RING TRIP CIRCUIT FOR SUBSCRIBER TELEPHONE

Abstract: Disclosed is an improved ring trip circuit for a subscriber telephone, for detecting an on-hook state or an off-hook state of the telephone. The ring trip circuit can be formed in a small size and can be formed by an LSI. The ring trip circuit includes a voltage detecting circuit (2), for detecting a voltage difference ( DELTA V) between both ends of a ringer sending resistor, having a first circuit (S1, S2, A1 SIMILAR B3) converting the voltage difference to a current difference, and further having a second circuit (F) converting the current difference to a voltage, the voltage difference varying in response to an on-hook state or an off-hook state of the telephone, and a state detecting circuit (3) for detecting the on-book state or the off-hook state in response to a value of the voltage from the voltage detecting circuit. The voltage detecting circuit is formed by a plurality of current mirror circuits, each of which includes at least two transistors. The state detecting circuit includes a comparator circuit (CP1) formed by a plurality of transistors.

Description: DESCRIPTION RING TRIP CIRCUIT FOR SUBSCRIBER TELEPHONE TECHNICAL FIELD The present invention relates to a ring trip circuit for a subscriber telephone, by which an on-hook state or an off-hook state of the telephone is detected. More particularly, it relates to an improved ring trip circuit which has a reduced size and can be formed by a large scale integrated circuit. BACKGROUND ART Ring trip circuits are provided in a telephone exchanger (or switching board) .for detecting an on-hook state or off-hook state of subscriber telephones. Each ring trip circuit is connected to a subscriber line, i.e., a ring trip circuit must be provided for each subscriber telephone. Since the telephone exchanger processes a large number of subscriber lines, for example, over 20,000 subscriber lines, a large number of ring trip circuits are provided in the telephone exchanger. The ring trip circuit of the prior art are bulky and is not suitable to be formed by a large scale integrated circuit (LSI) . Also, the ring trip circuits of the prior art are expensive. The prior art ring trip circuit will be described in more detail later with reference to the drawings. DISCLOSURE OF THE INVENTION An object of the present invention is to provide a ring trip circuit which can be formed in a small size and by a large scale integrated circuit. Another object of the present invention is to provide a ring trip circuit which can be manufactured at a low cost. According to a basic aspect of the present invention, there is provided a ring trip circuit for a subscriber telephone, including: a voltage detecting circuit, operatively connected between both ends of a ringer sending resistor operatively connected between the subscriber telephone and a ringer source, for detect- ing a voltage difference between both ends of the ringer sending resistor, and having a circuit converting the voltage difference to a current difference the voltage difference varying in response to whether the subscriber telephone is an on-hook state or an off-hook state; and a state detecting circuit, operatively connected to the voltage detecting circuit, for detecting the on-hook state or the off-hook state of the subscriber telephone in response to a value corresponding to the voltage differ¬ ence. The voltage detecting circuit is formed by a plurality of current mirror circuits, each of which includes at least two transistors, and the state detecting circuit includes a comparator circuit formed by a plurality of transistors, so that the ring trip circuit is formed in a small size and is adaptive to be formed by a large scale integrated circuit. According to one aspect of the present invention, the voltage detecting circuit includes a first current switching circuit operatively connected to one end of the ringer sending resistor, and having first to third ports, the first current switching circuit flowing a first current from the second port to the third port when a voltage at one end of the ringer sending resist is positive, or flowing the first current from the first port to one end of the ringer sending resistor through the third port when the voltage at one end of the ringer sending resistor is negative, a second current switching circuit operatively connected to another end of the ringer sending resistor, and having first to third ports, the second current switching circuit flowing a second current from the second port to the third port when another voltage at another end of the ringer sending resistor is positive, or flowing the second current from first port and flowing the same to another end of the ringer sending resistor through the third port when the another voltage at another end of the ringer sending resistor is negative, a first current mirror circuit having a first terminal connected to the first port of the first current switching circuit and a second terminal, a second current mirror circuit having a first terminal connected to the second port of the first current switching circuit and a second terminal connected to the second terminal of the first current mirror circuit, a third current mirror circuit having a first terminal connected to the first port of the second current switching circuit and a second terminal, a fourth current mirror circuit having a first terminal connected to the second port of the second current switch¬ ing circuit and a second terminal, a fifth current mirror circuit having a first terminal connected to the second terminal of the fourth current mirror circuit and a second terminal, and a sixth current mirror circuit having a first terminal connected to the second terminal of the third current mirror circuit and a second terminal. A commonly connected point of the second terminals of the first and second current mirror circuits, and the second terminals of the fifth and sixth current mirror circuits are commonly connected to output a current corresponding to a voltage difference between one and another ends of the ringer sending resistor. The voltage detecting circuit further includes a voltage conversion circuit, operatively connected to the common connected point, for converting the current to a voltage, outputting the same, and having an output terminal of the voltage detecting circuit. The first current switching circuit includes an npn-type transistor and a pnp-type transistor, a collector of the npn-type transistor is the first port, emitters of the transistors are commonly connected, a common connected point is operatively connected to one end of the ringer sending resistor, a collector of the npn-type transistor is the second port, and bases of the transistors are supplied with a first voltage. Also, the second current switching circuit having a circuit construction the same as the first switching circuit. Each of the current mirror circuits includes first and second npn-type transistors, a collector of the first transistor being the first terminal, a collector of the second transistor being the second terminal, bases of the transistors being commonly connected and connected to the collector of the first transistor, and • emitters of the transistors being supplied with a second voltage. Preferably, each of the current mirror circuits further includes a third pnp-type transistor, a base being connected to the collector of the first tran¬ sistor, an emitter being connected to the base common connected point of the first and second transistors, and a collector being supplied with a third voltage. Each of the current mirror circuits further includes a first resistor connected to the emitter of the first transistor, and a second resistor connected to the emitter of the second transistor. The voltage conversion circuit includes a voltage conversion resistor, operatively connected to the common connected point, for converting the current from the common connected point to a voltage and functioning as the output terminal of the voltage detecting circuit. The voltage conversion c ' ircuit further includes a capacitor parallel-connected to the voltage conversion resistor. The state detecting circuit includes a differential amplifier, a first input terminal connected to the output terminal of the voltage detecting circuit, a voltage limiter connected to the first input terminal of the differential amplifier, a threshold voltage supplying circuit connected to a second input terminal of the differential amplifier, and a current source circuit connected to the differential amplifier for supplying a constant current thereto. The ring trip circuit further include a first dropping resistor connected between one end of the ringer sending resistor and the third port of the first current switching circuit, and a second dropping resistor connected between another end of the ringer sending resistor and the third port of the second current switching circuit. The ring trip circuit further include a filtering capacitor connected between ends of the first and second dropping resistors, which ends are adjacent to the third ports of the first and second current switching circuits. The ring trip circuit further includes a scan output circuit connected to the state detecting circuit and outputting a digital detection signal in response to an on-hook state or an off-hook state. According to another aspect of the present inven¬ tion, the voltage detecting circuit includes a current difference detecting circuit, operatively connected between ends of ringer sending resistor, for detecting a difference of currents extracted from one end and another end of the ringer sending resistor, an absolute calculation circuit, operatively connected to the current difference detecting circuit, for calculating an absolute of the current difference, and a voltage conversion circuit, operatively connected to the absolute calculation, for converting the absolute current to a voltage, outputting the same, and having an output terminal of the voltage detecting circuit. The current difference detecting circuit includes a current inversion circuit, operatively connected to the one end of the ringer sending resistor, for inverting the first current extracted from one end of the ringer sending resistor, and an addition circuit, operatively connected to the current inversion circuit and another end of the ringer sending resistor, for adding the inverted first current and the second current flowing through another end of the ringer sending resistor. The current inversion circuit includes a first current switching circuit operatively connected to one end of the ringer sending resistor, and having first to third ports, the first current switching circuit flowing a first current from the second port to the third port when a voltage at one end of the ringer sending resistor is positive, or flowing the first current from the first port to one end of the ringer sending resistor through said third port when the voltage at the one end is negative, a first current mirror circuit having a first terminal connected to the first port of the first current switching circuit and a second terminal, and a second current mirror circuit having a first terminal connected to the second port of the first current switching circuit and a second terminal connected to the second terminal of the first current mirror circuit. The addition circuit includes a line operatively connected to another end of the ringer sending resistor, and another line connected to the common connected point of the second terminals of the first and second current mirror circuits, and connected to the line. The absolute calculation circuit includes a second current switching circuit connected to the addition circuit, and having first to third ports, the second current switching circuit flowing a third current, which is a difference current between the first current and the second current, extracted from another end of the ringer sending resistor, from the first port to said second port when another voltage at another end of the ringer sending resistor is positive, or flowing the third current from the second port to the third port when the another voltage is negative, and a third current mirror circuit having a first terminal connected to the third port of the second current switching circuit and a second terminal connected to the first port of the second current switching circuit. The voltage conversion circuit includes a fourth current mirror circuit having a first terminal connected to the first port of the second current switching circuit, and a second terminal, and a voltage conversion resistor connected to the second terminal of the fourth current mirror circuit and converting a current there- from to a voltage. BRIEF DESCRIPTION OF DRAWINGS Figures 1 and 2 are circuit diagrams of prior art ring trip circuits; Figs. 3a and 3b are graphs explaining states on-hook and off-hook states of a subscriber telephone; Figs. 4a and 4b are graphs explaining the detection of on-hook and off-hook states by a ring trip circuit; Fig. 5 is a circuit diagram of a first embodiment of a ring trip circuit in accordance with the present invention; Figs. 6a and 6b, and 7a and 7b are block diagrams explaining the operation of the ring trip circuit shown in Fig. 5; Fig. 8 is a block diagram of a modification of the first embodiment shown in Fig. 5; Fig. 9 is a circuit diagram of a second embodiment of a ring trip circuit in accordance with the present invention; and Fig. 10 is a block diagram of a modification of the second embodiment shown in Fig. 9. BEST MODE OF CARRYING OUT THE INVENTION Before describing the preferred embodiments of the present invention, examples of a prior art ring trip circuit are described with reference to the drawings. Figure 1 shows a prior art ring trip circuit of a -24 volt feed type. In Fig. 1, reference CR1 denotes a ringer source, reference R0 denotes a ringer sending resistor RO, and reference TEL denotes a subscriber telephone. The ringer trip circuit includes dropping resistors Rl and R2, an operational amplifier OPl, a resistor R3 connected between an inverting terminal of the operational amplifier OPl and an output terminal thereof, a capacitor Cl, a dropping resistor R4, and a capacitor C2. The ringer trip circuit also includes a comparator CPl. Voltages V and V, at both ends of the ringer sending resistor RO are supplied to the inverted and non-inverted input terminals of the operational amplifier OPl through the dropping resistors Rl and R2. The operational amplifier OPl cooperates with the resistor R3, and the resistor R4, and functions as a differential amplifier. Since an AC voltage having a high amplitude from the ringer source CRl is super¬ imposed on the DC feed voltage, and applied to the operational amplifier OPl, in order to avoid a malfunction due to the above high AC voltage, the capacitors Cl and C2 are connected to the resistors R3 and R4 in parallel, respectively, and function as low pass filters for rejecting the high AC voltage. The operational amplifier OPl calculates a voltage dif¬ ference ΔV between the above voltages V and V, : ΔV = V - V, . The comparator CPl compares the voltage difference ΔV with a threshold voltage V .. Figure 2 shows another prior art ring trip circuit of a -48 volt feed type. In Fig. 2, the connection of the telephone TEL, the ringer sending resistor RO and a ringer source CR2 is the reverse of that in Fig. 1, however, the configuration of the ring trip circuit, per se, is substantially the same as that of Fig. 1. The operation of the ring trip circuit shown in Fig. 2 will be described with reference to Figs. 3a and 3b, and Figs. 4a and 4b. Figures 3a and 3b show equivalent circuits of the subscriber telephone TEL when in an on-hook state and an off-hook state. When the telephone TEL is in the on-hook state, i.e., a call has been received, but the subscriber has not yet picked-up a transmitter and receiver for response, an internal impedance of the telephone TEL is large, as shown in Fig. 3a. The internal impedance is defined by a series- connected circuit of an internal resistor R, an internal inductor L and an internal capacitor C. Conversely, when the telephone TEL becomes in the off-hook state, and the transmitter and receiver is picked-up, the internal impedance of the telephone becomes small, as shown in Fig. 3b, the internal impedance is defined by only the internal resistor R. When in the on-hook state, the AC voltage from the ringer source CR2 is reduced by the internal capacitor, and the DC voltage from a voltage feeder (not shown) is also reduced, and therefore, the voltage difference between the ends of the ringer sending resistor RO is small, as shown in Fig. 4a. On the contrary, when in the off-hook state, the voltage difference is large and the AC voltage is superimposed on the DC voltage from the voltage feeder without reduction. The operational amplifier OPl provides the above DC voltage difference. The comparator CPl compares the DC voltage difference with the threshold voltage V , , and outputs a high level signal when the telephone TEL is in the off-hook state, and otherwise, outputs a low level signal. The high or low level signal indicates the off-hook state or the on-hook state. The ringer source CR2 outputs an AC voltage having a large amplitude of, for example, AC 100 V, and a low frequency, for example, approximately 16 to 20 Hz. As a result, the resistors Rl, R2, R3 and R4 must have a large resistance, and the capacitors Cl and C2 also must have a large capacitance. Below are example of the above resistors and the capacitors. resistance of Rl 1 MΩ resistance of R2 1 MΩ resistance of R3 100 kΩ resistance of R4: 100 kΩ capacitance of Cl: 1 « «F capacitance of C2: 1 μF As described above, since a large number of the ring trip circuits is provided in the telephone exchanger, the ring trip circuit must be as small as possible and as low cost as possible. In addition, the ring trip circui (s) should be formed by an LSI. However, the above large resistors and the large capacitors can not be substantially formed by the LSI. Thus, the prior art, ring trip circuit must be formed by individual circuit elements. This results in a bulky size and a high cost. The present invention overcomes the above defects- The principle of the detection of the on-hook state or the off-hook state, per se, is substantially the same as that described above. The present invention is intended to provide an improved ring trip circuit which introduces a new circuit technology devised to eliminate the large resistors and the large capacitors. Now, preferred embodiments of the present invention will be described. Figure 5 shows a circuit diagram of a first embodi¬ ment of a ring trip circuit in accordance with the present invention. In Fig. 5, the ringer source CR1, the ringer sending resistor R0, and the telephone TEL are same as those shown in Fig. 1. The feed voltage is also -24 V. These circuits, except for the telephone TEL, are provided in the telephone exchanger. The ringer trip circuit shown in Fig. 5 includes an input circuit 6, a voltage detecting circuit 2, a state detecting circuit 3, and a scan output circuit 1. The input circuit 6 includes a first dropping resistor R01 connected to one end a of the ringer sending resistor R0, and a second dropping resistor R02 connected to another end b_ of the ringer sending re¬ sistor R0. Preferably, a filtering capacitor CO is connected between the first and second resistors R01 and R02, as shown in Fig. 1. The voltage detecting circuit 2 includes a first current switching circuit SI connected to one end a_ of the ringer sending resistor through the resistor R01, and having first to third ports Pll, P12 and P13, and a second current switching circuit S2 connected to another end b of said ringer sending resistor through the resistor R02, and having first to third ports P21, P22 and P23. The voltage detecting circuit 2 also includes a first current mirror circuit Al having a first terminal connected to the first port Pll of the first current switching circuit SI and a second terminal, and a second current mirror circuit Bl having a first terminal connected to the second port P12 of the first current switching circuit SI and a second terminal connected to the second terminal of the first current mirror circuit. The voltage detecting circuit 2 further includes a third current mirror circuit A2 having a first terminal connected to the first port P21 of the second current switching circuit S2 and a second terminal, and a fourth current mirror circuit B2 having a first terminal connected to the second port P22 of said second current switching circuit S2 and a second terminal. Furthermore, the voltage detecting circuit 2 includes a fifth current mirror circuit A3 having a first terminal connected to the second terminal of the fourth current mirror circuit B2 and a second terminal, and a sixth current mirror circuit B3 having a first terminal connected to the second terminal of the third current mirror circuit A2 and a second terminal. A commonly connected point of the second terminals of the first and second current mirror circuits Al and Bl, and the second terminals of the fifth and sixth current mirror circuits A3 and B3 are commonly connected to output a current corresponding to a voltage difference between one and another ends a. and b_ of the ringer sending resistor. The voltage detecting circuit 2 further includes a voltage conversion circuit F having a fillering function and connected to the common connected point, for converting the current to a voltage, outputting the same, and having an output terminal of the voltage detecting circuit 2. The state detecting circuit 3 includes a differ¬ ential amplifier OPl, a first input terminal thereof connected to the output terminal c_ of the voltage detecting circuit, a voltage limiter D connected to the first input terminal of the differential amplifier, a threshold voltage supplying circuit SV connected to a second input terminal of the differential amplifier, and a current source circuit SI connected to the differen¬ tial amplifier for supplying a constant current thereto. The first current switching circuit SI includes series-connected npn-type transistor TR00 and a pnp-type transistor TR01. A collector of the transistor TR00 is the first port Pll. Emitters of the transistors are commonly connected. A connected point is connected to one end of the ringer sending resistor RO through the resistor R01. A collector of the transistor TR01 is the second port P12. Bases of the transistors TR00 and TR01 are supplied with a drive voltage V n _. Since the complementary operation transistors TR00 and TR01 are serially connected, and the third port P13, which is the common connected point of the emitters, is connected to one end a_ of the ringer sending resistor RO through the resistor R01, the first current switching circuit SI flows a first current I from the second port P12 to the third port P13 when a voltage V at one end a of the resistor RO is positive, or. flows the first current from the first port Pll through the third port P13 to the one end *a— * when the voltage Vcl at the one end "a— is negative. The second current switching circuit S2 has the same circuit construction as that of the first current switching circuit SI, and thus the operation thereof is similar to that of the first current switching circuit SI. Each of the first, third and fifth current mirror circuits Al, A2 and A3, for example, the first current mirror circuit Al, includes first and second npn-type transistors Q300 and Q301. A collector of the first transistor Q300 is the above first terminal, a collector of the second transistor Q301 is the second terminal, bases of the transistors are commonly connected and connected to the collector of the first transistor, and emitters of the transistors are supplied with a volt¬ age V--,. Each of the first to sixth current mirror circuits Al to B3 may further include a third pnp-type transistor, for example, Q302. A base of the transistor Q302 is connected to the collector of the first tran¬ sistor Q300, an emitter thereof is connected to the base common connected point of the first and second transistors Q300 and Q301, and a collector thereof is supplied with a third voltage V D _. The first current mirror circuit Al includes a first resistor R300 connected to the emitter of the first transistor Q300, and a second resistor R301 connected to the emitter of the second transistor Q301. Each of the second, fourth and sixth current mirror circuits Bl, B2 and B3 also has a circuit construction similar to that of the first current mirror circuit Al, except that, for example, a collector of a third pnp-type transistor Q314 in the second current mirror circuit Bl is grounded and emitters of the first and second transistors Q315 and Q316 are supplied with a voltage V βB . The current mirror circuit Al outputs an output current equal to an input current flowing through the first terminal, i.e., the collector of the transistor Q300, from the second terminal, i.e., the collector of the transistor Q301. By providing the third tran- sistor Q302, a balance of the input current and the output current is improved. Other current mirror circuits operate in the same way as above. The voltage conversion circuit F consists of parallel connected resistor R03 and capacitor COO. The resistor R03 converts the current passing through the common connected point c to a voltage ΔV. The resistor R03 also cooperates with the capacitor COO and functions as a filter. The differential amplifier CPl, functioning as the comparator, consists of two npn-type transistors TR04 and TR05 connected in parallel. The voltage limiter D is composed of series-con¬ nected transistors Q325 to Q328, each connected to function as a diode. The voltage limiter D shifts a level at a base of the transistor TR04 of the differen¬ tial amplifier OPl to protect the transistor TR04 against a high voltage from the point c_. The current source circuit SI is also formed by a current mirror circuit, consisting of transistors Q330 to Q332, and resistors R330 and R331. The threshold voltage supply circuit SV is composed of two series-connected dividing resistors R323 and R324 supplied with a constant voltage V r _, therebetween to supply a threshold voltage V , from a common connected point d of the resistors to the second input terminal of the differential amplifier, i.e. , a base of the transistor TR05. The circuit parameters of the circuits in Fig. 5 are shown below: resistance of the resistor RO: 600 Ω resistance of the resistor R01 and R02: 100 kΩ capacitance of the capacitor CO: 1 μF resistance of the resistors in each current mirror circuit, for example, R300 and R301: 2 kΩ resistance of the resistor R03: 100 kfi capacitance of the capacitor COO: 1 μF resistance of the resistor R330 20 kΩ resistance of the resistor R331 10 kΩ resistance of the resistor R323 3 kΩ resistance of the resistor R331 2 kΩ V. -24 VDC BB V + 5 VDC Cι_C_. VV. , : + 2 VDC th a 60 μA Compared with Figs. 1 and 5, the four large resistors Rl, R2 , R3 and R4 in Fig. 1 are reduced to two resistors R01 and R02, and the large capacitors Cl and C2 are reduced to one capacitor C00, or optionally, to two capacitors COO and CO. There are no large resistors or any capacitors, and thus the first to sixth current sources Al to A3, and Bl to B3 , the first and second current switch circuits SI and S2, the differential amplifier CPl, the voltage limiter D, the current source SI, and the threshold voltage supplying circuit SV, can be easily formed by an LSI. The scan output circuit 1 also can be formed by transistors and small resistors, and thus incorporated with the above circuits in the LSI. One LSI chip may include a plurality the above LSI circuits, and a plurality of LSI chips can be mounted on a printed circuit board (PCB) . - When the input circuit 6 is not provided with the capacitor CO, the input circuit 6 can be formed by the LSI. The voltage conversion circuit F should be provided outside of the LSI chip and the PCB, because the voltage conversion circuit 8 includes the large capacitor COO. Referring to Figs. 6a and 6b, and Figs. 7a and 7b, the operation of the ring trip circuit shown in Fig. 5 will be described. Figures 6a and 6b show the circuit condition when ' the telephone TEL is in the on-hook state. Figures 7a and 7b show the circuit condition when the telephone TEL is in the off-hook state. In the drawings, shaded waves represent ringer voltages. Also, terminals having small circles of the current mirror circuits Al to B3 represent the first terminals, other terminals having not small circles represent the second terminals. Figure 6a shows the circuit condition when the telephone TEL is in the on-hook state and a positive ringer voltage is supplied to the ringer sending resistor RO. In Fig. 6a, the voltage Va is higher than the voltage V__ supplied to the base of the transistor TR01 in the first current switching circuit SI, and thus the transistor TR01 is turned ON, allowing the first current I to flow from one end a of the ringer sending resistor RO to the first terminal of the second current mirror circuit Bl through the resistor R01 and the transistor TR01. The current mirror circuit Bl extracts a current equal to the first current Ia into the second terminal. Similarly, the voltage V. is higher than the voltage V__ supplied to the base of the transistor TR03 in the second current switching circuit S2, and thus the transistor TR03 is turned ON, allowing the second current I. to flow from another end b_ of the ringer sending resistor RO to the first terminal of the fourth mirror circuit B2 through the resistor R02 and the transistor TR03. The current mirror circuit B2 also extract a current equal to the second current I, into the second terminal from the first terminal of the fifth current mirror circuit A3. The current mirror circuit A3 outputs a current equal to the second current I. extracted from the first terminal, from the second terminal. Since the transistors TR00 and TR02 are turned OFF, the first and third current mirror circuits Al and A2 are in an inoperable state, and accordingly, the sixth current mirror circuit B3 is in an inoperable state. As a result, at point c_, the current I, from the second terminal of the fifth current mirror circuit A3 is subtracted from the current Ia from the second terminal of the second current mirror circuit Bl, i.e., ΔI = I,b - Ia is obtained. The current difference ΔI is converted to the voltage difference ΔV at the voltage converting circuit 8. Since the telephone TEL is in the on-hook state, both of the voltages V and V, are low level and approximately equal. Namely, V = V, , I = I. , ΔI = 0, and ΔV = 0. The voltage difference ΔV is lower than the threshold voltage V .. The comparator CPl detects the on-hook state, and outputs a low level signal of 0 V. Figure 6b shows the current condition when the telephone TEL is in the on-hook state, and a negative ringer voltage is supplied to the ringer sending re¬ sistor RO. In this case, the first, the third and the sixth current mirror circuits Al, A2 and B2 are operated. Conversely, the operation of the current mirror circuits Bl, B2, and A3 is stopped. At point £, the current Ia from the second terminal of the first current mirror circuit Al is subtracted from the current I,b flowing into the second terminal of the sixth current mirror circuit B3, i.e., 1 = 1a - I,b is obtained. As described above with reference to Fig. 6b, ΔI = 0, and V = 0. The comparator CPl also detects the on-hook state. Figure 7a shows the circuit condition when the telephone TEL is in the off-hook state and a positive ringer voltage is supplied. The operational condition of the current switching circuits SI and S2, and the current mirror circuits Al to B3 is same as that of Fig. 6a, however, in this case, the resistance R in Fig. 3b is approximately 100 Ω, and a high ringer voltage is supplied to the ringer sending resistor R0. As a result, a voltage difference between the volt¬ ages V and V, becomes large, and a large current difference ΔI = I.b - Ia (ΔI < 0) is obtained. The current difference ΔI is converted into the voltage difference ΔV having an amplitude higher than the threshold voltage V , at the voltage converting circuit 8. The comparator CPl detects the off-hook state. Figure 7b shows the circuit condition when the telephone TEL is in the off-hook state, and a negative voltage is supplied to the ringer sending resistor RO. The operational condition is same as that of Fig. 6b. A large current difference ΔI = I - L. (ΔI > 0) is obtained, and the off-hook state is detected. Compared with Figs. 7a and 7b, since a polarity of the voltage difference ΔV is reversed, and a single comparator CPl is provided, an absolute circuit should be provided at an input portion of the comparator CPl. The absolute circuit will be described with reference to Fig. 9. The scan output circuit 1 outputs a digital detec- tion signal in response to the on-hook state or the off-hook state. Figure 8 shows a modification of the ring trip circuit shown in Fig. 5. In Fig. 8, a voltage feeder V_ R is provided between the ground and the ringer source CR1, and another end b_ of the ringer sending resistor R0 is grounded. The ring trip circuit is similar to that shown in Fig. 5. Figure 9 shows a circuit diagram of a second embodiment of a ringer trip circuit of the present invention. The ringer trip circuit is of a -48 V feed type. In Fig. 9, the ring trip circuit includes an input circuit 6a similar to the input circuit 6 in Fig. 5, a - current difference detecting circuit 7a, an absolute calculation circuit 8a, a voltage conversion circuit 9a, a state detecting circuit 3a (not shown) including the comparator CPl, and a scan output circuit la (not shown) . The state detecting circuit 3a and the scan output circuit la are similar to those in Fig. 5. The current difference detecting circuit 7a in¬ cludes a current inversion circuit 71 for inverting the first current I extracted from one end a of the ringer sending resistor RO, and an addition circuit 72 for adding the inverted first current -I and the second current I, extracted from another end b of the ringer sending resistor RO. The current inversion circuit 71 includes a current switching circuit Sll, a first current mirror cir¬ cuit All, and a second current mirror circuit Bll. The current switching circuit Sll is composed of series-connected npn-type transistor TR703 and pnp-type transistor TR704. A collector of the npn-type tran¬ sistor TR703 is a first port PA1, emitters of the transistors are commonly connected, a common connected point PA2 is operatively connected to one end a of the ringer sending resistor RO, a collector of the npn-type transistor TR704 is a second port PA3, bases of the transistors are supplied with a voltage of V /2. The first current mirror circuit All is composed of transistors TR700 to TR702, and resistors 711 and 712. The second current mirror circuit Bll is composed of transistors TR705 to TR707, and resistors R713 and R714. The current switching circuit Sll, and the first and second current mirror circuits All and Bll corre¬ spond to the first current switching circuit SI and the first and second current mirror circuits Al and Bl, respectively. Accordingly, the current inversion circuit 71 outputs a first current Ia corresponding to a voltage Va at point a,- from a terminal commonly connected to collectors of the transistors TR701 and TR707, when Va < V__.,___■/2, as shown by solid lines, and otherwise, extracts the first current I into the terminal when V α > V rr ,/2, as shown by dotted lines. The addition circuit 72 is a point B commonly connected by a line connected to the terminal of the circuit 71 and another line connected to the point b through the resistor R02. Accordingly, the addition circuit 72 calculates a current difference ΔI = Ia - I.b The Absolute calculation circuit 8a calculates an absolute of the current difference: il- The absolute calculation circuit 8a includes a second current switching circuit S22 similar to the first current switching circuit Sll in the circuit 71, and a third current mirror circuit A12 similar to the second current mirror circuit Bll in the circuit 71. Note that there is not provided with a current mirror circuit correspond¬ ing to the first current mirror circuit All in the circuit 71. Accordingly, the absolute calculation circuit 8a always extracts an absolute current I equal to the current difference ΔI. The voltage conversion circuit 9a includes a fourth current mirror circuit A13 consisting of transis- tors TR913 to TR915, and resistors R901 and R902, and a filter F consisting of a dropping resistor R06 and a capacitor C03. The current mirror circuit A13 functions as a current inversion circuit for supplying a positive polarity of the current difference ΔI. The filter F always outputs a positive polarity of a voltage differ¬ ence ΔV. The voltage difference ΔV is varied in response to the state of the telephone TEL, as described with reference to Figs. 6a, 6b, 7a and 7b. The compara- tor CPl detects the on-hook state or the off-hook state. The resistances of the resistors R01, R02 and R06, and the capacitances of the capacitor CO3 are almost the same as those in Fig. 5. Other circuit parameters are also similar to those in Fig. 5. The " ring trip circuit shown in Fig. 9 is also formed by an LSI in the same way to the ring trip circuit shown in Fig. 5. Figure 10 shows a modification of the ring trip circuit shown in Fig. 9. In Fig. 10, an input cir¬ cuit 6b consists of resistors R04 to R07 and a capacitor CO2, formed as a ladder and functioning a filter and a voltage dropper. In Fig. 10, resistance of the resistors R04 to R07 is 100 kΩ, and the capacitance of the capacitor C02 is 1 μF. Many widely different embodiments of the present invention may be constructed without departing from the spirit and scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in this specifi¬ cation, except as defined in the appended claims. INDUSTRIAL APPLICABILITY The ring trip circuit of the present invention is provided in a telephone exchanger and is applicable for detecting an on-hook state or an off-hook state of a subscriber telephone, or the like.

Claims: CLAIMS 1. A ring trip circuit for a subscriber tele¬ phone, co prising: voltage detecting means (2) , operatively connected between both ends of a ringer sending resistor (RO) operatively connected between the subscriber telephone (TEL) and a ringer source (CR) , for detecting a voltage difference between said both ends of said ringer sending resistor, and having a circuit (SI, S2, Al ^ B3) converting the voltage difference to a current difference, said voltage difference varying in response to whether said subscriber telephone is in an on-hook state or an off-hook state; and state detecting means (3) , operatively connected to said voltage detecting means, for detecting the on-hook state or the off-hook state of said sub¬ scriber telephone in response to a value corresponding to said voltage difference, said voltage detecting means formed by a plurality of current mirror circuits, each of which includes at least two transistors, and said state detecting means including a comparator circuit (CPl) formed by a plurality of transistors, so that said ring trip circuit is formed in a small size and is adaptive to be formed by a large scale integrated circuit. 2. A ring trip circuit according to claim 1, wherein said voltage detecting means comprises a first current switching circuit (SI) operatively connected to one end (a) of said ringer sending resistor, and having first to third ports (Pll, P12, P13) r said first current switching circuit flowing a first current (I ) from said second port (P12) to said third port (P12) when a voltage (V ) at said one end is positive, or flowing the first current from said first port (Pll) to said one end through said third port (P13) when said voltage at said one end is negative, a second current switching circuit (S2) operatively connected to another end (b) of said ringer sending resistor, and having first to third ports (P21, P22, P23) , said second current switching circuit flowing a second current (I ) from said second port (P22) to said third port (P23) when another voltage (V, ) at said another end is positive, or flowing the second current from said first port (P21) to said another end through said third port (P23) when said another voltage at said another end is negative, a first current mirror circuit (Al) having a first terminal connected to said first port of said first current switching circuit and a second terminal, a second current mirror circuit (Bl) having a first terminal connected to said second port of said first current switching circuit and a second terminal connected to said second terminal of said first current mirror circuit, a third current mirror circuit (A2) having a first terminal connected to said first port of said second current switching circuit and a second terminal, a fourth current mirror circuit (B2) having a first terminal connected to said second port of said second current switching circuit and a second terminal, a fifth current mirror circuit (A3) having a first terminal connected to said second terminal of said fourth current mirror circuit (B2) and a second terminal, and a sixth current mirror circuit (B3) having a first terminal connected to said second terminal of said third current mirror circuit (A2) and a second terminal, a commonly connected point of said second terminals of said first and second current mirror circuits (Al, Bl) , and said second terminals of said fifth and sixth current mirror circuits (A3, B3) being commonly connected to output a current corresponding to a voltage difference between said one and another ends of said ringer sending resistor, and said voltage detecting means further comprising a voltage conversion circuit (9) , operatively connected to said common connected point, for converting said current to a voltage, outputting the same, and having an output terminal of said voltage detecting means. 3. A ring trip circuit according to claim 2, wherein said first current switching circuit (SI) comprises an npn-type transistor (TR00) and a pnp-type transistor (TR01) , a collector of said npn-type tran¬ sistor being said first port (Pll) , emitter of said transistors being commonly connected, a common connected point being operatively connected to said one end of said ringer sending resistor, a collector of said npn-type transistor being said second port (P12) , bases of said transistors being supplied with a first voltage (V DD ) , and wherein said second current switching circuit (S2) comprises an npn-type transistor (TR02) and a pnp-type transistor (TR03) , a collector of said npn-type transistor being said first port (P21) , emitters of said transistors being commonly connected, a common connected point being operatively connected to said another end of said ringer sending resistor, a collector of said npn-type transistor being said second port (P22) , bases of said transistors being supplied with said first voltage (V_ ) . 4. A ring trip circuit according to claim 3, wherein each of said first to sixth current mirror circuits (Al to B3) comprises first and second npn-type transistors (Q300, Q301) , a collector of said first transistor being said first terminal, a collector of said second transistor being said second terminal, bases of said transistors being commonly connected and connected to said collector of said first transistor, and emitters of said transistors being supplied with a second voltage (V DT ,) . 5. A ring trip circuit according to claim 4, wherein each of said first to sixth current mirror circuits (Al to B3) further comprises a third pnp-type transistor (Q302) , a base being connected to said collector of said first transistor, an emitter being connected to said base common connected point of said first and second transistors, and a collector being supplied with a third voltage (GND) . 6. A ring trip circuit according to claim 4 or 5, wherein each of said first to sixth current mirror circuits (Al to B3) further comprises a first resistor (R300) connected to said emitter of said first tran¬ sistor (Q300) and a second resistor (R301) connected to said emitter of said second transistor (Q301) . 7. A ringer trip circuit according to claim 2, wherein said voltage conversion circuit (9) comprises a voltage conversion resistor (R03) , operatively connected to said common connected point, for converting said current from said common connected point to a voltage and functioning as said output terminal of said voltage detecting means. 8. A ring trip circuit according to claim 7, wherein said voltage conversion circuit (9) comprises a capacitor (COO) parallel-connected to said voltage conversion resistor (R03) . 9. A ring trip circuit according to claim 8, wherein said voltage detecting means is formed as a large scaled integrated circuit, except for said parallel connected voltage conversion resistor (R03) and capacitor (COO) . 10. A ring trip circuit according to claim 2, wherein said state detecting means comprises a differential amplifier (OPl) , a first input terminal connected to said output terminal of said voltage detecting means, a voltage limiter (D) connected to said first input terminal of said differential amplifier, a threshold voltage supplying circuit (SV) connected to a second input terminal of said differential amplifier, and a current source circuit (SI) connected to said differential amplifier for supplying a constant current thereto. 11. A ring trip circuit according to claim 10, wherein said differential amplifier (OPl) comprises two npn-type transistors (TR04, TR05) connected in parallel, emitters thereat being commonly connected and connected to said current source circuit, a base of said first transistor (TR04) being said first input terminal, a base of said second transistor (TR05) being said second input terminal, a collector of said first transistor being an output terminal of said differential amplifier, and a collector of said second transistor being supplied with the second voltage (V cc ) , said voltage limiter (D) including at least one transistor (Q325 to Q328) functioning as a level shift diode, said threshold voltage supplying circuit (SV) including two series-connected resistors (R323, R324) supplied with a third voltage (V_. r ) therebetween to supply a threshold voltage (V , ) from a common connected point of said resistors to said second input terminal of said differential amplifier, and said current source circuit (SI) in¬ cluding a current mirror circuit. 12. A ring trip circuit according to claim 11, wherein said state detecting means is formed as a large scale integrated circuit. 13. A ring trip circuit according to claim 2, comprising a first dropping resistor (ROD connected between said one end of said ringer sending resistor (RO) and said third port (P13) of said first current switching circuit (SI) , and a second dropping resistor (R02) connected between said another end of said ringer sending resistor and said third port (P23) of said second current switching circuit (S2) . 14. A ring trip circuit according to claim 13, comprising a filtering capacitor (CO) connected between ends of said first and second dropping resistors (R01, R02) , said ends adjacent to said third ports (P13, P23) of said first and second current switching circuits. 15. A ring trip circuit according to claim 1, wherein said voltage detecting means comprises a current difference detecting cir¬ cuit (7) , operatively connected between said ends of ringer sending resistor (RO) , for detecting a difference (I,b ^ Ia) of currents (Ia , I.b) extracted from one end (a.) and another end (b) of said ringer sending resistor, an absolute calculation circuit (8) , operatively connected to said current difference detect- ing circuit, for calculating an absolute ( |I. - I |) of said current difference, and a voltage conversion circuit (9) , opera¬ tively connected to said absolute calculation, for converting said absolute current to a voltage, output- ting the same, and having an output terminal of said voltage detecting means. 16. A ring trip circuit according to claim 15, wherein said current difference detecting circuit (7) comprises a current inversion circuit (71) , operatively connected to said one end (a) of said ringer sending resistor, for inverting the first current (I 3) extracted from said one end (a_) of said ringer sending resistor, and an addition circuit (72) , operatively connected to said current inversion circuit and said another end (b) of said ringer sending resistor, for adding said inverted first current (-1a) and the second current (I,D) extracted from said another end (b) of said ringer sending resistor (RO) . 17. A ringer trip circuit according to claim 16, wherein said current inversion circuit (71) comprises a first current switching circuit (Sll) operatively connected to one end (a_) of said ringer sending resistor (RO) , and having first to third ports (PA1 to PA3) , said first current switching circuit flowing a first current (Ia) from said second port (PA2) to said third port (PA3) when a voltage (V 3) at said one end (a.) is positive, or flowing the first current from said first port (PAl) to said one end through said third port (A3) when said voltage at said one end is negative, a first current mirror circuit (All) having a first terminal connected to said first port of said first current switching circuit and a second terminal, and a second current mirror circuit (Bll) having a first terminal connected to said second port of said first current switching circuit and a second terminal connected to said second terminal of said first current mirror circuit, and wherein said addition circuit (72) comprises a line operatively connected to said another end (b_) of said ringer sending resistor, and another line connected to said common connected point of said second terminals of said first and second current mirror circuits (All, Bll) , and connected to said line. 18. A ring trip circuit according to claim 17, wherein said first current switching circuit (Sll) comprises an npn-type transistor (TR703) and a pnp-type transistor (TR704) , a collector of said npn-type tran¬ sistor being said first port (PAl) , emitters of said transistors being commonly connected., a common connected point being operatively connected to said one end of said ringer sending resistor, a collector of said npn-type transistor being said second port (PA3) , and bases of said transistors being supplied with a first voltage (V C( ,/2) , and each of said first and second current mirror circuits (All, Bll) comprises first and second npn-type transistors (TR700, TR701; TR706, TR707) , a collector of said first transistor being said first terminal, a collector of said second transistor being said second terminal, and bases of said transistors being commonly connected. 19. A ring trip circuit according to claim 18, wherein each of said first and second current mirror circuits (All, Bll) further comprises a third pnp-type transistor (TR702) , a base being connected to said collector of said first transistor, an emitter being connected to. said base common connected point of said first and second transistors, and a collector being connected to said collector of said first transistor. 20. A ring trip circuit according to claim 18, wherein each of said first and second current mirror circuit (All, Bll) further comprises a first resistor (R711) connected to said emitter of said first tran¬ sistor (TR700) and a second resistor (R712) connected to said emitter of said second transistor (TR701) . 21. A ring trip circuit according to claim 17, wherein said absolute calculation circuit (8) comprises a second current switching circuit (S22) connected to said addition circuit (72) , and having first to third ports (PCI to PC3) , said second current switching circuit flowing a third current , which is a difference current between said first current (I 3) and said second current (1^) extracted from said another end (b) of said ringer sending resistor, from said first port (PCI) to said second port (PC2) when another voltage (V, ) at said another end is positive, or flowing said third current from said second port (PC2) to said third port (PC3) when said another voltage is negative, and a third current mirror circuit (A12) having a first terminal connected to said third port (PC3) of said second current switching circuit (S22) and a second terminal connected to said first port (PCI) of said second current switching circuit (S22) . 22. A ring trip circuit according to claim 21, wherein said second current switching circuit (S22) comprises an npn-type transistor (TR801) and a pnp-type transistor (TR802) , a collector of said npn-type tran¬ sistor being said first port (PCI) , emitters of said transistors being commonly connected, a common connected point being connected to said addition circuit (72) , a collector of said npn-type transistor being said second port (PC2) , bases of said transistors being supplied with said first voltage (V c /2) , and wherein said third current mirror circuit (A12) comprises first and second npn-type transistors (TR804, TR805) , a collector of said first transistor being said first terminal, a collector of said second transistor being said second terminal, bases of said transistors being commonly connected and connected to said collector of said first transistor, and emitters of said transistors being supplied with a second voltage (GND) . 23. A ring trip circuit according to claim 20, wherein said third current mirror circuit (A12) further comprises a third pnp-type transistor (TR803) , a base being connected to said collector of said first tran¬ sistor, an emitter being connected to said base common connected point of said first and second transistors, and a collector being connected to said collector of said first transistor (TR804) . 24. A ring trip circuit according to claim 23, wherein each of said third current mirror circuit (A12) further comprises a first resistor (R801) connected to said emitter of said first transistor (TR804) and a second resistor (R802) connected to said emitter of said second transistor (TR805) . 25. A ring trip circuit according to claim 21, wherein said voltage conversion circuit (9) comprises a fourth current mirror circuit (A13) having a first terminal (PD1) connected to said first port (PCI) of said second current switching circuit (S22) , and a second terminal, and a voltage conversion resistor (R06) connected to said second terminal of said fourth current mirror circuit (A13) and converting a current therefrom to a voltage. 26. A ring trip circuit according to claim 25, wherein said fourth current mirror circuit (A13) com¬ prises first and second npn-type transistors (TR913, TR914) , a collector of said first transistor being said first terminal, a collector of said second transistor being said second terminal, bases of said transistors being commonly connected and connected to said collector of said first transistor, and emitters of said tran¬ sistors being supplied with a third voltage (V pr ,) . 27. A ring trip circuit according to claim 26, wherein said fourth current mirror circuit (A13) further comprises a first resistor (R901) connected to said emitter of said first transistor (TR913) and a second resistor (R902) connected to said emitter of said second transistor (TR914) . ' 28. A ring trip circuit according to claim 25, wherein said voltage conversion circuit (9) comprises a capacitor (C03) parallel connected to said voltage conversion resistor (R06) . 29. A ring trip circuit according to claim 1, comprising scan output means (1) connected to said state detecting means and outputting a digital detection signal in responses to said on-hook state or said off-hook state.

Inventors: IKETANI YOZO, ITO SHIN-ICHI, KINOSHITA KAZUMI, TAKATO KENJI, TOJO TOSHIRO, YAMAMOTO YUZO, IKETANI, YOZO, ITO, SHIN-ICHI, KINOSHITA, KAZUMI, TAKATO, KENJI, TOJO, TOSHIRO, YAMAMOTO, YUZO
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1988
Main classification: H04M19
IPC codes: H04M19