Current detector circuits

Abstract: A current detector circuit comprises a first detecting object (21); a first current detector (22) for detecting a current flowing through said first detecting object (21); a second detecting object (23); a second current detector (24) for detecting a current flowing through said second detecting object (23); and a monitor (25) including a first transistor (Tr1) having a control terminal connected to an output of said first current detector (22) and a second transistor (Tr2) having a second terminal connected to a first terminal of said first transistor (Trl) and a control terminal connected to an output of said second current detector (24), said first and second transistors (Tr1, Tr2) having identical characteristics. Such a current detector circuit has at most two transistors in series between the power supply lines, and uses no gate-drain-connected transistors. Thus, the circuit is capable of operating with a low-voltage power supply and with reduced power consumption. The circuit can be used in an amplifier to control a through current in an output section, and in a majority circuit for detecting signal transmission faults.

Description: The present invention relates to current detector circuits and to other circuitry such as amplifiers employing such current detector circuits. Recently, compact and low-power-consumption electronic apparatuses have been developed. Portable electronic devices are mostly driven by batteries. Home-use and office-use electronic apparatuses driven by, for example, AC 100 (117, 200 or 220) V usually employ semiconductor integrated circuits (LSIs) operating with a power source voltage of 5V. The portable devices driven by battery, however, usually operate with a power source voltage of 3V. The battery voltage decreases as the battery wears. LSIs in the portable devices must deal with such a fluctuation in the source voltage and a variety of source voltages ranging from 3 to 5V. Namely, the LSIs must be stable for a wide range of source voltages. To enable an analog circuit to operate over a wide range of source voltages, an operational amplifier plays an important role. An operational amplifier whose output section is constituted by a push-pull circuit may be used for heavy load or a high-speed operation in an analog-digital mixed signal LSI consisting of CMOSs. In this case, the operational amplifier must be particularly stable. It is required to provide an operational amplifier that shows stable performance for a wide range of source voltages. By the way, low-frequency amplifiers are classified into classes A, B, and AB. The A-class amplifiers involve a low signal distortion but large power consumption. The B-class amplifiers involve a large signal distortion but small power consumption. The AB-class amplifiers involve a small signal distortion and small power consumption. Amplifiers for driving speakers must provide a large current at low distortion. For this purpose, the A-class amplifiers consume large power even when there are no signals, and the B-class amplifiers cause a problem of crossover. Accordingly, the AB-class amplifiers are usually employed to drive speakers. To reduce power consumption, low-voltage power sourcesare widely used. It is necessary, therefore, to develop amplifiers capable of operating with a low-voltage power source. It is also desirable to provide a current detector circuit which can select the smaller of respective current values of currents flowing through two different detecting objects (detection devices) and which can operate with a low-voltage power source. The circuit may also control the currents to have predetermined values. Such a circuit may be employed in an amplifier which is required to operate with a low-voltage power source. An embodiment of a first aspect of the present invention can provide a current detector circuit comprising a first detecting object; a first current detector for detecting a current flowing through the first detecting object; a second detecting object; a second current detector for detecting a current flowing through the second detecting object; and a monitor including a first transistor having a control terminal connected to an output of the first current detector and a second transistor having a second terminal connected to a first terminal of the first transistor and a control terminal connected to an output of the second current detector, the first and second transistors having identical characteristics. Such a current detector circuit can select a smaller one of the two current values flowing through the first and second detecting objects. The circuit can be used to maintain the current flowing through the detecting objects at a predetermined value. An amplifier employing such a current detector circuit is capable of operating with a low source voltage. The current detector circuit may further comprise a first current source connected to a second terminal of the first transistor; and a feedback circuit for fixing currents flowing through the first and second detecting objects in response to a signal from the second terminal of the first transistor. The first detecting object may have a third transistor. The first current detector may include a fourth transistor having a control terminal connected to a control terminal of the third transistor; and a fifth transistor having second and control terminals connected to a second terminal of the fourth transistor and to the control terminal of the first transistor. The monitor may include a sixth transistor having a second terminal connected to the second terminal of the first transistor and a control terminal connected to the control terminal of the second transistor; and a seventh transistor having a control terminal connected to the control terminal of the first transistor and a second terminal connected to a first terminal of the sixth transistor. The first, second, fifth, sixth and seventh transistors may be made of a first conductivity type, and the third and fourth transistors are made of a second conductivity type. The first terminal may be a source, the second terminal may be a drain, and the control terminal may be a gate. The first terminal may be an emitter, the second terminal may be a collector, and the control terminal may be a base. The current detector circuit may constitute an operational amplifier, or a semiconductor integrated circuit. An embodiment of a second aspect of the present invention can provide an operational amplifier including a current detector embodying the first aspect of the invention, wherein said first and second detecting objects each comprise a drive transistor of an output section of the amplifier. An embodiment of a third aspect of the present invention can provide a majority circuit, for detecting faults in transmission of a plurality of digital signals each having the same signal level, including: a current detector circuit embodying the first aspect of the invention; a primary current source connected for causing a preselected primary current to flow through a first node of the majority circuit; a secondary current source connected for causing a preselected secondary current, weak relative to said preselected primary current, to flow along a current path between said first node and a second node of the majority circuit; and a plurality of switching transistors, each having a first terminal connected to said first node and a second terminal connected to said second node and a control terminal connected for receiving one of said digital signals, each said switching transistor being switchable by its received digital signal from an OFF state, in which no current flows therethrough, to an ON state in which a preselected switch current flows therethrough, said primary current being n times each said switch current, where n is the number of switching transistors of the said plurality; wherein: said first current detector is connected to said first node for detecting a difference between said primary current and a sum of said secondary current and any said switch currents that are flowing; and said second current detector is connected to said second node for detecting said sum; whereby a current flowing through said monitor is determined by a lower one of said detected difference and said detected sum. Reference will now be made, by way of example, to the accompanying drawings, in which Fig. 1 is a circuit diagram showing a previously-considered AB-class amplifier; Figs. 2A, 2B and 2C are diagrams for explaining the operation of the amplifier shown in Fig. 1 in which Fig. 2A is a table showing the input and output of a differential amplifier section of the amplifier, Fig. 2B shows differential inputs V+ and V- and currents at different parts, and Fig.2C shows differential inputs V+ and V- and currents I1 and I2 at different parts; Figs. 3A and 3B show another previously-considered AB-class amplifier, in which Fig. 3A is a circuit diagram generally showing the amplifier, and Fig. 3B is a circuit diagram showing a feedback system (current detector circuit) of the amplifier; Figs.4A and 4B are diagrams for explaining a principle of a current detector circuit embodying a first aspect of the present invention, in which Fig. 4A is a general view and Fig. 4B is a circuit diagram showing a modification of a monitor; Fig. 5 is a diagram for explaining another principle of a current detector circuit embodying the first aspect of the present invention; Fig. 6 is a diagram for explaining a still another principle of a current detector circuit embodying the first aspect of the present invention; Fig. 7 is a diagram for explaining a still another principle of a current detector circuit embodying the first aspect of the present invention; Figs. 8A and 8B show an AB-class amplifier according to a first embodiment of a second aspect of the present invention, in which Fig. 8A is a circuit diagram generally showing the amplifier, and Fig. 8B is a circuit diagram showing a current detector circuit of the amplifier; Fig. 9 is a diagram for explaining the operation of the first embodiment and a relationship between input voltages to a monitor and a through current; Fig. 10 is a circuit diagram showing a current detector circuit of an AB-class amplifier according to a second embodiment of the second aspect of the present invention; Fig. 11 is a circuit diagram showing parts of a circuit according to a third aspect of the present invention; and Figs. 12A and 12B are diagrams for explaining the operation of the Fig. 11 embodiment, in which Fig. 12A shows current values at different parts under a normal operation, and Fig.12B shows current values at the parts when a digital input signal Dk fails. First, problems of previously-considered current detector circuits will be explained. Fig. 1 shows a first previously-considered AB-class amplifier. The amplifier has a differential amplifier 11, constant current sources Irll and Ir12 each for supplying a current Iref, P-channel transistors Tr10 and Tr12, and N-channel transistors Tr11 and Tr13. Figures 2A, 28 and 2C are diagrams for explaining the operation of the amplifier shown in Fig. 1. Figure 2A is a table showing input and output levels V+ and V- and converted currents I1 and 12 of the differential amplifier 11. In the table, a reference mark G is the gain of the differential amplifier, and "*" is a multiplication operator used through this specification and accompanying drawings, in the following descriptions. Figure 2B shows relationships between the differential inputs V+ and V- and currents at different parts including a current Ip flowing through the P-channel transistor Tr10, a current In flowing through the N-channel transistor Tr11, and a weak through current (weak flowing current) It flowing to an output section of the amplifier. The through current It is the smaller of the currents Ip and In. Figure 2C shows relationships between the differential inputs V+ and V- and the currents I1 and I2. The through current It is necessary to cancel a crossover distortion. The through current must be as small as possible because it increases power consumption. The Fig. 1 amplifier is advantageous because it is simple. It, however, has a problem of large power consumption to keep the through current It above 0 because the through current It varies in response to a change in the characteristics of the transistors. To solve this problem, another AB-class amplifier, as shown in Figs. 3A and 3B, has been considered. Figure 3A is a circuit diagram generally showing the amplifier and Fig. 3B is a circuit diagram showing a feedback system (a current detector circuit) in the amplifier. In Fig. 3A, the amplifier has a differential amplifier 11, the current detector circuit 12, P-channel transistors Tr10 and Tr12, and N-channel transistors Tr11 and Tr13. In Fig. 3B, the current detector circuit 12 has a constant current source Ir1 for supplying a current Iref, P-channel transistors Tr2, Tr3, Tr8, Tr9, and Tr22, and N-channel transistors Tr1, Tr4, Tr6, Tr7, and Tr21. The feedback system (current detector circuit) carries out a negative feedback operation to supply a constant through current (constant flowing current) It to an output section (transistors Tr10 and Tr11) in the amplifier. The operation of the AB-class amplifier of Figs. 3A and 3B will be explained. The size of a transistor is defined as follows: Tr2/Tr1 = X Namely, the ratio of the size of a transistor Tr1 to that of a transistor Tr2 is X. The drain current of a transistor is proportional to the size of the transistor. When the transistors Tr1 and Tr2 are connected to each other in a current mirror configuration, a current flowing through the transistor Tr2 is X times a current flowing through the transistor Tr1. (i) Input Relationships between differential inputs V+ and V- and currents I1 and I2 are the same as those of the first prior art of Fig. 2A. (ii) Current detection The current detector circuit of Fig. 3B detects the through current It by the transistors Tr1 to Tr4, Tr21, and Tr22. The detecting operation will be explained. The sizes of the transistors are as follows: Tr10/Tr3 = Tr11/Tr21 = S Tr4/Tr1 = Tr22/Tr2 = T A current Ie is as follows: Ie = (1/S) * It Namely, the current Ie is proportional to the through current It. The transistors Tr4 and Tr22 operate in a saturation region, so that the following expressions are established: I(Tr4) = β (Vgl - Vp - Vthn) 2 /2 I(Tr22) = β (Vp - Vg2 - |Vthp|) 2 /2 I(Tr4) = I(Tr22) where Vp is a voltage at a node P, Vg1 and Vg2 are the gate voltages of the transistors Tr1 and Tr2, respectively, and Vthn and Vthp are the threshold voltages of N- and P-channel transistors, respectively. Accordingly, the voltages Vg1, Vg2, and Vp are uniquely determined according to the expressions (4) to (6). At this time, the following is established: Vp = Vq where Vq is a voltage at a node Q. This is because the expressions (4) to (6) are applicable to each of the transistors Tr1 and Tr2 and the voltages Vg1 and Vg2 are common to the transistors. I(Tr1) = I(Tr2) = (1/T) * Ie According to the expressions (3) and (8), the following is established: I(Tr1) = I(Tr2) = (1/S * T) * It Namely, the transistors Tr1 and Tr2 detect the through current It. (iii) General operation As explained in (2), a current flowing through the transistors Tr1 and Tr2 is (1/S * T) * It, so that the current follows the through current (flowing current) It. If the current It decreases due to some reason, a voltage Vr at a node R increases. This is because the current flowing through the transistors Tr1 and Tr2 is kept constant by the constant current source Ir1 connected in series. When the voltage Vr increases, currents flowing through the transistors Tr7 and Tr9 increase to change a voltage Va at a node A and a voltage Vb at a node B, thereby increasing the current Ie and through current It. Namely, a negative feedback operation is carried out to keep the through current It constant. The through current It is constant as follows: It = S * T * Iref The through current It is controllable by changing the ratio of the sizes of transistors. The AB-class amplifier of Figs. 3A and 3B always makes the through current It constant. The AB-class amplifier of Figs. 3A and 3B prevents a fluctuation in the through current It by negative feedback. The transistors Tr4 and Tr22 that form the circuit for detecting the through current It involve voltage drops corresponding to the threshold voltages vthn and Vthp. If these threshold voltages are each about 1V, they will cause a voltage drop of 2V. Accordingly, the Figs. 3A and 3B amplifier is incapable of operating with a low-voltage power source. Below, examples of a current detector circuit according to the present invention will be explained with reference to the accompanying drawings. Figures 4A, 4B, 5, 6 and 7 show principles of a first aspect of the present invention. To solve the problems described above, a current detector circuit embodying a first aspect of the present invention has, as shown in Fig. 4A, a first detecting object (detecting device) 21; a first current detector 22 for detecting a current flowing through the first detecting object 21; a second detecting object 23; a second current detector 24 for detecting a current flowing through the second detecting object 23; and a monitor 25. The monitor 25 includes a first transistor Tr1 having a control terminal connected to the output of the first current detector 22 and a second transistor Tr2 having a second terminal connected to a first terminal of the first transistor Tr1 and a control terminal connected to the output of the second current detector 24. The first and second transistors Tr1 and Tr2 have identical characteristics. The first detecting object of the current detector circuit preferably has a third transistor Tr10 as shown in Fig. 5. The first current detector 22 of the current detector circuit preferably has a fourth transistor Tr3 having a control terminal connected to a control terminal of the third transistor Tr10; and a fifth transistor Tr4 having second and control terminals connected to a second terminal of the fourth transistor Tr3 and to a control terminal of the first transistor Tr1, as shown in Fig. 6. The current detector circuit further preferably has a first current source Ir1 connected to a second terminal of the first transistor Tr1; and a feedback circuit 26 for fixing currents flowing through the first and second detecting objects 21 and 23 in response to a signal from the second terminal of the first transistor Tr1, as shown in Fig. 7. The monitor 25 of the current detector circuit preferably has a sixth transistor Tr2' having a second terminal connected to the second terminal of the first transistor Tr1 and a control terminal connected to the control terminal of the second transistor Tr2; and a seventh transistor Tr1' having a control terminal connected to the control terminal of the first transistor Tr1 and a second terminal connected to the first terminal of the sixth transistor Tr2', as shown in Fig. 4B. Preferably the first, second, fifth, sixth, and seventh transistors are of a first conductivity type, and the third and fourth transistors are of a second conductivity type. Each second terminal is preferably a drain or collector electrode, each control terminal a gate or base electrode, and each first terminal is a source or emitter electrode. An embodiment of a second aspect of the present invention can provide an amplifier employing the above current detector circuit. In the following explanation, the first, second, fifth, sixth, and seventh transistors Tr1, Tr2, Tr4, Tr2', and Tr1' are first conductivity type (N-channel) transistors, and the third and fourth transistors Tr10 and Tr3 are second conductivity type (P-channel) transistors. Accordingly, the second terminal is a drain electrode, the control terminal is a gate electrode, and the first terminal is a source electrode. In the current detector circuits shown in Figs. 4A to 7, the first current detector 22 detects a current flowing through the first detecting object 21, and the second current detector 24 detects a current flowing through the second detecting object 23. The monitor 25 generates a current in response to the currents detected by the first and second current detectors 22 and 24. The monitor 25 consists of the first and second transistors Tr1 and Tr2. When these transistors Tr1 and Tr2 have the same characteristics, the current flowing through the transistors Tr1 and Tr2 is determined according to a lower one of the input voltages to the transistors Tr1 and Tr2. When the input voltage Vg1 to the first transistor Tr1 is equal to the input voltage Vg2 to the second transistor Tr2, the transistors are equivalent to a transistor of half driving capacity. This will be explained. 1) When Vg1 = Vg2 = Vg The first transistor Tr1 operates in a saturation region, and therefore, the following is established: Ix = β (Vg - VA - Vth) 2 /2 The second transistor Tr2 operates in a triode region, and therefore, the following is established: Ix = β {2(Vg - Vth)VA - VA 2 }/2 According to the expressions (11) and (12), the following is established: Ix = β (Vg - Vth) 2 /4 2) When Vg1 > Vg2 The first transistor Tr1 operates in a triode region and the second transistor Tr2 operates in a saturation region, so that the following is established: Ix = β (Vg2 - Vth) 2 /2 3) When Vg1 < Vg2 The first transistor Tr1 operates in a saturation region and, therefore, the following is established: Ix = β (Vg1 - VA - Vth) 2 /2 Since the second transistor Tr2 operates in a triode region, the following is established: Ix = β {2(Vg2 - Vth)VA - VA 2 }/2 According to the expressions (15) and (16), the following is obtained: VA = [(Vf2 + Vf1) - {(Vf2 + Vf1) 2 - 2Vf1 2 } 1/2] 1/2 Accordingly, the following is established: Ix = β [Vf2 2 + (Vf2 - Vf1) {(Vf2 + Vf1) 2 - 2Vf1 2 } 1/2 ]/4 According to the expression (17), the following is obtained: 3 VA/ 3 Vf2 = [1 - (Vf2 + Vf1)/(Vf2 + Vf1) 2 - 2Vf1 2 } 1/2 ]/2 where Vf1 = Vg1 - Vth, and Vf2 = Vg2 - Vth When Vf1 is about 0, the following is obtained: VA ≒ 0, and ∂VA/∂Vf2 ≒ 0 At this time, the gate voltage Vg1 of the first transistor Tr1 controls the current Ix. According to the explanations in (1), (2), and (3) above, the current Ix is determined according to a lower one of the gate voltages of the first and second transistors Tr1 and Tr2. The Figs. 3A and 3B current detector circuit connects four transistors at the maximum in series between power sources. Among the transistors, two are gate-drain-connected transistors to cause at least a voltage drop of 2Vth. On the other hand, a current detector circuit embodying the present invention connects two transistors at the maximum (most) in series with no gate-drain-connected transistors. A current detector circuit embodying the present invention, therefore, is capable of operating with a low-voltage power source and with reduced power consumption. According to the current detector circuit of Fig. 4B, the monitor 25 has the first transistor Tr1, second transistor Tr2, sixth transistor Tr2', and seventh transistor Tr1'. This arrangement reduces fluctuations in a current flowing through the monitor when Vg1 > Vg2 and when Vg1 < Vg2. An amplifier employing a current detector circuit embodying the present invention is capable of operating with a low-voltage power source and with reduced power consumption. One such amplifier, employing a current detector circuit embodying the first aspect of the present invention, has a feedback circuit 26 responding to a signal from the second terminal (drain electrode) of the first transistor Tr1. The feedback circuit 26 carries out negative feedback on the first and second current detecting objects 21 and 23, to control the current flowing through the first and second detecting objects 21 and 23 to a constant value. This amplifier is of class AB and is capable of suppressing a fluctuation in a through current passing through the output section of the amplifier. Figures 8A and 8B show an AB-class amplifier according to a first embodiment of the present invention. Figure 8A is a circuit diagram generally showing the amplifier, and Fig. 8B is a circuit diagram showing a current detector circuit of the amplifier. The same parts as those of the second example of a prior art amplifier of Figs. 3A and 3B are represented with like reference marks. In Fig. 8A, the AB-class amplifier of this embodiment has a differential amplifier 11, the current detector circuit 28, P-channel transistors Tr10 and Tr12, and N-channel transistors Tr11 and Tr13. In Fig. 8B, the current detector circuit 28 has a first constant current source Ir1 for supplying a current Iref, P-channel transistors Tr3, Tr8, and Tr9, and N-channel transistors Tr1, Tr2, Tr4, Tr6, and Tr7. The current detector circuit 28 (feedback system) carries out a negative feedback operation to let a constant through current It flow to an output section involving the transistors Tr10 and Tr11 of the amplifier. The operation of the amplifier is the same as that of Figs. 3A and 3B except for the current detector circuit 28 for detecting the through current It. The P-channel transistor Tr10 is a first detecting object 21, and the N-channel transistor Tr11 is a second detecting object 23. The P-channel transistor Tr3 and N-channel transistor Tr4 form a first current detector 22. The N-channel transistors Tr1 and Tr2 form a monitor 25. Signal lines to the gate electrodes of the transistors Tr1 and Tr2 form a second current detector 24. The sizes of the transistors Tr1, Tr2, Tr3, and Tr4 are as follows: Tr11/Tr1 = Tr11/Tr2 = S Tr10/Tr3 = Tr11/Tr4 To satisfy the relational expression (22), output currents Ip and In and the input voltages Vg1 and Vg2 of the monitor 25 are as follows: when Ip = In, Vg1 = Vg2 when Ip > In, Vg1 > Vg2 when Ip < In, Vg1 < Vg2 Accordingly, the through currant It will be as shown in Fig. 9. In the figure, Iref is a current value provided by the first constant current source Ir1. The through current It changes in the range of S * Iref to 2 * S * Iref. By properly setting the current Iref and the ratio S of the transistor sizes, the AB-class amplifier is realized. When "Ip > In" changes to "Ip < In," a change in the through current It is advantageous because the through current It is large. The current detector circuit of Fig. 3B (not embodying the present invention) arranges the four transistors Tr3, Tr4, Tr21, and Tr22 in series between a power source Vcc and a ground GND. Two of these transistors are gate-drain-connected transistors. On the other hand, the current detector circuit 28 of the first embodiment arranges the two transistors Tr3 and Tr4 in series with no gate-drain-connected transistors. Accordingly, the amplifier of this embodiment is capable of operating with a low-voltage power source. A current flowing through the transistors Tr1 and Tr2 in the monitor 25 of Fig. 8B fluctuates between Vg1 > Vg2 and Vg1 < Vg2. Accordingly, the monitor 25 may be made up of N-channel transistors Tr1, Tr2, Tr2', and Tr1' as shown in Fig. 11B, to improve symmetry and flatten fluctuations in a current flowing through the monitor. Figure 10 shows a current detector circuit of an AB-class amplifier according to a second embodiment of the present invention. The general arrangement of this amplifier is the same as the amplifier of Fig. 8A. The current detector circuit of this embodiment consists of the current detector circuit 28 of the first embodiment of Fig. 8B plus a phase compensation circuit. According to the current detector circuit 28 of the first embodiment, a high-frequency signal provided to the node A may be shifted by 180 degrees (inverted) when it is passed through the transistors Tr3, Tr4, Tr1, and Tr7 to the node A (Tr3 - Tr4 - Tr1 - Tr7 - A), leading to positive feedback. This may cause oscillation. To prevent such high-frequency oscillation, the second embodiment employs a phase compensation circuit consisting of resistors R1 and R2 and capacitors C1 and C2. The modification of Fig. 4B of the monitor 25 is also applicable to the second embodiment. Figure 11 is a circuit diagram showing a majority circuit according to a third aspect of the present invention. The majority circuit sends a digital signal through several routes. Even if one of the routes inverts the signal due to a malfunction, a majority of the routes determine the output of the circuit. This circuit is highly reliable, and therefore, is adopted for processing, for example, railroad signals that must be reliable. The majority circuit has a second (primary) current source Ir2, a third (secondary) current source Ir3, N-channel transistors Til to Tin, TrA, TrB, Tr1, and Tr2. The transistor TrB forms a first current detector 22, the transistors (switching transistors) Ti1 to Tin and TrA form a second current detector 24, and the transistors Tr1 and Tr2 form a monitor 25. Digital input signals D1 to Dn of the same signal level (high or low) are supplied to the gate electrodes of the transistors Ti1 to Tin, respectively. The transistors Ti1 to Tin have identical characteristics. When a digital input signal Dk (k is one of 1 to n) is at high level, a transistor Tik passes a current (switch current) of I1 A. The second current source Ir2 provides a current of n * I1 A, and the third current source Ir3 provides a weak current of 12 A. When the digital input signals D1 to Dn are correct, i.e., when they are each at high or low level, one of the transistors TrA and TrB is cut off and no current flows to the monitor 25 (when D1 to Dn are L the weak current I2 is insufficient to cause the gate of transistor TrA to exceed a threshold voltage at which transistor Tr2 turns on). Accordingly, a current I (TrA) to the transistor TrA, a current I (TrB) to the transistor TrB, and a current Iout to the monitor 25 are as shown in Fig. 12A. If a digital input signal Dk (k is one of 1 to n) is inverted due to a fault, a current of I1 ± I2 A, i.e. approximately I1 A, flows to one of the transistors TrA and TrB and a current of S * I1 A proportional to the current I1 A flows to the monitor 25, as shown in Fig. 12B. As a result, the fault is detectable. Here, S is a transfer ratio. In theory, the majority circuit of this embodiment is capable of detecting a plurality of faults. In practice, however, the number of detectable faults is not so large due to the characteristics of the current detector circuit. The modification of the monitor 25 shown in Fig. 4B is also applicable to this embodiment. Although the first to third embodiments employ MOS transistors, the present invention may employ bipolar transistors constituting TTLs. The above described current detector circuits can be constituted as operational amplifiers, or as semiconductor integrated circuits. In a current detector circuit embodying the the present invention, the first current detector detects a current flowing through the first detecting object, and the second current detector detects a current flowing through the second detecting object. The outputs of the first and second current detectors are supplied to the first and second transistors, with the identical characteristics, of the monitor, respectively. The current flowing through the first and second transistors is determined by the lower one of the input voltages to the first and second transistors. Accordingly, the current detector circuit is realized by the two transistors at the maximum connected in series. This arrangement is capable of operating with a low-voltage power source and of reducing the power consumption. When the monitor consists of the first, second, sixth, and seventh transistors, fluctuations in the current flowing through the monitor are reduced. An amplifier embodying another aspect of the present invention employs any one of the current detector circuits embodying the first aspect of the present invention. Such an amplifier is capable of operating with a low-voltage power source and reduced power consumption. In one embodiment an amplifier employing the current detector circuit of the first aspect of the present invention includes a feedback circuit that receives an input signal from the second terminal (drain electrode) of the first transistor. The feedback circuit carries out a feedback operation on the first and second detectors, to control a current flowing through the first and second detectors. This AB-class amplifier is capable of suppressing fluctuations in a through current flowing to the output section of the amplifier. It should be understood that the present invention is not limited to the specific embodiments described in this specification, and many different embodiments of the present invention may be constructed without departing from the scope of the present invention as defined in the appended claims.

Claims: A current detector circuit comprising: a first detecting object (21); a first current detector (22) for detecting a current flowing through said first detecting object (21); a second detecting object (23); a second current detector (24) for detecting a current flowing through said second detecting object (23); and a monitor (25) including a first transistor (Tr1) having a control terminal connected to an output of said first current detector (22) and a second transistor (Tr2) having a second terminal connected to a first terminal of said first transistor (Tr1) and a control terminal connected to an output of said second current detector (24), said first and second transistors (Tr1, Tr2) having identical characteristics. A current detector circuit as claimed in claim 1, wherein said first detecting object (21) has a third transistor (Tr10). A current detector circuit as claimed in claim 2, wherein said first current detector (22) includes: a fourth transistor (Tr3) having a control terminal connected to a control terminal of said third transistor (Tr10); and a fifth transistor (Tr4) having second and control terminals connected to a second terminal of said fourth transistor (Tr3) and to the control terminal of said first transistor (Tr1). A current detector circuit as claimed in any one of claims 1 to 3, wherein said current detector circuit further comprises: a first current source (Ir1) connected to a second terminal of said first transistor (Tr1); and a feedback circuit (26) for controlling currents flowing through said first and second detecting objects (21, 23) in response to a signal from the second terminal of said first transistor (Tr1). A current detector circuit as claimed in any one of claims 1 to 4, wherein said monitor (25) includes: a sixth transistor (Tr2') having a second terminal connected to the second terminal of said first transistor (Tr1) and a control terminal connected to the control terminal of said second transistor (Tr2); and a seventh transistor (Tr1') having a control terminal connected to the control terminal of said first transistor (Tr1) and a second terminal connected to a first terminal of said sixth transistor (Tr2'). A current detector circuit as claimed in any one of claims 1 to 5, wherein said first, second, fifth, sixth, and seventh transistors (Tr1, Tr2, Tr4, Tr2', Tr1') have a first conductivity type, and said third and fourth transistors (Tr10, Tr3) have a second conductivity type. A current detector circuit as claimed in any one of claims 1 to 6, wherein said first terminal is a source, said second terminal is a drain, and said control terminal is a gate. A current detector circuit as claimed in any one of claims 1 to 6, wherein said first terminal is an emitter, said second terminal is a collector, and said control terminal is a base. A current detector circuit as claimed in any one of claims 1 to 8, wherein said current detector circuit constitutes an operational amplifier. A current detector circuit as claimed in any one of claims 1 to 9, wherein said current detector circuit constitutes a semiconductor integrated circuit. An amplifier including a current detector circuit as claimed in claim 4 or in any one of claims 5 to 8 when read as appended to claim 4, wherein said first and second detecting objects (21, 23) each comprise a drive transistor (Tr10, Tr11) of an output section of the amplifier. An amplifier as claimed in claim 11, wherein said drive transistor (Tr10) of said first detecting object (21) is connected between a first power supply line (Vcc) of the amplifier and an output node (Out), and said drive transistor (Tr11) of said second detecting object (23) is connected between said output node and a second power supply line (GND) of the amplifier. An amplifier as claimed in claim 11 or 12, wherein the current detector circuit (28) carries out a negative feedback operation so as to suppress a fluctuation in a through current (It) which flows in said output section and which serves to cancel a crossover distortion. An amplifier as claimed in any one of claims 11 to 13, further including a differential amplifier section (11) having inputs for receiving respective first and second differential signals (V+, V-) and also having a first output for sinking a current (I1) dependent upon a difference (V+-V-) between the received first and second differential signals and a second output for sourcing a current (I2) dependent upon the said difference, said first output being connected operatively to a control terminal of said drive transistor (Tr10) of one (21) of said first and second detecting objects (21, 23) for controlling a current (Ip) flowing therethrough, and said second output being connected operatively to a control terminal of said drive transistor (Tr11) of the other (23) of said first and second detecting objects (21, 23) for controlling a current (In) flowing therethrough. An amplifier as claimed in any one of claims 11 to 14, wherein said current detector circuit further includes a phase compensation circuit (R1, R2, C1, C2) connected operatively to said feedback circuit (26) for preventing positive feedback of high-frequency signals by said feedback circuit. A majority circuit, for detecting faults in transmission of a plurality of digital signals (D1 to Dn) each having the same signal level, including: a current detector circuit as claimed in claim 1; a primary current source (Ir2) connected for causing a preselected primary current (n*I1) to flow through a first node of the majority circuit; a secondary current source (Ir3) connected for causing a preselected secondary current (12), weak relative to said preselected primary current (n*I1), to flow along a current path between said first node and a second node of the majority circuit; and a plurality of switching transistors (Ti1 to Tin), each having a first terminal connected to said first node and a second terminal connected to said second node and a control terminal connected for receiving one of said digital signals (D1 to Dn), each said switching transistor being switchable by its received digital signal from an OFF state, in which no current flows therethrough, to an ON state in which a preselected switch current (I1) flows therethrough, said primary current being n times each said switch current, where n is the number of switching transistors of the said plurality; wherein: said first current detector (TrB) is connected to said first node for detecting a difference between said primary current (n*I1) and a sum of said secondary current (I2) and any said switch currents that are flowing; and said second current detector (TrA) is connected to said second node for detecting said sum; whereby a current (lout) flowing through said monitor (25) is determined by a lower one of said detected difference and said detected sum.

Inventors: KOBAYASHI OSAMU, MATSUDA ATSUSHI, YUASA TACHIO, KOBAYASHI, OSAMU, MATSUDA, ATSUSHI, YUASA, TACHIO
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1995
Main classification: H03K19
IPC codes: H03F3, H03F1