Abstract:
A rail-to-rail operational amplifier capable of reducing current consumption includes an amplification stage circuit including a first compensation output terminal and a second compensation output terminal, for generating an amplified signal according to an input signal, an output stage circuit coupled to the amplification stage circuit, for outputting the amplified signal, and a compensation circuit coupled to the amplification stage circuit and the output stage circuit. The compensation circuit includes a first voltage generator for generating a first voltage, a second voltage generator for generating a second voltage, a first compensation capacitor, a second compensation capacitor, and four switches named from a first switch to a fourth switch, wherein the first voltage is approximately a steady state voltage of the first compensation output terminal and the second voltage is approximately a steady state voltage of the second compensation output terminal.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a rail-to-rail operational amplifier, and more particularly, to a rail-to-rail operational amplifier capable of reducing current consumption by controlling bias of compensation capacitors. 
   2. Description of the Prior Art 
   With the advance of semiconductor technologies, operating voltages of integrated circuits become lower and lower. For the design of analog circuits, problems of insufficient input and output common-mode voltage of an operational amplifier often occur, and thus the operational amplifier needs to have a rail-to-rail common-mode input and output range to solve these problems. 
   Generally, a conventional operational amplifier has a two-stage structure, which includes a first stage amplification circuit (amplification stage), and a second stage output circuit (output stage). The first stage amplification circuit is utilized for increasing current or voltage gain of the operational amplifier, and the second stage output circuit is utilized for driving capacitive or resistive loads connected to the operational amplifier. In addition, since the operational amplifier may suffer loop instability problems, Miller compensation capacitors are commonly used to perform frequency compensation for improving loop stability. 
   Please refer to  FIG. 1 , which is a schematic diagram of a conventional operational amplifier  10 . The operational amplifier  10  is a rail-to-rail operational amplifier including an amplification stage circuit  11 , an output stage circuit  12  and a compensation circuit  13 . The operational amplifier  10  receives an input signal via a positive input terminal VP, outputs an amplified signal via an output terminal VOUT and forms a feedback path from the output terminal VOUT to a negative input terminal VN. The amplification stage circuit  11  includes a first differential input pair  110 , a second differential input pair  120 , a first current mirror  130 , a second current mirror  140  and a third current mirror  150 . The first differential pair  110  includes a pair of matched NMOS transistors MN 1  and MN 2  and a current source  11  coupled to sources of the transistors MN 1  and MN 2 , which is utilized for providing quiescent currents of the first differential input pair  110 . Similarly, the second differential pair  120  includes a pair of matched PMOS transistors MP 1  and MP 2  and a current source  12  coupled to sources of the transistors MP 1  and MP 2 , which is utilized for providing quiescent currents of the second differential input pair  120 . 
   The first current mirror  130  and the second current mirror  140  are active loads for the first differential input pair  110  and the second differential input pair  120 , respectively. The first current mirror  130  includes PMOS transistors MP 3 , MP 4 , MP 5  and MP 6 . The second current mirror  140  includes NMOS transistors MN 3 , MN 4 , MN 5  and MN 6 . Gates of the transistors MP 5 , MP 6 , MN 5  and MN 6  are all coupled to a bias VB. The third current mirror  150  is represented as current sources  13  and  14 , and is utilized for performing summation of output signals from the first differential input pair  110  and the second differential input pair  120  and outputting the summation result to the output stage circuit  12 . Operation of the current mirror is well-known to those skilled in the art and explanation is not given here. 
   The output stage circuit  12  is a class AB push-pull output circuit including a PMOS transistor MP 7  and an NMOS transistor MN 7 , in which gates of the transistors MP 7  and MN 7  are respectively coupled to a node E and a node F of the amplification stage circuit  11 . The compensation circuit  13  is coupled between the amplification stage circuit  11  and the output stage circuit  12  and includes switches S 1 -S 4  and compensation capacitors CM 1  and CM 2 . The switches S 1  and S 2  and the compensation capacitor CM 1  are coupled to a node A; the switch S 1  and the amplification stage circuit  11  are coupled to a node B. The switches S 3  and S 4  and the compensation capacitor CM 2  are coupled to a node C; the switch S 3  and the amplification stage circuit  11  are coupled to a node D. In the operational amplifier  10 , the compensation capacitors CM 1  and CM 2  are charged and discharged according to switching of the switches S 1 -S 4 , which helps loop stability. When the input signal of the operational amplifier  10  is in a transition state where the voltage of the input signal transits from a high level to a low level and from the low level to the high level, the switches S 1  and S 3  are turned off and the switches S 2  and S 4  are turned on such that the amplified signal does not pass through the compensation capacitors CM 1  and CM 2 . On the other hand, when the input signal of the operational amplifier  10  is in a steady state, the switches S 1  and S 3  are turned on and the switches S 2  and S 4  are turned off such that the amplified signals pass through the compensation capacitors CM 1  and CM 2 , for performing frequency compensation for improving loop stability. 
   Please note that, when the input signal is in the transition state, the node A is connected to a power supply terminal VDD and the node C is connected to a ground terminal GND. In this situation, a voltage level of the node A is not equal to a steady state voltage level of the node B, and a voltage level of the node C is not equal to a steady state voltage level of the node D. Therefore, when the input signal is in the steady state and the switches S 1  and S 3  are turned on, charge sharing occurs between the nodes A and B and between the nodes C and D so that the voltage level of the node B is higher than the normal steady state voltage level and discharging occurs on the node B; and the voltage level of the node D is lower than the normal steady state voltage level and charging occurs on the node D. The discharging occurs on the node B and the charging occurs on the node D increase current consumption of the transistors MP 6  and MN 6 . 
   From the above, the charging and discharging effects occurring on the nodes B and D when the input signal is in the steady state results in unnecessary current consumption and thereby extends the settling time of the operational amplifier  10 . In addition, switching of the switches S 1 -S 4  is controlled by an external circuit, which limits the application of the operational amplifier  10 . 
   SUMMARY OF THE INVENTION 
   It is therefore a primary objective of the claimed invention to provide a rail-to-rail operational amplifier capable of reducing current consumption. 
   The present invention discloses a rail-to-rail operational amplifier capable of reducing current consumption. The rail-to-rail operational amplifier comprises an amplification stage circuit, an output stage circuit and a compensation circuit. The amplification stage circuit comprises a first compensation output terminal, a second compensation output terminal, a first current output terminal and a second current output terminal, and is utilized for generating an amplified signal according an input signal of the rail-to-rail operational amplifier. The output stage circuit is coupled to the first current output terminal and the second current output terminal of the amplification stage circuit, and comprises an output terminal, for outputting the amplified signal. The compensation circuit is coupled to the amplification stage circuit and the output stage circuit, and comprises a first voltage generator, a second voltage generator, a first compensation capacitor, a second compensation capacitor, a first switch, a second switch, a third switch and a fourth switch. 
   The first voltage generator is utilized for generating a first voltage approximately equal to a steady state voltage of the first compensation output terminal of the amplification stage circuit. The second voltage generator is utilized for generating a second voltage approximately equal to a steady state voltage of the second compensation output terminal of the amplification stage circuit. The first compensation capacitor comprises a first terminal and a second terminal coupled to the output terminal of the output stage circuit. The second compensation capacitor comprises a first terminal and a second terminal coupled to the output terminal of the output stage circuit. 
   The first switch is coupled between the first compensation output terminal and the first terminal of the first compensation capacitor, and is utilized for selectively coupling the first compensation output terminal to the first terminal of the first compensation capacitor according to a first control signal. The second switch is coupled between the first voltage generator and the first terminal of the first compensation capacitor, and is utilized for selectively coupling the first voltage generator to the first terminal of the first compensation capacitor according to a second control signal. The third switch is coupled between the second compensation output terminal and the first terminal of the second compensation capacitor, and is utilized for selectively coupling the second compensation output terminal to the first terminal of the second compensation capacitor according to a third control signal. The fourth switch is coupled between the second voltage generator and the first terminal of the second compensation capacitor, and is utilized for selectively coupling the second voltage generator to the first terminal of the second compensation capacitor according to a fourth control signal. 
   These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of an operational amplifier according to the prior art. 
       FIG. 2  to  FIG. 5  are schematic diagrams of operational amplifiers according to embodiments of the present invention. 
       FIG. 6  is a schematic diagram of a signal generation device for generating control signals according to an embodiment of the present invention. 
       FIG. 7  is a schematic diagram of comparators shown in  FIG. 6 . 
   

   DETAILED DESCRIPTION 
   Please refer to  FIG. 2 , which is a schematic diagram of an operational amplifier  20  according to an embodiment of the present invention. The operational amplifier  20  is a rail-to-rail operational amplifier, and comprises an amplification stage circuit  21 , an output stage circuit  22  and a compensation circuit  23 . The operational amplifier  20  receives an input signal via a positive input terminal VP, outputs an amplified signal via an output terminal VOUT and forms a feedback path from the output terminal VOUT to a negative input terminal VN. The amplification stage circuit  21  comprises a first differential input pair  210 , a second differential input pair  220 , a first current mirror  230 , a second current mirror  240  and a third current mirror  250 , and is utilized for generating the amplified signal according to the input signal. The output stage circuit  22  is a push-pull output circuit. The output stage circuit  22  comprises a PMOS transistor MP 7 , an NMOS transistor MN 7  and an output terminal VOUT and is utilized for outputting the amplified signal. Current sources  11 - 14 , PMOS transistors MP 1 -MP 7  and NMOS transistors MN 1 -MN 7  in the amplification stage circuit  21  and the output stage circuit  22  are similar to those in the amplification stage circuit  11  and the output stage circuit  12  shown in  FIG. 1 . 
   The amplification stage circuit  21  comprises a first compensation output terminal, a second compensation output terminal, a first current output terminal and a second current output terminal, which correspond to nodes B, D, E and F respectively. Gates of the transistors MP 7  and MN 7  are respectively coupled to the nodes E and F. The compensation circuit  23  is coupled to the nodes B, D and the output terminal VOUT, and comprises a first voltage generator VG 1 , a second voltage generator VG 2 , compensation capacitors CM 1  and CM 2  and switches S 1 , S 2 , S 3  and S 4 . The first voltage generator VG 1  is utilized for generating a first voltage V 1 , which is approximately equal to a steady state voltage of the node B. The second voltage generator VG 2  is utilized for generating a second voltage V 2 , which is approximately equal to a steady state voltage of the node D. The compensation capacitor CM 1  comprises a first terminal coupled to the switch S 2  and a second terminal coupled to the output terminal VOUT. The compensation capacitor CM 2  comprises a first terminal coupled to the switch S 4  and a second terminal coupled to the output terminal VOUT. In the operational amplifier  20 , the compensation capacitors CM 1  and CM 2  are charged and discharged according to switching of switches S 1 -S 4 , which helps loop stability. 
   Connections and operations of the switches S 1 -S 4  are further described as follows. The switch S 1  is coupled between the node B and the first terminal of the compensation capacitor CM 1 , and is utilized for selectively coupling the node B to the first terminal of the compensation capacitor CM 1  according to a control signal VCTR 1 . The switch S 2  is coupled between the first voltage generator VG 1  and the first terminal of the compensation capacitor CM 1 , and is utilized for selectively coupling the first voltage generator VG 1  to the first terminal of the compensation capacitor CM 1  according to a control signal VCTR 2 . The switch S 3  is coupled between the node D and the first terminal of the compensation capacitor CM 2 , and is utilized for selectively coupling the node D to the first terminal of the compensation capacitor CM 2  according to a control signal VCTR 3 . The switch S 4  is coupled between the second voltage generator VG 2  and the first terminal of the compensation capacitor CM 2 , and is utilized for selectively coupling the second voltage generator VG 2  to the first terminal of the compensation capacitor CM 2  according to a control signal VCTR 4 . 
   When the input signal of the operational amplifier  20  is in a transition state, which is a state the input signal transits from a high level to a low level and transits from the low level to the high level, the switches S 1  and S 3  are turned off respectively according to the control signals VCTR 1  and VCTR 3 , and the switches S 2  and S 4  are turned on respectively according to the control signals VCTR 2  and VCTR 4 . In this situation, the amplified signal outputted from the amplification stage circuit  21  does not pass through the compensation capacitors CM 1  and CM 2 . When the input signal of the operational amplifier  20  is in a steady state, the switches S 1  and S 3  are turned on respectively according to the control signals VCTR 1  and VCTR 3 , and the switches S 2  and S 4  are turned off respectively according to the control signals VCTR 2  and VCTR 4 , thus the amplified signal passes through the compensation capacitors CM 1  and CM 2  and frequency compensation is performed. 
   Note that, in the embodiment of the present invention, the first voltage V 1  generated by the first voltage generator VG 1  is approximately equal to the steady state voltage of the node B, and the second voltage V 2  generated by the second voltage generator VG 2  is approximately equal to the steady state voltage of the node D. Therefore, when the input signal of the operational amplifier  20  is in a steady state, a voltage level of the node A approaches the voltage level of the node B and a voltage level of the node C approaches the voltage level of the node D, so that charge sharing does not occur between the nodes A and B and between the nodes C and D. In other words, there is no charging or discharging on the nodes B and D, which helps decreasing unnecessary current consumption of the transistors MP 6  and MN 6 . Compared with the prior art, the embodiment of the present invention decreases current consumption of the operational amplifier  20  and does not increase the settling time. 
   Please note that, the present invention aims to control the voltages generated by the first voltage generator VG 1  and the second voltage generator VG 2  to avoid charging and discharging occurring on the nodes B and D when the input signal is in the steady state. Embodiments of the first voltage generator VG 1  and the second voltage generator VG 2  are described as follows, and those skilled in the art can make alterations and modifications accordingly. 
   Please refer to  FIG. 3 , which is a schematic diagram of an operational amplifier  30  according to an embodiment of the present invention.  FIG. 3  further illustrates an embodiment of the first voltage generator VG 1  and the second voltage generator VG 2 . Similar to the operational amplifier  20 , the operational amplifier  30  comprises an amplification stage circuit  31 , an output stage circuit  32  and a compensation circuit  33 . As shown in  FIG. 3 , the first voltage generator VG 1  comprises PMOS transistors MP 8  and MP 9  and the second voltage generator VG 2  comprises NMOS transistors MN 8  and MN 9 . The transistor MP 8  has a source coupled to a power supply terminal VDD, and a gate and a drain both coupled to the switch S 2 , in which gate voltage and drain voltage are both equal to the first voltage V 1  which is equal to the steady state voltage of the node B. The transistor MP 9  has a source coupled to the gate and the drain of the transistor MP 8 , a drain coupled to a ground terminal GND and a gate coupled to a bias VB in the amplification stage circuit  31 . The transistor MN 8  has a source coupled to the ground terminal GND, a gate and a drain both coupled to the switch S 4 , in which gate voltage and drain voltage are both equal to the second voltage V 2  which is equal to the steady state voltage of the node D. The transistor MN 9  has a source coupled to the gate and the drain of the transistor MN 8 , a drain coupled to the power supply terminal VDD and a gate coupled to the bias VB. 
   Operations of the switches S 1 -S 4  in the compensation circuit  33  is similar to those in the compensation circuit  23  shown in  FIG. 2 . Therefore, when the input signal of the operational amplifier  30  is in a steady state, a voltage level of the node A approaches the voltage of the node B and a voltage level of the node C approaches the voltage of the node D. That is, there is no charging or discharging on the nodes B and D. Current consumption of the transistors MP 6  and MN 6  in the operational amplifier  30  is therefore decreased. 
   Pleas refer to  FIG. 4 , which is a schematic diagram of an operational amplifier  40  according to an embodiment of the present invention.  FIG. 4  further illustrates another embodiment of the first voltage generator VG 1  and the second voltage generator VG 2 . Similar to the operational amplifier  20 , the operational amplifier  40  comprises an amplification stage circuit  41 , an output stage circuit  42  and a compensation circuit  43 . Different from  FIG. 3 , the first voltage generator VG 1  and the second voltage generator VG 2  are respectively implemented by unit gain operational amplifiers OP 1  and OP 2  in which an output voltage is equal to an input voltage. Operations of the switches S 1 -S 4  in the compensation circuit  43  is similar to those in the compensation circuit  23  shown in  FIG. 2 . Therefore, when an input voltage VIN_ 1  of the unit gain operational amplifier OP 1  is designed to be approximately equal to the steady state voltage of the node B and an input voltage VIN_ 2  of the unit gain operational amplifier OP 2  is designed to be approximately equal to the steady state voltage of the node D, charging sharing does not occur between the nodes A and B and between the nodes C and D when the input signal is in the steady state. In other words, there is no charging or discharging on the nodes B and D; current consumption of the transistors MP 6  and MN 6  in the operational amplifier  40  is therefore decreased. 
   Note that, operations of the switches S 1 -S 4  in the operational amplifiers  20 ,  30  and  40  is one of embodiments of the present invention, and those skilled in the art can make alterations and modifications accordingly. Please refer to  FIG. 5 , which is a schematic diagram of an operational amplifier  50  according to an embodiment of the present invention. Similar to the operational amplifier  30  in  FIG. 3 , the operational amplifier  50  comprises an amplification stage circuit  51 , an output stage circuit  52  and a compensation circuit  53 . The difference is that, in  FIG. 5 , the first voltage generator VG 1  only comprises the transistor MP 8  (without MP 9 ) and the second voltage generator VG 2  only comprises the transistor MN 8  (without MN 9 ). 
   Furthermore, operations of the switches S 1 -S 4  in the operational amplifier  50  is different from that in the operational amplifiers  20 ,  30  and  40 . When the input signal of the operational amplifier  50  is in a transition state from a low level to a high level, the switches S 1  and S 4  are turned on respectively according to the control signals VCTR 1  and VCTR 4 , and the switches S 2  and S 3  are turned off respectively according to the control signals VCTR 2  and VCTR 3 . On the other hand, when the input signal is in a transition state from the high level to the low level, the switches S 1  and S 4  are turned off respectively according to the control signals VCTR 1  and VCTR 4 , and the switches S 2  and S 3  are turned on respectively according to the control signals VCTR 2  and VCTR 3 . In addition, when the input signal is in a steady state, the switches S 1  and S 3  are turned on respectively according to the control signals VCTR 1  and VCTR 3 , and the switches S 2  and S 4  are turned off respectively according to the control signals VCTR 2  and VCTR 4 , thus the amplified signal passes through the compensation capacitors CM 1  and CM 2  and the amplified signal is therefore compensated. 
   As mentioned previously, switching of the switches S 1 -S 4  in a conventional amplifier is controlled by an external circuit. In comparison, the embodiment of the present invention further generates the control signals of the switches according to internal signals of the operational amplifier. Please refer to  FIG. 6 , which is a schematic diagram of a signal generation device  60  according to an embodiment of the present invention. The signal generation device  60  is installed in the operational amplifier, such as the operational amplifier  20 ,  30  or  40 , for generating the control signals VCTR 1 -VCTR 4 . 
   Take the operational amplifier  20  in the  FIG. 2  as an example to describe the signal generation device  60  as follows. The signal generation device  60  comprises a first comparator  600 , a second comparator  602  and a NOR gate  604 . The first comparator  600  is utilized for comparing a voltage level of the node E with a reference voltage Vref_A, for generating a comparison value OUT_A. Similarly, the second comparator  602  is utilized for comparing a voltage level of the node F with a reference voltage Vref_B, for generating a comparison value OUT_B. The NOR gate  604  is utilized for performing a logic operation on the comparison values OUT_A and OUT_B for generating a switch signal SW. The switch signal SW is utilized for generating the control signals VCTR 1 -VCTR 4 . 
   Criteria for the switch signal SW are described as follows. When the input signal of the operational amplifier  20  transits from the low level to the high level, the voltage level of the node E shall be decreased for increasing the voltage level of the output terminal VOUT. When the voltage level of the node E is lower than the reference voltage Vref_A, the switch signal SW has to control the switches S 1  and S 3  to turn off and control the switches S 2  and S 4  to turn on to make the compensation circuit  23  open, so that the amplified signal does not pass through the compensation circuit  23  and the slew rate of the operational amplifier  20  is therefore enhanced. Similarly, when the input signal of the operational amplifier  20  transits from the high level to the low level, the voltage level of the node F shall be increased for decreasing the voltage level of the output terminal VOUT. When the voltage level of the node F is higher than the reference voltage Vref_B, the switch signal SW has to control the switches S 1  and S 3  to turn off and control the switches S 2  and S 4  to turn on to make the compensation circuit  23  open to enhance the slew rate. 
   When the input signal of the operational amplifier  20  is in the steady state, the node E is in the steady state and the voltage level of the node E is higher than the reference voltage Vref_A; the node F is also in the steady state and the voltage level of the node F is lower than the reference voltage Vref_B. In this situation, the switch signal SW has to control the switches S 1  and S 3  to turn on and control the switches S 2  and S 4  to turn off. As a result, the amplified signal passes through the compensation capacitors CM 1  and CM 2  for frequency compensation, and the circuit will be stable. 
   In order to generate the switch signal SW and generate the control signals VCTR 1 -VCTR 4  accordingly, operations of the first comparator  600 , the second comparator  602  and the NOR gate  604  shall be implemented according to the above. 
   Please refer to  FIG. 7 , which is a schematic diagram of the first comparator  600  and the second comparator  602 . In  FIG. 7 , the first comparator  600  comprises PMOS transistors MP 10 , MP 11  and an NMOS transistor MN 10 . The transistor MP 10  has a source coupled to the power supply terminal VDD, a gate, and a drain coupled to the gate. The transistor MP 11  has a source coupled to the gate and the drain of the transistor MP 10 , a gate coupled to the node E, and a drain coupled to the NOR gate  604 , in which drain voltage of the transistor MP 11  is the comparison value OUT_A. The transistor MN 10  has a drain coupled to the drain of the transistor MP 11  and the NOR gate  604 , a gate having a voltage equal to the reference voltage Vref_A, and a source coupled to the ground terminal GND. 
   The second comparator  602  comprises a PMOS transistor MP 12 , NMOS transistors MN 11  and MN 12 . The transistor MP 12  has a source coupled to the power supply terminal VDD, a gate having a voltage equal to the reference voltage Vref_B, and a drain coupled to the NOR gate  604 , in which drain voltage of the transistor MP 12  is the comparison value OUT_B. The transistor MN 11  has a drain coupled to the drain of the transistor MP 12  and the NOR gate  604 , and a gate coupled to the node F. The transistor MN 12  has a drain and a gate both coupled to the source of the transistor MN 11 , and a source coupled to the ground terminal GND. 
   Please refer to  FIG. 6  and  FIG. 7 . When the voltage level of the node E is lower than the reference voltage Vref_A and is low enough, the transistor MP 10  is turned on and the comparison value OUT_A is pulled to a high level. When the voltage level of the node F is higher than the reference voltage Vref_B and is high enough, the transistor MN 12  is turned on and the comparison value OUT_B is pulled to a low level. When the above two conditions occur, the NOR gate  604  performs a logic operation on the comparison values OUT_A and OUT_B to generate the switch signal SW. In this situation, the control signals VCTR 1 -VCTR 4  generated according to the switch signal SW can control the switches S 1  and S 3  to turn off and control the switches S 2  and S 4  to turn on. On the other hand, when the input signal of the operational amplifier  20  is in the steady state, the nodes E and F are also in the steady state and therefore the transistors MP 10  and MN 12  are turn off. In this situation, the control signals VCTR 1 -VCTR 4  can control the switches S 1  and S 3  to turn on and control the switches S 2  and S 4  to turn off. 
   From the above, the control signals VCTR 1 -VCTR 4  are generated according to the switch signal SW, which is an internal signal generated by the circuit of the first comparator  600 , the second comparator  602  and the NOR gate in the operational amplifier according to the embodiment of the present invention. Please note that, the first comparator  600  and the second comparator in  FIG. 7  is one of embodiments of the present invention, and the switch signal SW can be generated by other circuits in the operational amplifier. 
   In conclusion, the embodiment of the present invention controls the bias of the compensation capacitors of the operational amplifier to avoid charging and discharging occurring between the amplification stage circuit and the compensation capacitor when the input signal is in the steady state. In addition, the embodiment of the present invention generates the control signals according to the internal signals in the operational amplifier instead of the external signals. Compared with the prior art, the embodiment of the present invention avoids unnecessary current consumption and enhances flexibility of the use of the operational amplifier. 
   Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.