Abstract:
An output stage circuit includes: a first transistor, including a first terminal coupled to a first node, a second terminal coupled to an output terminal, a third terminal coupled to an input terminal for receiving an input voltage, and a fourth terminal coupled to a first power terminal for receiving a first voltage; a second transistor, including a first terminal coupled to a second node, a second terminal coupled to the output terminal, a third terminal coupled to the input terminal for receiving the input voltage, and a fourth terminal coupled to ground; and a current source, coupled to the output terminal for providing a constant current.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an output stage circuit, and more particularly, to an output stage circuit capable of eliminating the body effect and applied in a half supply voltage structure. 
     2. Description of the Prior Art 
     An operational amplifier (op-amp) is a basic circuit component, frequently used in analog integrated circuits. For reducing power consumption, the conventional operational amplifier circuit is utilized in a partition supply voltage structure. An illustration of this structure is shown in  FIG. 1 , which is a schematic diagram of a conventional operational amplifier circuit utilizing a partition supply voltage structure. The operational amplifier circuit  10  comprises an operational amplifier OP 1  and an operational amplifier OP 2 . The operational amplifier OP 1  receives a first voltage VDD through a first power terminal PW 1  and receives a second voltage VDD_H through a second power terminal PW 2 . The operational amplifier OP 2  receives the second voltage VDD_H through the second power terminal PW 2  and is coupled to ground GND through a third power terminal PW 3 . In such a condition, if the second voltage VDD_H is half the first voltage VDD, the operational amplifiers OP 1  and OP 2  are utilized in a half supply voltage structure: the operational amplifier circuit  10  is a half supply voltage operational amplifier. Supply voltage of the operational amplifier OP 1  is within a range from the voltage VDD to the voltage ½ VDD. Supply voltage of the operational amplifier OP 2  is within a range from the voltage ½ VDD to ground. In such a condition, the output interval of the operational amplifier OP 1  is within a range from the voltage VDD to the voltage ½ VDD and the output interval of the operational amplifier OP 2  is within a range from the voltage ½ VDD to ground. As a result, the power consumption of the operational amplifier circuit  10  can be significantly reduced. 
     Although the above circuit utilizing the partition supply voltage structure may reduce power consumption, the operational amplifier circuit may work abnormally due to the body effect. Please refer to  FIG. 2 , which is a schematic diagram of the operational amplifier OP 1  shown in  FIG. 1 . As shown in  FIG. 2 , the operational amplifier OP 1  comprises an output stage circuit  202 . The output stage circuit  202  consists of a transistor NOUT and a transistor POUT, wherein the transistor NOUT and the transistor POUT are in a cascaded formation. The base of the transistor NOUT is coupled to the lowest voltage of the operational amplifier circuit  10  (i.e. to ground) and the supply voltage interval is within a range from the voltage VDD to the voltage ½ VDD (i.e. the source voltage of the transistor NOUT is ½ VDD). In such a condition, the transistor NOUT has the body effect, such that the threshold voltage of the transistor NOUT increases. For the output stage circuit  202 , which is utilized for providing a huge current to drive post-stage loading in the operational amplifier OP 1 , the transistor NB 1  may be cut off when the gate voltage of the transistor NOUT is significantly increased due to the serious body effect. As a result, the output current of the output stage  202  may be limited to an extremely small current, such that the operational amplifier OP 1  works abnormally. 
     Please refer to  FIG. 3 , which is a schematic diagram of the operational amplifier OP 2  shown in  FIG. 1 . As shown in  FIG. 3 , the operational amplifier OP 2  consists of a transistor NOUT and a transistor POUT, wherein the transistor NOUT and the transistor POUT are in a cascaded formation. The base of the transistor POUT is coupled to the highest voltage of the operational amplifier circuit  10  (i.e. to the voltage VDD). Since the supply voltage interval of the operational amplifier OP 2  is within a range from the voltage ½ VDD to ground, the source voltage of the transistor POUT is ½ VDD. In such a condition, the threshold voltage of the transistor POUT is increased due to the body effect. The transistor PB 1  may be cut off when the gate voltage of the transistor POUT decreases significantly due to the serious body effect, such that the operational amplifier OP 2  works abnormally. 
     In the prior art, an independent P-well and independent N-well provided by special processes are used for eliminating the body effect generated when utilizing the above partition supply voltage structure. Utilizing these special processes, however, causes the manufacturing cost of the integrated circuit to be greatly increased, which is not ideal for the designer of the integrated circuit. How to eliminate the body effect generated by utilizing the partition supply voltage structure without using the special processes has therefore become a problem to be solved in the industry. 
     SUMMARY OF THE INVENTION 
     Therefore, the present invention provides an output stage circuit utilized in a half supply voltage structure, which is capable of eliminating the body effect. 
     The present invention discloses an output stage circuit. The output stage circuit comprises a first transistor, comprising a first terminal coupled to a first node, a second terminal coupled to an output terminal, a third terminal coupled to an input terminal for receiving an input voltage, and a fourth terminal coupled to a first power terminal for receiving a first voltage; a second transistor, comprising a first terminal coupled to a second node, a second terminal coupled to the output terminal, a third terminal coupled to the input terminal for receiving the input voltage, and a fourth terminal coupled to ground; and a current source, coupled to the output terminal for providing a constant current. 
     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 a conventional operational amplifier utilizing the partition supply voltage structure. 
         FIG. 2  is a schematic diagram of the operational amplifier shown in  FIG. 1 . 
         FIG. 3  is another schematic diagram of the operational amplifier shown in  FIG. 1 . 
         FIG. 4  is a schematic diagram of an output stage circuit according to an embodiment of the present invention. 
         FIG. 5A  and  FIG. 5B  are voltage-current characteristic diagrams of the transistor according to an embodiment of the present invention. 
         FIG. 6  is a schematic diagram of an output stage circuit according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Please refer to  FIG. 4 , which is a schematic diagram of an output stage circuit  40  according to an embodiment of the present invention. The output stage circuit  40  is an output stage circuit utilized in the operational amplifier OP 1  shown in  FIG. 1 . It is assumed that the output stage circuit  40  is utilized in a half supply voltage structure and the supply voltage is within a range from the voltage VDD to the voltage ½ VDD, i.e. the second voltage VDD_H is the voltage ½ VDD. The output stage circuit  40  is utilized for outputting an output voltage VOUT according to an input voltage VIN of an input terminal IN, and transmitting the output voltage VOUT through an output terminal OUT. As shown in  FIG. 4 , the output stage circuit  40  comprises transistors  400  and  402  and a current source  404 . The transistors  402  and  404  are cascaded. The transistor  400  is a P-type MOS, for providing a current ID_P to the output terminal OUT. The transistor  402  is an N-type MOS, for providing a current ID_N to the output terminal OUT. The sources of the transistors  400  and  402  are coupled to the first power terminal PW 1  and a second power terminal PW 2 , respectively, for receiving the first voltage VDD and the second voltage VDD_H. The base of the transistors  400  and  403  are coupled to the first power terminal and ground, respectively. The current source  404  is a constant current source coupled to the output terminal OUT, for providing a constant current I_BIAS. 
     Since the base of the transistor  402  and the source of the transistor  402  respectively receive different voltages, the transistor  402  would have the body effect. The output stage circuit  40  needs to continuously generate a constant current when the input voltage VIN is at a normal biasing point. In such a condition, the transistor  402  is cut off. The required constant current is therefore provided by the constant current I_BIAS of the current source  404 . When the input voltage VIN increases, the output voltage VOUT decreases. In such a condition, the transistor  402  is turned on for providing additional current to decrease the output voltage VOUT. As a result, the combination of the transistor  402  with the body effect and the current source  404  is equivalent to a transistor without the body effect. Via the co-operation of the transistor  402  and the current source  404 , the output stage circuit  40  with the body effect can normally generate a biasing current, and the transient charging/discharging behavior thereof also works normally and has a driving capability not limited by said biasing current. 
     Further, since the present invention adds the current source  404  at the drain of the transistor  402  and the current source  404  replaces the transistor  402  to generate the constant current required when the output stage circuit  40  operates in a steady state, the transistor  402  is cut off when the output stage circuit  40  operates in the steady state. The gate voltage of the transistors  400  and  402  increases when the output stage circuit  40  needs to discharge an external loading, such that the transistor  402  is turned on for discharging. The discharging current is therefore not limited by the constant current I_BIAS generated by the current source  404 . When the external loading is discharged to a certain voltage level, the gate voltage of the transistor  402  decreases to the original biasing point and cuts off the transistor  402 . As can be seen above, the combination of the transistor  402  with the body effect and the current source is equivalent to a transistor without the body effect. 
     Please refer to  FIG. 5A  and  FIG. 5B , wherein  FIG. 5A  is a voltage-current characteristic diagram of the transistor  402 . A first curve C 1  is a characteristic curve of the transistor  402  without the body effect. The second curve C 2  is a characteristic curve of the transistor  402  with the body effect. As shown in the first curve C 1 , the transistor  402  generates a constant current I_BIAS when the transistor  402  is at the normal biasing point. The output current ID_N increases with the input voltage VIN. As shown in the second curve C 2 , the body effect results in the current of the transistor  402  being limited to an extremely small current when the transistor  402  is in the normal biasing point. The output stage circuit  40  therefore cannot work normally. Please refer to  FIG. 5B . Via the current source  404  providing the constant current I_BIAS, the second curve C 2  is shifted upward (i.e. configuring the current source  404 ) and the required biasing current of the output stage circuit  40  can be achieved. The second curve C 2  is therefore equivalent to the first curve C 1 . The transient charging/discharging behavior of the combination of the current source and the transistor  402  with the body effect is equivalent to the transient charging/discharging behavior of transistor  402  without the body effect. In other words, through configuring the current source  404  in the output stage circuit  40 , the output stage circuit  40  with the body effect can normally generate a biasing current and can provide driving capability without being limiting by the biasing current, such that the influence generated by the body effect can be eliminated. 
     Please refer to  FIG. 6 , which is a schematic diagram of an output stage circuit  60  according to another embodiment of the present invention. Since components annotated with the same numerals in  FIG. 6  and in  FIG. 4  have similar operational methods and functions, a detailed description and connecting methods thereof are not described herein for brevity. The output stage circuit  60  is an output stage circuit utilized in the operational amplifier OP 2  shown in  FIG. 1 , for outputting an output voltage VOUT at an output terminal OUT according to an input voltage VIN of an input terminal IN. Assume the output stage circuit  60  is utilized in a half supply voltage structure, and the supply voltage is within a range from the voltage ½ VDD to ground, i.e. the second voltage VDD_H is the voltage ½ VDD. As shown in  FIG. 6 , the output stage circuit  60  comprises transistors  600  and  602  and a current source  604 . The current source  604  is a constant current source coupled to the output terminal OUT, for providing a constant current I_BIAS. In contrast with the output stage circuit  40  shown in  FIG. 4 , the sources of the transistors  600  and  602  are coupled to the second power terminal PW 2  and to ground, respectively, for receiving the second voltage VDD_H and ground voltage. The base of the transistors  400  and  403  are coupled to the first power terminal PW 1  and to ground GND, respectively. 
     The output stage circuit  60  needs to provide a constant current when the input voltage VIN is at a normal biasing point. In such a condition, the transistor  600  is cut off and the required constant current is provided by the constant current I_BIAS generated by the current source  604 . When the input voltage VIN decreases, the output voltage VOUT increases. In such a condition, the transistor  600  is turned on for providing additional current to increase the output voltage VOUT. The combination of the transistor  600  with the body effect and the current source  604  is equivalent to the transistor  600  without the body effect. Detailed charging/discharging behavior of the output stage circuit  60  can be known by referring to the description of the output stage circuit  40 , and is therefore not described herein for brevity. Via the co-operation of the transistor  600  and the current source  604 , the output stage circuit  60  can normally generate the biasing current and the charging/discharging behavior thereof can work normally when the output voltage circuit  60  is a half supply voltage. The driving capability of the output stage circuit  60  is therefore not limited by the biasing current. 
     The objective of the present invention is to eliminate the body effect of the output stage circuit utilized in a half supply voltage via configuring a constant current source in the output stage circuit. According to different applications, those skilled in the art can conceive appropriate alternations and modifications. For example, the gate of the transistor  400  and the gate of the transistor  402  can be coupled to different input terminals, as long as the output stage circuit  40  can generate the proper output voltage VOUT. Such modifications also fall within the scope of the present application. 
     To sum up, when operating in a half supply voltage structure, a prior art output stage circuit needs special process for providing an independent P-well and an independent N-well, in order to avoid the body effect. In comparison, the output stage circuit of the present application utilizes a constant current source for assisting operations of the output stage circuit, such that the output stage circuit with the body effect can normally generate a biasing current and charging/discharging behavior thereof will work normally when utilized in a half supply voltage structure. The driving capability of the output stage circuit of the present invention is therefore not limited by the biasing current. As a result, the output stage circuit of the present invention can completely eliminate the influence of the body effect. Moreover, the present invention does not need special processes for providing the independent P-well and N-well, thereby reducing manufacturing costs of the integrated chip. 
     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. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.