Patent Publication Number: US-7907013-B2

Title: Class AB output stage with programmable bias point control

Description:
FIELD OF THE INVENTION 
     The present invention is related generally to a class AB output stage and, more particularly, to the bias point control of a class AB output stage. 
     BACKGROUND OF THE INVENTION 
     Conventionally, a class AB output stage is used to drive big MOS transistors in power applications. As shown in  FIG. 1 , a typical class AB output stage  10  includes a driver  12  to provide two drive signals UH and UL, and two bias voltage sources  16  and  18  to provide bias voltages VOS 1  and VOS 2 , respectively, which are used to level shift the drive signals UH and UL to produce the gate voltages for driving a pair of high side transistor MP and low side transistor MN serially connected between a power supply Vcc and a ground terminal GND, in order to supply a current for a load RL connected to an output LX. Typically, the driver  12  uses an operational amplifier  14  to produce the drive signals UH and UL according to an input signal Vin and a feedback signal from the output LX. The bias voltages VOS 1  and VOS 2  are the key to the quiescent current control and total harmonic distortion (THD) of this circuit. The quiescent current refers to the current consumed by this circuit from the power supply Vcc under loadless condition, i.e., without the load RL. For driving a resistive load RL, there are many factors, including crossover distortion, power consumption through the big MOS paths and loop stability, must be taken into consideration at the same time for the design of the class AB output stage  10 . The target of the design region is shown in  FIG. 2 , in which the X-axis represents the bias voltages VOS 1  and VOS 2 , the left Y-axis represents the THD, the right Y-axis represents the quiescent current IQ, the curve  20  represents the relationship between the THD and the bias voltages VOS 1  and VOS 2 , and the curve  22  represents the relationship between the quiescent current IQ and the bias voltages VOS 1  and VOS 2 . As shown by the curves  20  and  22 , as the bias voltages VOS 1  and VOS 2  increase, the THD decreases whereas the quiescent current IQ increases; contrarily, for smaller quiescent current IQ, the THD is greater. A dash circle  24  marks the ideal design region of the bias voltages VOS 1  and VOS 2 , where it may have lower quiescent current IQ and better THD performance at the same time. However, this design region  24  is quite sensitive to process variation and as a result, the bias voltages VOS 1  and VOS 2  may deviate from the design region  24  due to the process variation in real applications. Therefore, solutions are needed for the bias voltages VOS 1  and VOS 2  to return to the target design region  24  against process variation. 
     U.S. Pat. No. 5,481,213 to Johnson has proposed a cross-conduction prevention circuit for power amplifier output stage, which may get a best THD performance and a reasonable quiescent current if the width/length size of some MOS transistors in a fill-in circuit is well designed. However, this solution requires adding the fill-in circuit and an extra control circuit into the output stage and thus results in a relatively complicated structure. Moreover, the fill-in circuit and the extra control circuit may be interfered by each other, thus leading to system instability. On the other hand, the bias point of this output stage is not programmable and thus cannot be adjusted to reduce or remove the error resulted from process variation. 
     Therefore, a simpler class AB output stage with programmable bias point control is desired. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a class AB output stage with programmable bias point control. 
     Another object of the present invention is to provide a programmable bias point control method for a class AB output stage. 
     According to the present invention, to drive a pair of serially connected high side and low side transistors, a class AB output stage includes a driver to generate a first drive signal and a second drive signal, a first bias voltage source to provide a first bias voltage to level shift the first drive signal to thereby produce a level shifted first drive signal to drive the high side transistor, a second bias voltage source to provide a second bias voltage to level shift the second drive signal to thereby produce a level shifted second drive signal to drive the low side transistor, and a control circuit to provide a control signal to adjust the first and second bias voltages. 
     According to the present invention, for a class AB output stage to drive a pair of serially connected high side and low side transistors, a programmable bias point control method generates a first drive signal and a second drive signal, level shifts the first and second drive signals with a first bias voltage and a second bias voltage to produce a level shifted first drive signal and a level shifted second drive signal to drive the high side and low side transistors, respectively, and adjusts the first and second bias voltages with a control signal. 
     According to the present invention, the operation point of the class AB output stage will automatically return to a target operation region by the control signal adjusting the first and second bias voltages, and the bias point of the class AB output stage is programmable by setting a parameter in the control circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects, features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a circuit diagram of a typical class AB output stage; 
         FIG. 2  is a schematic diagram showing the operation region of a conventional class AB output stage; 
         FIG. 3  is a circuit diagram of an embodiment according to the present invention; and 
         FIG. 4  is a schematic diagram showing the operation region of a class AB output stage according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An embodiment according to the present invention is shown in  FIG. 3 , in which a class AB output stage  30  includes a driver  32  to generate drive signals UH and UL, two controlled bias voltage sources  36  and  38  to provide adjustable bias voltages VOS 1  and VOS 2  to level shift the drive signals UH and UL, respectively, and a control circuit  40  to provide a control signal i 3  to adjust the bias voltages VOS 1  and VOS 2 , and the level shifted drive signals are then used to drive a pair of serially connected high side transistor MP and low side transistor MN, respectively, between a power supply Vcc and a ground terminal GND. The diver  32  includes an operational amplifier  34  which has two inputs to receive an input signal Vin and a feedback signal from the node LX, respectively, and two outputs to provide the opposite-in-phase drive signals UH and UL, respectively. In the control circuit  40 , a transistor MP 2  is common gated to the high side transistor MP to mirror the current therein to generate a current Id 1 , a transistor MN 2  is common gated to the low side transistor MP to mirror the current therein to generate a current Id 2 , a controlled current source  42  providing a current io is serially connected to the transistor MP 2 , a controlled current source  44  providing a current io is serially connected to the transistor MN 2 , and a logic circuit  46  generates the control signal i 3  according to the difference i 1  between the currents Id 1  and io and the difference i 2  between the currents Id 2  and io, i.e., i 3  is a function of i 1  and i 2 . The transistor MP 2  is proportional to the high side transistor MP in size such that the current Id 1  is proportional to the current in the high side transistor MP with a predetermined ratio therebetween. Likewise, the transistor MN 2  is proportional to the low side transistor MN in size such that the current Id 2  is proportional to the current in the low side transistor MP with a predetermined ratio therebetween. Both the currents io provided by the current sources  42  and  44  are externally adjustable. 
       FIG. 4  is a schematic diagram showing the operation region of the class AB output stage  30 , in which the X-axis represents the bias voltages VOS 1  and VOS 2  or the currents io, the left Y-axis represents the THD, and the right Y-axis represents the quiescent current IQ, the curve  50  represents the relationship between the THD and the bias voltages VOS 1  and VOS 2 , the curve  52  represents the relationship between the quiescent current IQ and the bias voltages VOS 1  and VOS 2 , and the region B represents an ideal design region. The operation of the class AB output stage  30  is described by referring to FIGS,  3  and  4 . It may greatly increase the quiescent current IQ of the high side transistor MP and low side transistor MN at first, by giving great bias voltages VOS 1  and VOS 2  provided by the bias voltage sources  36  and  38 , in order to get better THD performance, where the operation point is in a region A to the right of the region B. In this case, it can be ensured that, even in the presence of process variation, the bias points are located to the right of the region B. However, the overlarge quiescent current IQ is not preferred. Thus, the currents io provided by the current sources  42  and  44  are adjusted in such a way that the bias points are shifted from the region A to the region B. The parameter io is programmable, and a larger io will lead to a larger quiescent current IQ but better THD performance. In a case, the logic circuit  46  selects between the differences i 1  and i 2 , e.g., the smaller one thereof, as the control signal i 3  to adjust the bias voltages VOS 1  and VOS 2  provided by the bias voltage sources  36  and  38 . 
     If the differences i 1  and i 2  are both large, then i 3  is large, and it will decrease the bias voltages VOS 1  and VOS 2  and thereby reduce the quiescent current IQ of the high side transistor MP and low side transistor MN. The ratio Id 1 /Id 2  will also become smaller. Finally, i 1 , i 2 , io and i 3  will reach a balance point. In other words, the bias point of the class AB output stage  30  is programmable by setting the currents io provided by the current sources  42  and  44 . The above illustrates that the bias voltages VOS 1  and VOS 2  are set great at first to locate the bias point in a region A to the right of the region B. In another embodiment, it may set the bias voltages VOS 1  and VOS 2  small at first to locate the bias point in a region to the left of the region B, and then adjusts the parameter io to be smaller to shift the bias point to return to the region B. 
     By adjusting the parameter io, the class AB output stage  30  may adjust the bias voltages VOS 1  and VOS 2  to shift the bias point to the design region B. In particular, by setting the bias point in a higher region or a lower position at first, followed with adjusting the parameter io to shift the bias point, the process variation issue is removed. According to the present invention, only a control circuit  40  is added, without introducing circuit interference, it may remain system stability. 
     While the present invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and scope thereof as set forth in the appended claims.