Operational amplifiers and current detector circuits

Abstract: An operational amplifier has a differential amplification section (100), an output section (300), a control signal generation unit (1), and a drive control unit (2). The output section (300) has a first transistor (T6) of a first conductivity type and a second transistor (T7) of a second conductivity type that is opposite to the first conductivity type. The first and second transistors are connected in series between a first power source unit (V DD ) and a second power source unit (Vss), and the first transistor (T6) is driven according to an output of the differential amplification section (100). The control signal generation unit (1) is used to detect a current flowing through the first transistor (T6) and generate a control signal (S) in response to the detected current. The drive control unit (2) is used to drive the second transistor (T7) in response to the control signal (S).

Description: The present invention as it is defined in the appended claims relates to amplifiers and in particular, but not exclusively, to operational amplifiers, for example operational amplifiers having output sections constituted by push-pull 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. Japanese Unexamined Patent Publication (Kokai) No. 1-137808 and europear publication EP-A-0 318 396 discloses an operational amplifier which may be considered to comprise: a differential amplification section; and an output section having a first transistor of a first conductivity type and a second transistor of a second conductivity type that is opposite to the first conductivity type, the first and second transistors being connected in series between a first power source means and a second power source means, the first transistor being driven according to an output of the different amplification section. This operational amplifier is capable of maintaining the bias voltages of the first and second transistors that form a push-pull circuit, with respect to a fluctuation of, for example, ±5% in a source voltage of 5V. This operational amplifier, however, is incapable of stabilizing operation for a wide range of source voltages, for example, from 3V to 5V. To deal with a fluctuation in a bias voltage, the operation point of the output section is usually of class A or AB. It is, therefore, difficult to reduce power consumption. If the operation point of the output section is of class B, the first and second transistors may be cut off due to a fluctuation in a bias voltage, to thereby cause distortion. By the way, low-frequency amplifiers are classified into classes A, B and AB. The A-class amplifiers involve a lower 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 sources are widely used. It is necessary therefore, to develop amplifiers capable of operating with a low-voltage power source. It is accordingly desirable to provide an operational amplifier that operates stably for a wide range of source voltages. An operational amplifier embodying a first aspect of the present invention is characterised by: a control signal generation unit for detecting a current flowing through the first transistor and generating a control signal in response to the detected current; and a drive control unit for controlling the second transistor to increase or decrease a current flowing through the second transistor if the current flowing through the first transistor detected by the control signal generation unit decreases or increases, respectively. The control signal generation unit may have a third transistor of the first conductivity type having a first terminal connected to the first power source unit and a control terminal connected to the output of the differential amplification section as well as to the control terminal of the first transistor; and the drive control unit may have fourth and fifth transistors of the first conductivity type having first terminals connected to the first power source unit, a sixth transistor of the second conductivity type having a first terminal connected to the second power source unit and a control terminal and a second terminal connected to the second terminal of the fifth transistor as well as to the control terminal of the second transistor, and constant current source, the control terminal and second terminal of the fourth transistor being connected to the second power source unit through the constant current source, to the control terminal of the fifth transistor, and to the second terminal of the third transistor, to receive the control signal from the control signal generation unit. The first transistor may be connected to the third transistor in a current mirror configuration, the fourth transistor may be connected to the fifth transistor in a current mirror configuration, and the second transistor may be connected to the sixth transistor in a current mirror configuration. The control signal generation unit may have a third transistor of a first conductivity type having a first terminal connected to the first power source unit and a control terminal connected to the output of the differential amplification section as well as to the control terminal of the first transistor; and the drive control unit may have fourth, fifth, and sixth transistors of the second conductivity type having first terminals connected to the second power source unit, and a constant current source, the second terminal of the fifth transistor, the second terminal and control terminal of the sixth transistor, and the control terminal of the second transistor being connected to the first power source unit through the constant current source, the second terminal and control terminal of the fourth transistor and the control terminal of the fifth transistor being connected to the second terminal of the third transistor, to receive the control signal from the control signal generation unit. The first transistor may be connected to the third transistor in a current mirror configuration, the fourth transistor may be connected to the fifth transistor in a current mirror configuration, and the second transistor may be connected to the sixth transistor in a current mirror configuration. The drive control unit may control the second transistor to pass a current if a current flowing through the first transistor detected by the control signal generation unit is larger than a predetermined value. The control signal generation unit may have a third transistor of the first conductivity type having a first terminal connected to the first power source unit and a control terminal connected to the output of the differential amplification section as well as to the control terminal of the first transistor; and the drive control unit may have a load unit, a fourth transistor of the first conductivity type having a first terminal connected to the control terminal of the first transistor as well as to the control terminal of the third transistor and a second terminal connected to the control terminal of the second transistor as well as to the second power source unit through the load unit, and a first constant current source, the second terminal of the third transistor and the control terminal of the fourth transistor being connected to the second power source unit through the first constant current source. The operational amplifier may further comprise a second constant current source arranged between the second power source unit and a node between the second terminal of the first transistor and the second terminal of the second transistor. The first transistor may be connected to the third transistor in a current mirror configuration. The differential amplification section may have third and fourth transistors of the second conductivity type, fifth and sixth transistors of the first conductivity type, and a constant current source, the first terminals of the third and fourth transistors being connected to the second power source unit through the constant current source, the control terminals of the third and fourth transistors receiving first and second inputs, respectively, the first terminals of the fifth and sixth transistors being connected to the first power source unit, the control terminals of the fifth and sixth transistors being connected to the second terminal of the fifth transistor as well as to the second terminal of the third transistor, the output of the differential amplification section being provided from a node between the second terminal of the sixth transistor and the second terminal of the fourth transistor. In the output section, the first terminal of the first transistor may be connected to the first power source unit, the control terminal of the first transistor may be connected to the output of the differential amplification section, the first terminal of the second transistor may be connected to the second power source unit, the control terminal of the second transistor may be connected to the output of the drive control unit, and a node between the second terminal of the first transistor and the second terminal of the second transistor may provide the output of the operational amplifier. The first conductivity type may be a P-channel type, the second conductivity type may be an N-channel type, the transistors may be MIS transistors, the first terminal may be a source, the second terminal may be a drain, and the control terminal may be a gate. The first conductivity type may be a PNP type, the second conductivity type may be an NPN type, the transistors may be bipolar transistors, the first terminal may be an emitter, the second terminal may be a collector, and the control terminal may be a base. The operational amplifier may constitute a semiconductor integrated circuit. According to a second aspect of the present invention there is provided an amplifier comprising: an output section having a first transistor of a first conductivity type and a second transistor of a second conductivity type that is opposite to said first conductivity type, said first and second transistors being connected in series between a first power source means and a second power source means, said first transistor being driven according to a driving signal; characterised by: a control signal generator for detecting a current flowing through said first transistor and generating a control signal in response to the detected current; and a drive controller for controlling said second transistor to increase or decrease a current flowing through said second transistor if the current flowing through said first transistor detected by said control signal generator decreases or increases, respectively. Reference will now be made, by way of example, to the accompanying drawings, in which: Fig. 1 is a circuit diagram showing an operational amplifier according to a prior art; Fig. 2 is a circuit diagram showing an operational amplifier according to another prior art; Fig. 3 is a block diagram showing a principle of an operational amplifier embodying the present invention; Fig. 4 is a circuit diagram showing an operational amplifier according to a first embodiment of the present invention; Fig. 5 is a circuit diagram showing an operational amplifier according to a second embodiment of the present invention; Fig. 6 is a circuit diagram showing an operational amplifier according to a third embodiment of the present invention; and Fig. 7 is a circuit diagram showing an operational amplifier according to a fourth embodiment of the present invention. For a better understanding of the preferred embodiments, the problems of an operational amplifier according to the prior art will be explained, with reference to Figs. 1 and 2. Figure 1 is a circuit diagram showing an operational amplifier according to a prior art. The operational amplifier consists of a differential amplification section 100, a level shift section 200, and an output section 300. The operational amplifier includes a high-potential power source line (first power source means) V DD and a low-potential (ground level) power source line (second power source means) Vss. The differential amplification section 100 has N-channel MOS transistors T1 and T2, P-channel MOS transistors T3 and T4, and a constant current source CS1. The gate of the transistor T1 receives a signal input from an inverting input /IN, and the gate of the transistor T2 receives a signal input from a non-inverting input IN. A node between the transistors T2 and T4 provides the output of the differential amplification section 100, which is supplied to the level shift section 200. The level shift section 200 has an N-channel MOS transistor T5 and a constant current source CS0. The output section 300 has a P-channel MOS transistor T6 and an N-channel MOS transistor T7. The transistors T6 and T7 form a push-pull circuit. The gate of the transistor T6 receives the output of the differential amplification section 100. The gate of the transistor T7 receives a level-shifted output from the level shift section 200 through a node between the source of the transistor T5 and the constant current source CS0. The back gate (substrate) of the transistor T5 receives the low-potential (ground level) source voltage Vss. In the operational amplifier of Fig. 1, the level shift section 200 shifts the level of an output signal in response to a fluctuation in the source voltage V DD . The amount of the shift, however, does not correctly follow the fluctuation in the source voltage V DD . This results in fluctuating a bias voltage (a gate-source voltage VGS minus a threshold voltage Vth) of each of the transistors T6 and T7 of the output section 300, to thereby change currents passing through the transistors T6 and T7 and fluctuate the power source current and other characteristics of the operational amplifier. Namely, the bias voltage is changed in response to the source voltage V DD . The bias voltage of the transistor T7 is changed when the source voltage V DD varies. This change is fed back to the transistor T6 that forms the push-pull circuit with the transistor T7, to thereby shift the bias voltage of the transistor T6. Figure 2 is a circuit diagram showing an operational amplifier according to another prior art. This is an improvement of the operational amplifier of Fig. 1 and is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 1-137808 and European Publication EP-A-0 318 396. The operational amplifier of Fig. 2 has a level shift section 200 having an N-channel MOS transistor T5' and a current source CS0', which changes a current in response to a fluctuation in a source voltage. This operational amplifier has a differential amplification section 100 and an output section 300, which are the same as those of the operational amplifier of Fig. 1. The operational amplifier of Fig. 2 is capable of maintaining the bias voltages of transistors T6 and T7 that form a push-pull circuit, with respect to a fluctuation of, for example, ±5% in a source voltage of 5V. This operational amplifier, however, is incapable of stabilizing operation for a wide range of source voltages, for example, from 3V to 5V. To deal with a fluctuation in a bias voltage, the operation point of the output section 300 is usually of class A or AB. It is, therefore, difficult to reduce power consumption. If the operation point of the output section 300 is of class B, the transistors T6 and T7 may be cut off due to a fluctuation in a bias voltage, to thereby cause distortion. Below, embodiments of an operational amplifier according to the present invention will be explained with reference to Figs. 3 to 7. Figure 3 is a block diagram showing the principle of an operational amplifier embodying the present invention. As shown in Fig. 3, the operational amplifier has a differential amplification section 100 and an output section 300. The output section 300 has a first transistor T6 of first conductivity type and a second transistor T7 of second conductivity type that is opposite to the first conductivity type. The transistors T6 and T7 are connected in series between a first power source V DD and a second power source Vss. The first transistor T6 is driven according to the output of the differential amplification section 100. The amplifier also has a control signal generator 1 for detecting a current flowing through the first transistor T6 and generating a control signal S in response to the detected current, and a drive controller 2 for driving the second transistor T7 in response to the control signal S. The output section 300 of the operational amplifier embodying the present invention has the first transistor T6 of first conductivity type and the second transistor T7 of second conductivity type that is opposite to the first conductivity type. The transistors T6 and T7 are connected in series between the first and second power sources V DD and Vss. The control signal generator 1 detects a current flowing through the first transistor T6 and generates a control signal S in response to the detected current. The drive controller 2 drives the second transistor T7 in response to the control signal S. The first and second transistors T6 and T7 form a push-pull circuit. The control signal S generated by the control signal generator 1 is proportional to a current flowing through the first transistor T6. The drive controller 2 receives the control signal S, detects a decrease in the control signal S, and drives the second transistor T7. In this way, the operational amplifier embodying the present invention detects a current flowing through the first transistor T6, and if there is a decrease in the current flowing through the first transistor T6, increases a current flowing through the second transistor T7. Even if a source voltage fluctuates, a current flowing through the second transistor T7 will be unchanged to provide stable operation independent of a source voltage. In this way, the operational amplifier embodying the present invention stably operates for a wide range of source voltages. Figure 4 is a circuit diagram showing an operational amplifier according to the first embodiment of the present invention. The operational amplifier consists of a control signal generator 1, a drive controller 2, a differential amplification section 100, and an output section 300. The differential amplification section 100 and output section 300 are the same as those of the prior arts of Figs. 1 and 2. The operational amplifier includes a high-potential power source line (first power source means) V DD and a low-potential (ground level) power source line (second power source means) Vss. The differential amplification section 100 has N-channel MOS transistors T1 and T2, P-channel MOS transistors T3 and T4, and a constant current source CS1. The gate of the transistor T1 is connected to an inverting input /IN, and the gate of the transistor T2 is connected to a non-inverting input (positive logic input) IN. A node between the transistors T2 and T4 provides the output of the differential amplification section 100. An end of the constant current source CS1 is connected to the source of the transistor T1 and the source of the transistor T2. The other end of the constant current source CS1 is connected to the low-potential power source line Vss. The output section 300 has two transistors T6 and T7 connected in series between the high-potential power source line V DD and the low-potential power source line Vss, to form a push-pull circuit. The transistor T6 is a P-channel MOS transistor having a source connected to the power source line V DD , and the transistor T7 is an N-channel MOS transistor having a source connected to the power source line Vss. A node between the drain of the transistor T6 and the drain of the transistor T7 provides the output OUT of the operational amplifier. The control signal generator 1 has a P-channel MOS transistor TA having a source connected to the high-potential power source line V DD . The gate of the transistor TA is connected to the gate of the transistor T6 of the output section 300, which receives the output of the differential amplification section 100. Namely, the transistors TA and T6 are connected to each other in a current mirror configuration. A current Ia flowing through the transistor TA is proportional to a current I6 flowing through the transistor T6, i.e., Ia = k1 * I6. Here, k1 to k7 are constants. The drive controller 2 has P-channel MOS transistors TB and TC having sources connected to the high-potential power source line V DD , an N-channel MOS transistor TD having a gate and drain connected to the drain of the transistor TC, and a constant current source CS2. The gate and drain of the transistor TB are connected to the low-potential power source line Vss through the constant current source CS2, to the gate of the transistor TC, and to the drain of the transistor TA of the control signal generator 1, to receive a control signal S from the control signal generator 1. The transistors TB and TC are connected to each other in a current mirror configuration. A current Ib flowing through the transistor TB is proportional to a current Ic flowing through the transistor TC, i.e., Ic = k2 * Ib. The transistor TA of the control signal generator 1 generates the control signal S proportional to a current flowing through the transistor T6 of the output section 300. The signal S corresponds to the current Ia flowing through the transistor TA. The drive controller 2 receives the signal S, detects a decrease in the signal S (current Ia), and drives the transistor T7 of the output section 300 accordingly. The transistor TD and the transistor T7 of the output section 300 are connected to each other in a current mirror configuration, so that a current I7 flowing through the transistor T7 is proportional to the current Ic flowing through the transistor TD, i.e., I7 = k3 * Ic. As explained above, the current Ia (control signal S) flowing through the transistor TA is proportional to the current I6 flowing through the transistor T6, the current Ib flowing through the transistor TB is proportional to the current Ic flowing through the transistor TC, and the current Ic is proportional to the current I7 flowing through the transistor T7. Namely, Ia = k1 * I6, Ic = k2 * Ib, and I7 = k3 * Ic. Ia + Ib = Is2 (constant), so that I7 + k3 * k2 * (Is2 - k1 * I6) = k3 * k2 * (Is2 - k1) * k2 * k3 * I6. Here, k1, k2, and k3 are constants determined according to the physical sizes of the transistors, and therefore, are independent of a source voltage. Accordingly, the amplitude of the current I7 flowing through the transistor T7 is independent of the source voltage. In this way, the current I6 flowing through the transistor T6 of the output section 300 is indirectly detected by the transistor TA connected to the transistor T6 in a current mirror configuration, and the transistor TA generates the control signal S accordingly. The control signal S is supplied to the drive controller 2 which increases a control voltage to the gate of the other transistor T7 of the output section 300 if the current flowing through the transistor T6 decreases, to thereby increase a current passing through the transistor T7. When the current passing through the transistor T6 increases, the drive controller 2 drops the control voltage to the gate of the transistor T7 of the output section 300, to thereby decrease the current I7 passing through the transistor T7. This embodiment establishes a relationship of I7 ≤ k2 * k3 * Is2, to provide a current limit function on sink current. In this way, the operational amplifier of the first embodiment stably operates for a wide range of source voltages. Figure 5 is a circuit diagram showing an operational amplifier according to a second embodiment of the present invention. This operational amplifier has a differential amplification section 100 having transistors T1 to T4 and an output section 300 having transistors T6 and T7. These sections 100 and 300 are the same as those of the first embodiment of Fig. 4 and of the prior arts of Figs. 1 and 2. A control signal generator 1 has a P-channel MOS transistor TE having a source connected to a high-potential power source line V DD . A drive controller 2 has a resistor (load means) R, a P-channel MOS transistor TF having a drain connected to a low-potential power source line Vss through the resistor R, and a constant current source CS3. The output of the differential amplification section 100 is connected to the gate of the transistor T6 of the output section 300, the gate of the transistor TE of the control signal generator 1, and the source of the transistor TF of the drive controller 2. The drain of the transistor TE is connected to the gate of the transistor TF and to the power source line Vss through the constant current source CS3. A node between the drain of the transistor TF and the resistor R provides the output of the drive controller 2, which is connected to the gate of the transistor T7 of the output section 300. The transistor TE of the control signal generator 1 is connected to the transistor T6 of the output section 300 in a current mirror configuration, so that a current Ie passing through the transistor TE is proportional to a current I6 passing through the transistor T6, i.e., Ie = k4 * I6. According to the second embodiment, the current Ie (control signal S) passing through the transistor TE is proportional to the current I6 passing through the transistor T6, so that Ie = k4 * I6. When Ie > Is3, a voltage V1 at a node N1 at the drain of the transistor TE, the gate of the transistor TF, and an end of the constant current source CS3 is as high as the source voltage V DD . Accordingly, the transistor TF is OFF, and a voltage V3 at a node N3 among the drain of the transistor TF, the gate of the transistor T7, and an end of the resistor R is low. Consequently, the transistor T7 of the output section 300 is OFF and passes no current. When 16 < Is3/k4, i.e., when Ie < Is2 to make V1 < V2 - Vth(TF), the transistor TF passes a current If to increase the voltage V3 at the node N3, to thereby increase a voltage drop due to the resistor R. As a result, the-other transistor T7 of the output section 300 passes a current I7. Here, the reference mark V2 is a voltage at a node N2, i.e., the output voltage of the differential amplifier 100, and Vth(TF) is the threshold voltage of the transistor TF. The above operation is independent of the magnitude of a source voltage, and therefore, the current I7 flowing through the transistor T7 is independent of the source voltage. According to the embodiment of Fig. 5, a constant current source CS4 is inserted between the output OUT of the operational amplifier and the low-potential power source line Vss. The constant current source CS4 passes a current corresponding to the current I6 flowing through the transistor T6 at a usual operation point (V+IN = V-IN), and draws a current from the output OUT of the operational amplifier when the transistor T7 is OFF. In this way, the second embodiment indirectly detects the current I6 passing through the transistor T6 of the output section 300, using the transistor TE of the control signal generator 1 connected to the transistor T6 in a current mirror configuration, and generates the control signal S. The control signal S is supplied to the drive controller 2, so that, when the current flowing through the transistor T6 decreases to below a predetermined value, the other transistor T7 of the output section 300 is turned ON to pass the current I7 through the transistor T7. When the current flowing through the transistor T6 is above the predetermined value, the transistor T7 is kept OFF and passes no current. Accordingly, the operational amplifier of the second embodiment stably operates for a wide range of source voltages. Figure 6 is a circuit diagram showing an operational amplifier according to a third embodiment of the present invention. This operational amplifier has a differential amplification section 100 including transistors T1 to T4 and an output section 300 including transistors T6 and T7. These sections 100 and 300 are the same as those of the operational amplifiers mentioned above. According to the third embodiment of Fig. 6, a control signal generator 1 has a P-channel MOS transistor TG having a source connected to a high-potential power source line V DD . A drive controller 2 has N-channel MOS transistors TH, TI, and TJ having sources connected to a low-potential power source line Vss, and a constant current source CS5. The output of the differential amplification section 100 is supplied to the gate of the transistor T6 of the output stage 300 and the gate of the transistor TG of the control signal generator 1. The drain of the transistor TG is connected to the gate and drain of the transistor TH and the gate of the transistor TI. An end of the constant current source CS5 is connected to the power source line V DD , and the other end of the constant current source CS5 is connected to the drain of the transistor TI, the drain and gate of the transistor TJ, and the gate of the transistor T7. The transistors TG and T6 are connected to each other in a current mirror configuration, so that a current Ig passing through the transistor TG is proportional to a current I6 passing through the transistor T6, i.e., Ig = k5 * I6. The transistors TH and TI are connected to each other in a current mirror configuration, so that a current Ii passing through the transistor TI is proportional to a current Ih passing through the transistor TH, i.e., Ii = k6 * Ih. The transistors TJ and T7 are connected to each other in a current mirror configuration, so that a current I7 passing through the transistor T7 is proportional to a current Ij passing through the transistor Tj, i.e., I7 = k7 * Ij. In this way, the current I6 passing through the transistor T6 is proportional to the current Ig passing through the transistor TG, the current Ih passing through the transistor TH is proportional to the current Ii passing through the transistor TI, and the current Ij passing through the transistor TJ is proportional to the current I7 passing through the transistor T7. Namely, Ig = k5 * I6, Ii = k6 * Ih, and I7 = k7 * Ij. Ii + Ij = Is5 (constant), and therefore, I7 = k7 * (Is5 - k5 * k6 * I6) = K7 * Is5 - k5 * K6 * k7 * I6. Here k5 to k7 are constants. Accordingly, the current I7 passing through the transistor T7 is independent of the source voltage. The third embodiment employs the transistor TG (control signal generator 1) connected to the transistor T6 in a current mirror configuration, to indirectly detect the current I6 passing through the transistor T6 of the output section 300 and generate the control signal S. The control signal S is supplied to the drive controller 2 so that, when the current passing through the transistor T6 decreases, a control voltage applied to the gate of the transistor T7 of the output section 300 is increased to increase the current I7 passing through the transistor T7. When the current passing through the transistor T6 increases, the control voltage applied to the gate of the other transistor T7 is decreased to decrease the current I7 passing through the transistor T7. According to the third embodiment, 17 ≤ k7 * Is5, to realize a current limit function on a sink current. As explained above, the operational amplifier according to the third embodiment stably operates for a wide range of source voltages. Figure 7 is a circuit diagram showing a modification of the operational amplifier of Fig. 6. This modification employs PNP bipolar transistors instead of the P-channel MOS transistors of Fig. 6, and NPN bipolar transistors instead of the N-channel MOS transistors of Fig. 6. The operational amplifier of Fig. 7 has a differential amplification section 100' and an output section 300' corresponding to the differential amplification section 100 and output section 300 of Fig. 6. The bipolar transistors T10 to T40, T60, and T70 and a constant current source CS10 correspond to the MOS transistors T1 to T4, T6, and T7 and constant current source CS1 of Fig. 6. The bipolar transistor TG0 of a control signal generator 1', the bipolar transistors TH0, TI0, and TJ0 and a constant current source CS50 of a drive controller 2' correspond to the MOS transistor TG, MOS transistors TH, TI, and TJ, and constant current source CS5 of Fig. 6. The operational amplifiers of the embodiments mentioned above may be made not only of MOS (MIS) transistors but may also be bipolar transistors or other devices. Further, the above described operational amplifiers can be constituted as semiconductor integrated circuits. As explained above, an operational amplifier embodying the present invention is capable of securing the performance thereof for a wide range of source voltages. Many different embodiments of the present invention may be constructed without departing from the scope of the present invention as it is defined in the appended claims, and it should be understood that the present invention is not limited to the specific embodiments described in this specification, except as defined in the appended claims.

Claims: An operational amplifier comprising: a differential amplification section (100); an output section (300) having a first transistor (T6) of a first conductivity type and a second transistor (T7) of a second conductivity type that is opposite to said first conductivity type, said first and second transistors being connected in series between a first power source means (V DD ) and a second power source means (V SS ), said first transistor (T6) being driven according to an output of said differential amplification section (100); characterised by: a control signal generator (1) for detecting a current flowing through said first transistor (T6) and generating a control signal (S) in response to the detected current; and a drive controller (2) for controlling said second transistor (T7) to increase or decrease a current flowing through said second transistor (T7) if the current flowing through said first transistor (T6) detected by said control signal generator (1) decreases or increases, respectively. An operational amplifier as claimed in claim 1, wherein: said control signal generator (1) has a third transistor (TA; TE; TG) of said first conductivity type having a first terminal connected to said first power source means (V DD ) and a control terminal connected to the output of said differential amplification section (100) as well as to the control terminal of said first transistor (T6). An operational amplifier as claimed in claim 1, wherein: said drive controller (2) has fourth, fifth and sixth transistors (TH, TI, TJ) of said second conductivity type having first terminals connected to said second power source means (V SS ), and a constant current source (CS5), the second terminal of said fifth transistor (TI), the second terminal and control terminal of said sixth transistor (TJ), and the control terminal of said second transistor (T7) being connected to said first power source means (V DD ) through said constant current source (CS5), the second terminal and control terminal of said fourth transistor (TH) and the control terminal of said fifth transistor (TI) being connected to an output terminal of said control signal generator (1), to receive the control signal (S) from said control signal generator (1). An operational amplififer as claimed in claim 3, wherein a current flowing through said first transistor (T6) is proportional to a current flowing through said control signal generator (1), said fourth transistor (TH) is connected to said fifth transistor (TI) in a current mirror configuration, and said second transistor (T7) is connected to said sixth transistor (TJ) in a current mirror configuration. An operational amplifier as claimed in claim 1, wherein said drive controller (2) controls said second transistor (T7) to increase a current thereof if a current flowing through said first transistor (T6) detected by said control signal generator (1) is decreased; and controls said second transistor (T7) to decrease the current thereof if the current flowing through said first transistor (T6) detected by said control signal generator (1) is increased. An operational amplifier as claimed in claim 1, wherein: said drive controller (2) has a first load means (R), a fourth transistor (TF) of said first conductivity type having a first terminal connected to the control terminal of said first transistor (T6) as well as to an input terminal of said control signal generator (1) and a second terminal connected to the control terminal of said second transistor (T7) as well as to said second power source means (V SS ) through said first load means (R), and said drive controller (2) also has a second load means (CS3), an output terminal of said control signal generator (1) and the control terminal of said fourth transistor (TF) being connected to said second power source means (V SS ) through said second load means (CS3). An operational amplifier as claimed in claim 1, wherein said operational amplifier further comprises a constant current source (CS4) arranged between said second power source means (V SS ) and a node (OUT) between the second terminal of said first transistor (T6) and the second terminal of said second transistor (T7). An operational amplifier as claimed in claim 6, wherein a current flowing through said first transistor (T6) is proportional to a current flowing through said control signal generator (1). An operational amplifier as claimed in claim 1, wherein: said drive controller (2) has fourth and fifth transistors (TB, TC) of said first conductivity type having first terminals connected to said first power source means (V DD ), and a constant current source (CS2), the control terminal and second terminal of said fourth transistor (TB) being connected to said second power source means (V SS ) through said constant current source (CS2), to the control terminal of said fifth transistor (TC), and to an output terminal of said control signal generator (1), in order to receive the control signal (S) from said control signal generator (1). An operational amplifier as claimed in claim 9, wherein a current flowing through said first transistor (T6) is proportional to a current flowing through said control signal generator (1), and said fourth transistor (TB) is connected to said fifth transistor (TC) in a current mirror configuration. An operational amplifier as claimed in claim 9, wherein: said drive controller (2) further has a sixth transistor (TD) of said second conductivity type having a first terminal connected to said second power source means (V SS ) and a control terminal and a second terminal connected to the second terminal of said fifth transistor (TC) as well as to the control terminal of said second transistor (T7). An operational amplifier as claimed in claim 11, wherein said second transistor (T7) is connected to said sixth transistor (TD) in a current mirror configuration. An operational amplifier as claimed in any one of the preceding claims, wherein, in said output section (300), the first terminal of said first transistor (T6) is connected to said first power source means (V DD ), the control terminal of said first transistor (T6) is connected to the output of said differential amplification section (100), the first terminal of said second transistor (T7) is connected to said second power source means (V SS ), the control terminal of said second transistor (T7) is connected to the output of said drive control means (2), and a node (OUT) between the second terminal of said first transistor (T6) and the second terminal of said second transistor (T7) provides the output of said operational amplifier. An operational amplifier as claimed in any one of the preceding claims, wherein: said differential amplification section (100) has third and fourth transistors (T1, T2) of said second conductivity type, fifth and sixth transistors (T3, T4) of said first conductivity type, and a constant current source (CS1), the first terminals of said third and fourth transistors (T1, T2) being connected to said second power source means (V SS ) through said constant current source (CS1), the control terminals of said third and fourth transistors (T1, T2) receiving first and second inputs (/IN, IN), respectively, the first terminals of said fifth and sixth transistors (T3, T4) being connected to said first power source means (V DD ), the control terminals of said fifth and sixth transistors (T3, T4) being connected to the second terminal of said fifth transistor (T3) as well as to the second terminal of said third transistor (T1), the output of said differential amplification section (100) being provided from a node between the second terminal of said sixth transistor (T4) and the second terminal of said fourth transistor (T2). An operational amplifier as claimed in any one of the preceding claims, wherein said first conductivity type is a p-channel type, said second conductivity type is an N-channel type, said transistors are MIS transistors, said first terminal is a source, said second terminal is a drain, and said control terminal is a gate. An operational amplifier as claimed in any one of claims 1 to 14, wherein said first conductivity type is a PNP type, said second conductivity type is an NPN type, said transistors are bipolar transistors, said first terminal is an emitter, said second terminal is a collector, and said control terminal is a base. An operational amplifier as claimed in any one of claims 1 to 14, wherein said first conductivity type is an N-channel type, said second conductivity type is a P-channel type, said transistors are MIS transistors, said first terminal is a source, said second terminal is a drain and said control terminal is a gate. An operational amplifier as claimed in any one of claims 1 to 14, wherein said first conductivity type is an NPN type, said second conductivity type is a PNP type, said transistors are bipolar transistors, said first terminal is an emitter, said second terminal is a collector, and said control terminal is a base. An operational amplifier as claimed in any one of the preceding claims, wherein said operational amplifier constitutes a semiconductor integrated circuit. An amplifier comprising: an output section (300) having a first transistor (T6) of a first conductivity type and a second transistor (T7) of a second conductivity type that is opposite to said first conductivity type, said first and second transistors being connected in series between a first power source means (V DD ) and a second power source means (V SS ), said first transistor (T6) being driven according to a driving signal; characterised by: a control signal generator (1) for detecting a current flowing through said first transistor (T6) and generating a control signal (S) in response to the detected current; and a drive controller (2) for controlling said second transistor (T7) to increase or decrease a current flowing through said second transistor (T7) if the current flowing through said first transistor (T6) detected by said control signal generator (1) decreases or increases, respectively. An amplifier as claimed in claim 20, wherein: said control signal generator (1) has a third transistor (TA; TE; TG) of said first conductivity type having a first terminal connected to said first power source means (V DD ) and a control terminal which receives said driving signal as well as the control terminal of said first transistor (T6). An amplifier as claimed in claim 20, wherein: said drive controller (2) has fourth and fifth transistors (TB, TC) of said first conductivity type having first terminals connected to said first power source means (V DD ), and a constant current source (CS2), the control terminal and second terminal of said fourth transistor (TB) being connected to said second power source means (V SS ) through said constant current source (CS2), to the control terminal of said fifth transistor (TC) and to an output terminal of said control signal generator (1), in order to receive the control signal (S) from said control signal generator (1). An amplifier as claimed in claim 22, wherein: said drive controller (2) further has a sixth transistor (TD) of said second conductivity type having a first terminal connected to said second power source means (V SS ) and a control terminal and second terminal connected to the second terminal of said fifth transistor (TC) as well as to the control terminal of said second transistor (T7). An amplifier as claimed in claim 22, wherein a current flowing through said first transistor (T6) is proportional to a current flowing through said control signal generator (1), and said fourth transistor (TB) is connected to said fifth transistor (TC) in a current mirror configuration. An amplifier as claimed in claim 23, wherein said second transistor (T7) is connected to said sixth transistor (TD) in a current mirror configuration. An amplifier as claimed in any one of claims 20 to 25, being an operational amplifier.

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