CURRENT MIRROR ARRANGEMENT

Abstract: The invention relates to a current mirror arrangement for monitoring a bidirectional current in a signal path. The arrangement comprises a first semiconductor switch (2) with a control electrode (3) and main current path electrodes (4, 5). The arrangement further comprises a resistor (R1) arranged between the control electrode (3) and the signal path (1), a resistor (R3) arranged between a main current path electrode (4) and the signal path (1), and a resistor (R2) arranged in the signal path (1). The current mirror arrangement further comprises a second semiconductor switch (6) which comprises a control electrode (7) and main current path electrodes (8, 9) connected to the first semiconductor switch (2) in the signal path (1) to implement a bipolar current mirror function irrespective of the direction of the current (Ix) in the signal path (1).

Description: Current mirror arrangement The invention relates to a current mirror arrangement for monitoring a bidirectional current flow- ing in a signal path, said arrangement comprising a first semiconductor switch comprising a control electrode and main current path electrodes, the current of said control electrode being arranged to control the current flowing through said main current path electrodes, a resistor arranged between the control electrode and the signal path, a resistor arranged between a main current path electrode and the signal path, and a resistor arranged in the signal path. A current mirror is a device which generates a certain output current as a function of an input current. A current mirror arrangement can be used in all electronic devices which require bidirectional current mirroring. Current mirroring is needed, for example, in the examination of the current of signalling conductors in analog telephone exchanges. By the use of a current mirror, information on a current flowing in a monitored conductor can be transferred, for example, to a certain resistor from which it is examined, for example by means of a comparator, whether the current flowing in the conductor concerned is greater than a given limit value. Current mirror arrangements of the prior art employ two different circuits or couplings for measuring currents flowing in different directions. The known current mirrors require thus separate arrangements for the measurement of currents flowing in different direc¬ tions. The known solutions comprise, for example, a transistor the base and emitter of which are coupled to the conductor to be measured, i.e. the signal path, by means of resistors. In addition, the signal path is provided with a measuring resistor, and a diode is coupled over the base-emitter junction in order that a current flowing in the other direction would not cause overvoltage in this junction. An alternative solution is wide voltage division which is implemented with resistors and to which a comparator, for example, is coupled. The known solutions have many drawbacks, as they do not render it possible to measure currents flow¬ ing in different directions by the same current mirror arrangement in a signal path, such as a signalling con¬ ductor. The object of the invention is to provide a new type of current mirror arrangement by means of which the problems associated with the known solutions can be avoided. This is achieved with a current mirror arrange¬ ment of the invention which is characterized in that the arrangement further comprises a second semiconductor switch which is known per se and comprises a control electrode and main current path electrodes, that the control electrode of the second semiconductor switch is coupled to that main current path electrode of the first semiconductor switch which is coupled through the resistor to the signal path, that a main current path electrode of the second semiconductor switch is coupled to the control electrode of the first semiconductor switch and through the resistor to the signal path, and that certain other main current path electrodes of each semiconductor switch are coupled together. The current mirror arrangement of the invention is based on the idea that by suitable coupling of the second semiconductor switch it is possible to achieve a so-called bipolar current mirror. The solution according to the present invention has several advantages, as the arrangement of the inven- tion is capable of bipolar measuring, i.e. it can measure a current irrespective of its direction. In addition, the new arrangement does not require protect¬ ing diodes. In the following, the invention will be described in greater detail with reference to the accom¬ panying drawing, in which Figure 1 illustrates a current mirror arrange¬ ment of the invention. In the case of Figure 1, a current mirror arrangement is disposed in connection with a monitored signal path 1. The signal path may be, for example, a DC signalling conductor in an analog telephone exchange. For the monitoring of a bidirectional current I-- in the signal path 1, the current mirror arrangement comprises a first semiconductor switch 2 comprising a control electrode 3 and main current path electrodes 4 and 5. The switch is preferably an NPN semiconductor tran¬ sistor, as in the figure, or a PNP semiconductor tran- sistor. The control electrode 3 refers to a base B, and the main current path electrodes 4 and 5 refer to an emitter E and a collector C. As is characteristic of transistors, the current of the control electrode 3 controls the current, flowing through the main current path electrodes 4 and 5, i.e. the measuring current I m in the figure, which is a function of the current I x of the signal path. The arrangement further comprises a resistor Rl coupled between the control electrode 3 of the first transistor, or semiconductor switch 2, and the signal path 1. In addition, the arrangement comprises a resistor R3 coupled between a main current path electrode, or emitter 4, and the signal path 1, and a resistor R2, or measuring resistor, coupled in the signal path 1. The current mirror arrangement further com¬ prises a second semiconductor switch 6 which is known per se and which is preferably a transistor as the first semiconductor switch 2. The second semiconductor switch, or transistor, naturally comprises a control electrode 7, or a base B, and main current path electrodes 8 and 9, or an emitter E and a collector C. According to the invention, the control electrode 7 of the second semi¬ conductor switch 6 is coupled to that main current path electrode 4, or emitter, of the first semiconductor switch 2 which is coupled through a resistor R3 to the signal path 1. Further, a main current path electrode, or emitter 8, of the second semiconductor switch 5 is coupled to the control electrode 3, or base B, of the first semiconductor switch 2, and via the resistor Rl to the signal path. In addition, certain other main current path electrodes 5 and 9, i.e. collectors C in practice, of each semiconductor switch 2 and 6 are coupled together. The aim of the resistor R2, or measuring resistor, is to provide a difference of voltage over the resistor, by means of which voltage difference a current is generated to either one of the transistors 2 and 6, depending on the direction of the current I x in the signal path. Let us first examine a case where the direction of the current I x is from point (a) to point (b), i.e. from left to right. In this case, the current I x causes a voltage drop in R2, which leads to a voltage difference between (a) and (b), i.e. (a) becomes more positive. When the voltage U(R2) of the measuring resistor R2 exceeds the base-emitter voltage of the transistor 2, a control voltage/current is generated from point (a) through the resistor Rl to the base B, or control electrode 3, of the first transistor, or semiconductor switch 2. The control current flows through the base-emitter junction of the transistor 2 to R3 and further to point (b). The second transistor 6 is biased to the reverse direction. The collectors of the transistors are coupled together in a collector line, or measuring current line J. The measuring current line comprises a resistor R4. A measuring current I n is generated by a voltage source U(s) when the transistor 2 switches on the current as it receives a control signal to its control electrode, or base B. The measuring current line J, or collector line, conducts a current which provides an equilibrium. The ratio between the resistors R2 and R3 determines the current transfer coefficient between the currents I x and I B . The voltage U(s) is higher than the voltage U(x) of the signal path when NPN transistors are concerned and vice versa when PNP transistors are concerned. In a preferred embodiment of the invention, the resistor R2 in the signal path 1 and the resistor Rl which connects the main current path electrode 8, or emitter E, of the second semiconductor switch 6 to the signal path 1 are arranged to connect the current I m in the measuring current line J, or collector line, common to the semiconductor switches 2 and 6 through the main current electrodes 4 and 5 of the first semiconductor switch 2. If the current in the signal line 1 flows to the opposite direction, whereby (b) is more positive, the transistor 6 receives the control current from point (b) via R3 to its control electrode 7, or base B, where- by the collector line, or measuring current line J, is coupled. In this preferred embodiment, the main current electrodes 5 and 9, now collectors C, of the semicon¬ ductor switches, or transistors 2 and 6, are coupled together in the measuring current line J. In this case, the resistor R2 in the signal path 1 and the resistor R3 which connects the main current path electrode 4, or emitter E, of the first semiconductor switch 2 to the signal path 1 are arranged to connect the current I m in the measuring current line J by means of the control electrode 7 of the second semiconductor switch through the main current electrodes 8 and 9 of the second semi¬ conductor switch. In the preferred embodiment of the invention, the semiconductor switches 2 and 6 are substantially analogous, and the resistances of the resistors Rl and R3 which connect the main current electrodes 4 and 8 of the semiconductor switches 2 and 6 to the signal path 1 are substantially of the same magnitude. This embodi¬ ment simplifies the coupling. It is .essential in the solution of the inven¬ tion that in the signal path 1, e.g. in a signalling circuit, the same current I ra can be used to express the current I x irrespective of its direction. It is essen¬ tial to the operation that the current flowing in the measuring resistor R2 causes a voltage drop between the resistors Rl and R3. Depending on the direction of the current I x , the voltage U(R2) of the measuring resistor R2 exceeds the base-emitter voltage of either one of the transistors, whereby the measuring current I n of the collector line flows through this transistor. Although the invention has been described above with reference to the examples according to the accom¬ panying drawing, it will be clear that the invention is not restricted to them but can be modified in many ways within the inventive concept disclosed in the appended claims.

Claims: Claims 1. A current mirror arrangement for monitoring a bidirectional current flowing in a signal path ( 1 ) , said arrangement comprising - a first semiconductor switch (2) comprising a control electrode (3 ) and main current path electrodes (4, 5 ), the current of said control electrode (3) being arranged to control the current (I B ) flowing through said main current path electrodes (4, 5), - a resistor (Rl) arranged between the control electrode (3) and the signal path (1 ) , - a resistor (R3) arranged between a main current path electrode (4) and the signal path ( 1 ) , and - a resistor (R2) arranged in the signal path (1), c h a r a c t e r i z e d in that the current mirror arrangement further comprises a second semiconductor switch ( 6 ) which is known per se and comprises a control electrode (7 ) and main current path electrodes (8, 9), that the control electrode (7 ) of the second semicon¬ ductor switch ( 6 ) is coupled to that main current path electrode (4) of the first semiconductor switch ( 2) which is coupled through the resistor (R3) to the signal path (1), that a main current path electrode ( 8 ) of the second semiconductor switch ( 6 ) is coupled to the control electrode (3 ) of the first semiconductor switch (2 ) and through the resistor (Rl) to the signal path (1), and that certain other main current path electrodes (5, 9) of each semiconductor switch (2, 6) are coupled together. 2. A current mirror arrangement according to claim 1, c h a r a c t e r i z e d in that said other main current path electrodes (5, 9) of the semiconductor switches (2, 6) are coupled together in a measuring current line ( J) , and that the resistor (R2 ) in the signal path 1 and the resistor (R3) which connects a main current path electrode (4) of the first semicon- ductor switch (2 ) to the signal path (1 ) are arranged to connect the current ( I m ) in the measuring current line (J) through the main current path electrodes (8, 9 ) of the second semiconductor switch ( 6) . 3. A current mirror arrangement according to claim 2, c h a r a c t e r i z e d in that the resistor (R2) in the signal path (1) and the resistor (Rl ) which connects the main current path electrode ( 8 ) of the second semiconductor switch (6) to the signal path (1) are arranged to connect the current ( I B ) in the measuring current line (J) common to the semiconductor switches (2, 6) by means of the control electrode (3) of the first semiconductor switch (2) through the main current path electrodes (4, 5) of the first semicon¬ ductor switch (2) . 4. A current mirror arrangement according to any one of the preceding claims, c h a r a c t e r ¬ i z e d in that the semiconductor switches (2, 6) are substantially analogous, and that the resistances of the resistors (R3, Rl ) which connect certain main current electrodes (4, 8) of the semiconductor switches (2, 6) to the signal path ( 1 ) are substantially of the same magnitude.

Inventors: LOUKUSA JARMO, VARANKA KAUKO, LOUKUSA, JARMO, VARANKA, KAUKO
Applicants: LOUKUSA JARMO, NOKIA TELECOMMUNICATIONS OY, VARANKA KAUKO, LOUKUSA, JARMO, VARANKA, KAUKO
Application year: 1993
Main classification: H03F3
IPC codes: H03F3