS. Generator  120  has an oscillator  122  that is connected to relay  116 , and a negative voltage source  124  that is connected between oscillator  122  and ground. Voltage source  124  outputs a DC voltage of −48V, while oscillator  122  outputs an AC voltage. The AC voltage has a frequency of 20-30 Hz, an amplitude of 65-95V RMS, and a zero current crossing at the −48V DC bias when tip voltage TIP and ring voltage RING are terminated in an AC load. Ring relay  118 , in turn, is connected to ground.  
           [0008]    Further, circuit  100  includes a control circuit  126  that is connected to relays  116  and  118 , and a ring trip monitor circuit  130 . As further shown in FIG. 1A, ring trip monitor circuit  130  includes a comparator  132  that has a first input  134 A, a second input  134 B, and an output  136  that outputs a comparator voltage VC to control circuit  126 . In addition, circuit  130  has a resistor R 1  that is connected between tip node N 2  and input  134 A of comparator  132 , and a resistor R 2  that is connected between ring node N 1  and input  134 B of comparator  132 .  
           [0009]    Ring trip monitor circuit  130  also has a resistor R 3  that is connected to terminal  114  on one side and both input  134 B and resistor R 2  on the other side, and a resistor R 4  that is connected to terminal  112  on one side and both input  134 A and resistor R 1  on the other side.  
           [0010]    Resistors R 3 /R 2  and R 1 /R 4  function as voltage dividers. (Line feed resistors LFR 1  and LFR 2  function as current limiting fuses, and are monitored by the ring trip monitor circuit for loop current. In addition, line feed resistors LFR 1  and LFR 2  are small in value compared to resistors R 1 -R 4 .) Resistors R 2  and R 3  are equal in value. Resistor R 1  is slightly smaller than resistor R 4 , and resistor R 3  is slightly larger than resistor R 2 . As a result, the voltage divider forms an input voltage V(+IN) on input  134 A of comparator  132  and an input voltage V(−IN) on input  134 B of comparator  132  that are offset from one another.  
           [0011]    Further, ring trip monitor circuit  130  has a first capacitor C 1  that is connected between input  134 B of comparator  132  and ground, a second capacitor C 2  that is connected between input  134 A of comparator  132  and ground, and a capacitor C 3  that is connected between inputs  134 A and  134 B of comparator  132 .  
           [0012]    Capacitor C 1  and resistor R 3 , and capacitor C 2  and resistor R 4  function as low pass filters which keep the ring signal RS within the common mode range of comparator  132 , while capacitor C 3  limits the rate of change of the input voltages V(+IN) and V(−IN) on inputs  134 A and  134 B.  
           [0013]    FIGS.  1 B 1 - 1 B 3  show timing diagrams that illustrate the operation of telephone circuit  100 . FIG. 1B 1  shows the tip voltage TIP and the ring voltage RING that are output from telephone circuit  100 . FIG. 1B 2  shows the input voltages V(+IN) and V(−IN) that are input to comparator  132 , while FIG. 1B 3  shows the comparator voltage VC output from comparator  132 .  
           [0014]    As shown in FIG. 1B 1 , at time t0 telephone  115  is on hook and no incoming call is present. In this state, no current flows out the tip node N 2 , through telephone  115 , and back to the ring node N 1 . As a result, the ring voltage RING is equal to −48V, and the tip voltage TIP is equal to ground.  
           [0015]    In addition, as shown in FIG. 1B 1 , due to the voltage divider provided by resistors R 1 -R 4 , input voltage V(+IN) is slightly more positive than input voltage V(−IN) at this time which, as shown in FIG. 1B 3 , causes comparator  132  to output the comparator voltage VC as a logic high (at 5V).  
           [0016]    At time t1, a call is placed to telephone  115 . Control circuit  126  detects the incoming call and outputs control signals CS 1  and CS 2  to relays  116  and  118 , respectively, to open relays  116  and  118 . As a result, the ring signal RS is connected to line feed resistor LFR 1  and resistor R 4  (and is output to telephone  115  via the ring node N 1 ), while line feed resistor LFR 2  and resistor R 3  are connected to ground.  
           [0017]    As further shown in FIG. 1B 2 , when relay  116  connects the ring signal RS to resistor R 4 , the input voltages V(+IN) and V(−IN) begin to oscillate substantially in phase, with input voltage V(+IN) continuing to have a slightly higher voltage than input voltage V(−IN). As a result, as shown in FIG. 1B 3 , comparator  132  continues to output the comparator voltage VC with a logic high (at 5V). From time t1 to time t2, only an AC current flows out to telephone  115  via twisted pair TP.  
           [0018]    When telephone  115  is answered at time t2, the off hook condition causes the AC current and a DC current I to flow from ground through line feed resistor LFR 2  out the tip node N 2  to telephone  115 , and back via the ring node N 1  to battery terminal  112 . As shown in FIG. 1B 1 , the DC current I causes the tip voltage TIP to become more negative and the ring voltage to become more positive.  
           [0019]    When telephone  115  is answered, the impedance of the line changes from the AC impedance of the ringer in telephone  115  to the DC impedance of the line plus telephone  115 , which is much lower than the AC impedance. The AC ring voltage and the DC battery voltage divide per Ohm&#39;s Law across the line feed resistors LFR 1  and LFR 2 , the resistance of the line, and the DC resistance of telephone  115 .  
           [0020]    As shown in FIG. 1B 2 , this changes the input voltages V(+IN) and V(−IN) so that input voltage V(−IN) is now more positive than input voltage V(+IN). As shown in FIG. 1B 3 , the changes in the input voltages V(+IN) and V(−IN) are detected by comparator  132  which, at time t3, changes the logic state of the comparator voltage VC to a logic low (represented as ground). The logic low is detected by controller  126  which then changes the logic states of the control signals CS 1  and CS 2 , thereby closing relays  116  and  118  at time t4.  
           [0021]    One problem with telephone circuit  100  is that capacitors C 1 -C 3  are quite large. As a result, capacitors C 1 -C 3  consume a significant amount of circuit board space. One approach to reducing the size of circuit  100  is to use a telephone circuit that utilizes a differential amplifier.  
           [0022]    [0022]FIG. 2A shows a circuit diagram that illustrates a prior art telephone circuit  200  that utilizes a differential amplifier. As shown in FIG. 2A, circuit  200  includes a battery  210  that has a first terminal  212 A that outputs a first voltage, such as −48V, and a second terminal  212 B that outputs a second voltage, such as ground. In addition, battery  210  has a reference terminal  214  that outputs a reference voltage VREF, such as +1.5V. Further, circuit  200  includes a line feed resistor LFR 1  that is connected to terminal  212 A, and a line feed resistor LFR 2  that is connected to terminal  212 B.  
           [0023]    Circuit  200  also includes a ring relay  216  that is connected to resistor LFR 1  and a ring node N 1 , and a ring relay  218  that is connected to resistor LFR 2  and a tip node N 2 . Circuit  200  outputs a ring voltage RING on ring node N 1 , and a tip voltage TIP on tip node N 2 . Ring node N 1  and Tip node N 2  are connected to a telephone  217  via a twisted pair TP.  
           [0024]    Further, circuit  200  includes a ring generator  220  that outputs a ring signal RS. Generator  220  has a voltage source that outputs a DC voltage of −48V, and an oscillator that outputs an AC signal that has a frequency of 20-30 Hz, an amplitude of 65-95V RMS, and a zero current crossing at the −48V DC bias when tip voltage TIP and ring voltage RING are terminated in an AC load. Further, circuit  200  includes a control circuit  226  that is connected to relays  216  and  218 , and a ring trip monitor circuit  230 .  
           [0025]    As further shown in FIG. 2A, ring trip monitor circuit  230  includes a differential amplifier  232  that has a positive input  234 A, a negative input  234 B, and an output  236  that outputs a voltage VC to control circuit  226 . In addition, circuit  230  has a resistor RB 1  that is connected between ring generator  220  and the positive input  234 A of amplifier  232 , and a resistor RB 2  that is connected between input  234 B of amplifier  232  and ground. Circuit  230  also has a resistor RB 3  that is connected between relay  216  and the negative input  234 B of amplifier  232 , and a resistor RB 4  that is connected to the positive input  234 A of amplifier  232  and resistor RB 1 .  
           [0026]    In addition, circuit  230  includes a sense resistor RN 1  that is connected between resistor RB 3  and generator  220 , and a sense resistor RN 2  that is connected between resistor RB 4  and ground. Resistors RN 1  and RB 3  are connected to a first intermediate node NM 1 , and an intermediate voltage V(−IN) is measured at node NM 1 . Resistors RN 2  and RB 4  are connected to a second intermediate node NM 2 , and an intermediate voltage V(+IN) is measured at node NM 2 .  
           [0027]    Further, ring trip monitor circuit  230  has a feedback resistor RF 1  that is connected between output  236  and input  234 B, and a feedback resistor RF 2  that is connected between input  234 A and reference terminal  214 . In addition, resistors RF 1  and RF 2  divide down the voltage to the common mode range of amplifier  232 .  
           [0028]    FIGS.  2 B 1 - 2 B 3  show timing diagrams that illustrate the operation of telephone circuit  200 . FIG. 2B 1  shows the tip voltage TIP and the ring voltage RING that are output from telephone circuit  200 . FIG. 2B 2  shows the intermediate voltages V(+IN) and V(−IN), while FIG. 2B 3  shows the differential voltage VC output from differential amplifier  232 .  
           [0029]    As shown in FIG. 2B 1 , at time t0 telephone  217  is on hook and no incoming call is present. In this state, no current flows out the tip node N 2 , through telephone  217 , and back to the ring node N 1 . As a result, the ring voltage RING is equal to −48V, and the tip voltage TIP is equal to ground. In addition, as shown in FIG. 2B 2 , intermediate voltage V(+IN) is equal to ground, while intermediate voltage V(−IN) oscillates about −48V with positive peaks.  
           [0030]    Resistors RB 1 -RB 4 , which are equal in value (e.g., 2MΩ), function as voltage dividers which, in part, define the voltages on inputs  234 A and  234 B. Line feed resistors LFR 1  and LFR 2  function as current limiting fuses, and are small, equal in value (e.g., 150Ω), and monitored by the ring trip monitor circuit for loop current. Resistors RF 1  and RF 2  are also equal (e.g., 68KΩ). Thus, as shown in FIG. 2B 3 , to insure that the voltages on inputs  234 A and  234 B remain equal, amplifier  232  sets the value of output voltage VC equal to the reference voltage VREF (e.g., +1.5V).  
           [0031]    At time t1, a call is placed to telephone  217 . Control circuit  226  detects the incoming call and outputs control signals CS 1  and CS 2  to relays  216  and  218 , respectively, to open relays  216  and  218 . As a result, the intermediate voltage V(−IN) is connected to the ring node N 1  (and output to the telephone via the ring node N 1 ), while the tip node N 2  is connected to ground via sense resistor RN 2 .  
           [0032]    When relay  216  connects the intermediate voltage V(−IN) to the ring node N 1 , a small current flows through sense resistor RN 2 . The small current causes the intermediate voltage V(+IN) to oscillate slightly, thereby causing the output of differential amplifier  232  to oscillate slightly around the positive logic high voltage (e.g., +1.5V).  
           [0033]    When telephone  217  is answered at time t2, the off hook condition causes a DC current I to flow from ground through line feed resistor LFR 2  and sense resistor RN 2  out the tip node N 2  to telephone  217 , and back via the ring node N 1 . As shown in FIG. 2B 1 , the DC current I causes the tip voltage TIP to begin oscillating, while the magnitude of the oscillating ring voltage RING falls slightly.  
           [0034]    As shown in FIG. 2B 2 , the changes also cause the intermediate voltage V(+IN) to begin oscillating, and the magnitude of the intermediate voltage V(−IN) to fall slightly. As shown in FIG. 2B 3 , the changes in the intermediate voltages V(+IN) and V(−IN) cause amplifier  232  to begin to oscillate the differential voltage VC to insure that the voltages on inputs  234 A and  234 B remain equal.  
           [0035]    The average DC value of the differential voltage VC is offset when telephone  217  is answered (off-hook), the magnitude of the offset depending on the battery, loop length, and load. When the average DC value exceeds a detection threshold that is set in control circuit  236  for a predefined period of time, controller  226  detects a ring trip and changes the logic states of the control signals CS 1  and CS 2 , thereby closing relays  216  and  218  at time t3.  
           [0036]    One problem with telephone circuit  200  is that ring trip monitor circuit  230  is always connected to the ring generator, and always drawing current. Even though the values of resistors RB 1 -RB 4  are quite high, in large phone exchanges one ring generator may be shared by hundreds of lines. Thus, the cumulative current drawn is significant.  
           [0037]    Another short coming of this circuit is that to keep the load on the ring generator to a minimum, resistors RB 1 -RB 4  are quite high in value. This causes the ring trip to be less stable due to offset currents and voltages from differential amplifier  232 , and variations due to temperature, humidity, and manufacturing processes.  
           [0038]    Thus, there is a need for a telephone circuit with a ring trip monitor circuit that is small in size, uses fewer components, and is less power demanding.  
         SUMMARY OF THE INVENTION  
         [0039]    The present invention provides a telephone circuit that has a small, low-power ring trip monitor. The telephone circuit includes a ring trip monitor circuit that has an amplifier with a positive input, a negative input, and an output. The ring trip monitor circuit also has a reference resistor that is connected to the positive input and a voltage reference, and a feedback resistor that is connected to the negative input and the output.  
           [0040]    The ring trip monitor circuit further has a first voltage divider that has a node connected to the positive input and a node connected to a tip node. In addition, the ring trip monitor circuit has a second voltage divider that has a node connected to the negative input and a node connected to the ring node.  
           [0041]    Further, the telephone circuit includes a first relay that is connected to a node of the first voltage divider, and a second relay that is connected to a node of the second voltage divider. The telephone circuit additionally includes a ring signal generator that is connected between the first relay and ground.  
           [0042]    A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description and accompanying drawings which set forth an illustrative embodiment in which the principals of the invention are utilized. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0043]    [0043]FIG. 1A is a circuit diagram illustrating a prior-art telephone circuit  100 .  
         [0044]    FIGS.  1 B 1 - 1 B 3  are timing diagrams illustrating the operation of telephone circuit  100 . FIG. 1B 1  shows the tip signal TIP and the ring signal RING that are output from telephone circuit  100 . FIG. 1B 2  shows the input signals IN+ and IN− that are input to comparator  132 , and FIG. 1B 3  shows the output of comparator  132 .  
         [0045]    [0045]FIG. 2A is a circuit diagram illustrating a prior art telephone circuit  200  that utilizes an differential amplifier.  
         [0046]    FIGS.  2 B 1 - 2 B 3  are timing diagrams illustrating the operation of telephone circuit  200 . FIG. 2B 1  shows the tip voltage TIP and the ring voltage RING that are output from telephone circuit  200 . FIG. 2B 2  shows the intermediate voltages V(+IN) and V(−IN), while FIG. 2B 3  shows the differential voltage VC output from differential amplifier  232 .  
         [0047]    [0047]FIG. 3A is a circuit diagram illustrating an example of a telephone circuit  300  in accordance with the present invention.  
         [0048]    FIGS.  3 B 1 - 3 B 3  are timing diagrams illustrating the operation of telephone circuit  300 . FIG. 3B 1  shows the tip voltage TIP and the ring voltage RING that are output from telephone circuit  300 . FIG. 3B 2  shows the terminal voltages V(−IN) and V(+IN), while FIG. 3B 3  shows the differential voltage VC output from amplifier  332 . 
     
    
     DETAILED DESCRIPTION  
       [0049]    [0049]FIG. 3A shows a circuit diagram that illustrates an example of a telephone circuit  300  in accordance with the present invention. As shown in FIG. 3A, circuit  300  includes a battery  310  that includes a first terminal  312 A that has a first intermediate voltage V(−IN), such as −48V, and a second terminal  312 B that has a second intermediate voltage V(+IN), such as ground.  
         [0050]    Battery  310  also has a reference terminal  314  that outputs a reference voltage VREF such as, for example, 1.5V. (Battery  310  can include circuitry that protects the battery from lightning strikes and other high energy conditions.) In addition, circuit  300  includes a ring relay  316  that is connected to first terminal  312 A, and a ring relay  318  that is connected to second terminal  312 B and ground.  
         [0051]    In addition, circuit  300  has a line feed resistor LFR 1  that is connected between first terminal  312 A and a ring node N 1 , and a line feed resistor LFR 2  that is connected between second terminal  312 B and a tip node N 2 . (The line feed resistor values are generally slightly larger than a standard value resulting in a shorter supervision range.) Circuit  300  outputs a ring voltage RING on ring node N 1 , and a tip voltage TIP on tip node N 2 . Ring node N 1  and Tip node N 2  are connected to a telephone  319  via a twisted pair TP.  
         [0052]    Circuit  300  also includes a ring signal generator  320  that outputs an oscillating ring signal RG. Generator  320  has an oscillator  322  that is connected to relay  316 , and a negative voltage source  324  that is connected between oscillator  322  and ground. Oscillator  322  outputs an AC signal that has a frequency of, for example, 20-30 Hz and an amplitude of, for example, 65-95V RMS, while voltage source  324  outputs a DC voltage of, for example, −48V. Further, circuit  300  includes a control circuit  326  that is connected to relays  316  and  318 , and a ring trip monitor circuit  330 .  
         [0053]    As further shown in FIG. 3A, ring trip monitor circuit  330  includes a differential amplifier  332  that has a positive input  334 A, a negative input  334 B, and an output  336  that outputs a differential voltage VC to control circuit  326 . In addition, circuit  330  has a first resistor R 1  that is connected between tip node N 2  and input  334 A of amplifier  332 , and a second resistor R 2  that is connected between ring node N 1  and input  334 B of amplifier  332 .  
         [0054]    Ring trip monitor circuit  330  also has a third resistor R 3  that is connected to terminal  312 B through the normally closed contacts of ring relay  318  on one side and both input  334 B and resistor R 2  on the other side. In addition, ring trip monitor circuit  330  has a fourth resistor R 4  that is connected to terminal  312 A through the normally closed contacts of ring relay  316  on one side and both input  334 A and resistor R 1  on the other side. Further, ring trip monitor circuit  330  has a fifth resistor R 5  that is connected between reference terminal  314  and input  334 A, and a sixth resistor R 6  that is connected between output  336  and input  334 B.  
         [0055]    FIGS.  3 B 1 - 3 B 3  show timing diagrams that illustrate the operation of telephone circuit  300 . FIG. 3B 1  shows the tip voltage TIP and the ring voltage RING that are output from telephone circuit  300 . FIG. 3B 2  shows the terminal voltages V(−IN) and V(+IN), while FIG. 3B 3  shows the differential voltage VC output from amplifier  332 .  
         [0056]    As shown in FIG. 1B 1 , at time t0 telephone  319  is on hook and no incoming call is present. In this state, there is no current flowing out the tip node N 2 , through telephone  319 , and back to the ring node N 1 . As a result, the intermediate and ring voltages V(−IN) and RING are both equal to −48V DC, and the intermediate and tip voltages V(+IN) and TIP are both equal to ground.  
         [0057]    Resistors R 3 /R 2  and R 1 /R 4 , which are equal in value (e.g., 400KΩ), function as voltage dividers which, in part, define the voltages on inputs  334 A and  334 B. Line feed resistors LFR 1  and LFR 2  function as current limiting fuses, and are small, equal in value (e.g., 100Ω), and monitored by the ring trip monitor circuit for loop current.  
         [0058]    Resistors R 5  and R 6  are also equal. Thus, as shown in FIG. 3B 3 , to insure that the voltages on inputs  334 A and  334 B remain equal, amplifier  332  sets the value of output voltage VC equal to the reference voltage VREF (e.g., +1.5V). Further, resistors R 3  and R 6  divide down the voltage to the common mode range of amplifier  332  to prevent the differential voltage VC output from amplifier  332  from exceeding the input and output voltage range.  
         [0059]    At time t1, a call is placed to telephone  319 . Control circuit  326  detects the incoming call and outputs control signals CS 1  and CS 2  to relays  316  and  318 , respectively, to open relays  316  and  318 . As a result, the ring signal RG is connected to line feed resistor LFR 1  and resistor R 4  (and is output to telephone  319  via the ring node N 1 ), while line feed resistor LFR 2  and resistor R 3  are connected to ground.  
         [0060]    As shown in FIGS.  3 B 1  and  3 B 2 , when relay  316  connects the oscillating voltage to line feed resistor LFR 1 , the intermediate voltage V(−IN) and the ring voltage RING are slightly out of phase. (The intermediate voltage V(+IN) and the tip voltage TIP remain equal to ground.) As shown in FIG. 3B 3 , when the intermediate voltage V(−IN) and the ring voltage RING are slightly out of phase, the differential voltage VC oscillates slightly about the reference voltage VREF.  
         [0061]    When telephone  319  is answered at time t2, the off hook condition causes a DC current I to flow from ground through line feed resistor LFR 2  out the tip node N 2  to telephone  319 , and back via the ring node N 1  to battery terminal  312 . As shown in FIG. 3B 1 , the DC current I causes the tip voltage TIP to begin oscillating, while the magnitude of the oscillating ring voltage RING falls slightly. This is due to the change from the relatively high AC impedance of the ringer in telephone  319  to the DC resistance of telephone  319  when telephone  319  was taken off hook.  
         [0062]    As shown in FIG. 3B 3 , the changes in the ring and tip voltages RING and TIP cause amplifier  332  to begin to oscillate the differential voltage VC. During the ring state between times t1 and t2, very little AC current and no DC current is flowing from the ring generator to the telephone to ground. Therefore, as shown in FIG. 3B 3 , there is little output from differential amplifier  332 . At time t2, telephone  319  goes off hook. When this happens, both AC and DC current begin to flow.  
         [0063]    From time t2 to time t3, the difference voltage across resistors LFR 1  and LFR 2  is much higher, resulting in a much larger signal at differential output VC. In addition, the AC signal is DC offset by the current from the ring generator  320  DC source  324 . The average DC value of the differential voltage VC is offset when telephone  319  is answered (off-hook), the magnitude of the offset depending on the battery, loop length, and load.  
         [0064]    A detection threshold is set in control circuit  336 . When the average DC value exceeds the threshold for a predefined period of time, controller  326  detects a ring trip and changes the logic states of the control signals CS 1  and CS 2 , thereby closing relays  316  and  318  at time t3. The ring trip time (t3-t2) is a function of where in the ring cycle telephone  319  is answered (off-hook), the ring frequency, the ring voltage AC and DC, the loop length, and the ringer load.  
         [0065]    One advantage of telephone circuit  300  is that telephone circuit  300  does not require capacitors as does circuit  100 . The capacitors in circuit  100  are quite large and occupy a significant amount of circuit board space. Thus, by eliminating the capacitors, the present invention allows smaller circuit boards to be utilized or, alternately, more circuitry can be incorporated on the same sized circuit board.  
         [0066]    Another advantage of telephone circuit  300  is that ring trip monitor circuit  330  has no standby power consumption as does telephone circuit  200 . Unlike circuit  200 , circuit  300  provides a load to ring signal generator  320  only when the ring signal RG is placed on the line. As a result, a smaller power supply can be utilized, and less cooling is required.  
         [0067]    A further advantage is that the present invention can be used on both balanced and unbalanced lines. Unbalanced ringing is primarily used in North America and is defined as an AC voltage which has a frequency in the range 15-60 Hz (typically 20-30 Hz) that is superimposed on a battery (typically −48V). The superimposed AC is normally applied to the ring lead with the tip lead providing a ring ground return.  
         [0068]    Balanced ringing occurs when the superimposed signals are simultaneously applied to the ring and tip leads 180° out of phase with each other. Each AC source is balanced ring-to-ground and tip-to-ground. Further, each AC source is DC offset so that the ring lead is generally more negative than the tip lead.  
         [0069]    Other advantages of the present invention are that a ring trip sense can be done under normal loop closure as well as a ring ground fault or tip power fault. Further, circuit  300  does not require a separate high wattage fault tolerant sensing resistor. In addition, resistors R 1 -R 4  of circuit  300  can have lower values, resulting in more reliable and stable operation.  
         [0070]    It should be understood that the above description is an example of the present invention, and various alternatives to the embodiment of the invention described herein may be employed in practicing the invention. Thus, it is intended that the following claims define the scope of the invention and that methods and structures within the scope of these claims and their equivalents be covered thereby.