CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation of U.S. patent application Ser. No. 09/516,961 (Atty. Docket No. 60178.300401) entitled Low Voltage Rail-to-Rail CMOS Output Stage, filed on Feb. 29, 2000. This application is related to U.S. patent application Ser. No. 09/516,008 (Atty. Docket No. 60178.300301) entitled Low Voltage Rail-to-Rail CMOS Input Stage, filed on an even day herewith on behalf of Troy Stockstad, the disclosure of which is incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    Operational amplifiers in current electronic devices are provided with an output stage for driving additional devices connected to the amplifier in a particular application. To be suitable for broad application, it is preferable to provide such output stages with various characteristics, such as a relatively large and symmetrical output swing, preferably rail-to-rail. It is also desirable to have the output able to both source and sink a substantial amount of current in order to drive loads having a significant capacitive component. In addition, the output should dissipate a relatively low quiescent power to minimize power consumption when not driving such loads. Obviously, other characteristics such as stability, manufacturability, etc. are also important considerations.  
           [0003]    Most prior art output stages capable of operating at one volt are push-pull class A output stages. In this case either the pull-up or the pull-down device is a current source, and the other device is configured in a common-source configuration. This results in a high level of power dissipation to drive large output loads.  
           [0004]    To minimize power dissipation in the output stage, class AB stages are often used. Such stages have relatively low quiescent power dissipation, yet are capable of driving large amounts of current.  
           [0005]    In bipolar technologies, low voltage push-pull output stages generally rely on controlling base current drive to the output transistors. Since bipolar transistors are current driven devices, the output current of the device can be controlled if the base is driven with a controlled current source. Since collector current is exponentially dependent upon the base-emitter voltage, a large change in output current can be realized for small changes in the base-emitter voltage. Thus, in a bipolar design capable of operating at one volt, a circuit may be designed to control the base current drive of the device, yet still achieve a high output current. However, in a CMOS circuit such techniques are not effective since the amount of output current is strictly a function of the amount of voltage between the gate and the source of the device (V GS ).  
           [0006]    CMOS push-pull output stages are generally designed such that one transistor is driven directly from the input of the output stage, and a complimentary transistor is driven by an output network. However, conventional CMOS output networks are problematic in that a conventional CMOS output network may or may not drive the complimentary transistor hard enough to create a symmetrical output. This problem is further increased at low voltages, such as at one volt.  
           [0007]    Prior art FIG. 1 is a schematic diagram of a conventional CMOS output stage  100 . The conventional output stage  100  includes P-channel transistor  102  and N-channel transistor  104  set in a push-pull configuration. In addition, output stage  100  includes P-channel transistor  106  and N-channel transistors  108 ,  110 , and  112 , as well as current source  114 .  
           [0008]    Conventional output stage  100  is an example of a IV CMOS push-pull output stage. Essentially, the drains of the P-channel transistor  102  and the N-channel transistor  104  are coupled together. In addition, the source of the P-channel transistor  102  is coupled to the positive power supply V CC , while the source of the N-channel transistor  104  is coupled to the negative power supply V EE . In this manner, the conventional output stage  100  achieves near rail-to-rail performance, until a load is placed at the output.  
           [0009]    In order to provide negative drive capability, the conventional output stage  100  must be operated at a high quiescent current. Current source  114 , along with the area ratios of NMOS transistors  108  and  104 , set the maximum sink current capability of the output stage. The output sink current in NMOS  104  is controlled by replica PMOS transistor  106 , which controls the bias to NMOS transistors  110  and  112 . NMOS  110  then modulates the bias to output NMOS  104 . The output drive capability of circuit  100  is not symmetrical, since the drain current in PMOS  102  is limited only by its V GS , while the NMOS  104  can only deliver I 114 ·((W/L 104 )/(W/L 108 )). This limits the type of applications that will function properly with output stage  100 .  
           [0010]    In view of the foregoing, what is needed is an output stage that provides near rail-to-rail performance, which does not require a high quiescent current to provide negative drive capability. Moreover, the output stage should be capable of operating from low supply voltages, such as slightly more than a single V GS  voltage.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention address this need by providing an output stage that provides essentially rail-to-rail performance, and operates from supply voltages down to slightly more than a single V GS  voltage. In one embodiment, an output stage suitable for low voltage operation and capable of providing an essentially symmetrical rail-to-rail output voltage is disclosed. The output stage includes a first field effect device having a first drain, a first gate, and a first source coupled to a power supply V CC . The output stage further includes a second field effect device complimentary to the first field effect device, having a second drain, a second gate, and a second source coupled to a power supply having a nominal voltage of V EE . Further, the second drain is coupled to the first drain. Also included in the output stage is an output sink network coupled to the second field effect device. The output sink network drives the second field effect device such that a product of a current in the first field effect device and a current in the second field effect device is essentially equal to a predetermined constant during operation of the output stage.  
           [0012]    In another embodiment, a method for providing an output signal from an output stage of a low voltage amplifier capable of providing a substantially rail-to-rail output voltage is disclosed. The method comprises providing an input signal to a first field effect device having a first drain, a first gate, and a first source coupled to a power supply V CC . Next, a second complimentary field effect device is driven utilizing an output sink network such that the product of the current in the first field effect device and the current in the second field effect device is essentially equal to a predetermined constant during operation of the amplifier.  
           [0013]    In yet another embodiment, an application specific integrated circuit (ASIC) having an output stage for a low voltage operational amplifier is disclosed. The ASIC includes a first field effect device having a first drain, a first gate, and a first source coupled to a power supply V CC . The ASIC further includes a second field effect device complimentary to the first field effect device, having a second drain, a second gate, and a second source coupled to a power supply having a nominal voltage of V EE . Further, the second drain is coupled to the first drain. Also included in the ASIC is an output sink network coupled to the second field effect device. The output sink network drives the second field effect device such that the product of the current in the first field effect device and the current in the second field effect device is essentially equal to a predetermined constant during operation of the output stage.  
           [0014]    An operational amplifier output stage is disclosed in a further embodiment of the present invention. The operational amplifier output stage includes a push-pull output network that receives a first input signal and a second input signal, the first input signal being provided by an input signal V IN . Also included in the operational amplifier output stage is an output sink network that provides the second input signal to the push-pull output network.  
           [0015]    Finally, an operational amplifier suitable for operating on low input voltage and capable of providing a substantially symmetrical rail-to-rail output voltage is disclosed. The operational amplifier includes an input stage and an output stage coupled to the input stage. Further, the output stage includes an output sink network.  
           [0016]    Advantageously, the present invention provides essentially rail-to-rail performance, and does not require a high quiescent current to provide negative drive capability. Furthermore, the output stage of the present invention is capable of operating from a low supply voltage of slightly more than a single V GS  voltage.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    The invention, together with further advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which:  
         [0018]    Prior Art FIG. 1 is a schematic diagram of a conventional output stage;  
         [0019]    [0019]FIG. 2 is a block diagram showing an operational amplifier, in accordance with one embodiment of the present invention;  
         [0020]    [0020]FIG. 3 is a block diagram of an output stage, in accordance with an embodiment of the present invention;  
         [0021]    [0021]FIG. 4 is a schematic diagram of an output stage, in accordance with one aspect of the present invention; and  
         [0022]    [0022]FIG. 5 is a schematic diagram of an output stage in accordance with another aspect of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]    An invention is disclosed for providing an output stage that achieves essentially symmetrical rail-to-rail performance, and can operate with a voltage supply of slightly more than a single V GS  voltage. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to those skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known process steps have not been described in detail in order not to unnecessarily obscure the present invention.  
         [0024]    [0024]FIG. 1 was described in terms of the prior art. FIG. 2 is a block diagram showing an operational amplifier  200 , in accordance with one embodiment of the present invention. The operational amplifier  200  includes an input stage  202  and an output stage  204 .  
         [0025]    In operation, the input stage  202  receives a differential input signal V IN . The input stage  202  then converts the differential input signal into a single output stage input signal, and then supplies the output stage input signal to the output stage  204 . The output stage  204  receives the output stage input signal and converts it to an amplified output voltage V O .  
         [0026]    The output stage  204  provides essentially rail-to-rail performance, and is capable of operating with a voltage supply as low as essentially a single V GS  voltage. As described in greater detail subsequently, the output stage  204  utilizes an output sink network to achieve this functionality.  
         [0027]    [0027]FIG. 3 is a block diagram of an output stage  204 , in accordance with an embodiment of the present invention. The output stage  204  includes a push-pull output  300  and output sink network  302 . In use, the push-pull output  300  receives two input signals. One signal is received from the source V IN , the other signal is received from the output sink network  302 .  
         [0028]    As shown in FIG. 3, one side of the push-pull output  300  is driven directly by the source signal V IN , while the other side is controlled by the output sink network  302 . The result is an output stage  204  that provides a symmetrical rail-to-rail output when driven at one volt.  
         [0029]    Referring next to FIG. 4, an output stage  400  is shown, in accordance with one embodiment of the present invention. The output stage  400  includes an output sink network  302 , and a push-pull output  300  having a P-channel transistor  402  and an N-channel transistor  404 . The source of the P-channel transistor  402  is coupled to V CC , while the source of the N-channel transistor  404  is coupled to V EE . Finally, the drain of both the P-channel transistor  402  and the N-channel transistor  404  are coupled together.  
         [0030]    In use, the P-channel transistor  402  is driven directly by the source voltage V IN , while the N-channel transistor  404  is driven by the output sink network  302 . To provide a push-pull output, the current in the P-channel transistor  402  and the N-channel transistor  404  are always equal to a constant when multiplied together.  
         [0031]    Thus, the present invention drives the P-channel transistor  402  directly with the source voltage V IN , and uses the output sink network to drive the N-channel transistor such that the product of the current in the P-channel transistor  402  and the N-channel transistor  404  is always equal to a predetermined constant. In other words, when the current in the P-channel transistor  402  is increased, the current in the N-channel transistor  404  is decreased, and vice-versa. It will be apparent to those skilled in the art that a similar approach is to connect voltage V IN  to the gate of NMOS transistor  404 , and have an output source network drive PMOS  402 .  
         [0032]    [0032]FIG. 5 is a schematic diagram of an output stage  500 , in accordance with one aspect of the present invention. The output stage  500  includes a push-pull output  300  having P-channel transistor  402  and N-channel transistor  404 , an output sink network  302 , and P-channel transistors  502 ,  504 , and  506 .  
         [0033]    The P-channel transistor  402  is configured in a common source configuration. P-channel transistor  502  is configured to replicate P-channel transistor  402  in order to track the current in transistor  402  at a predetermined ratio, such as 6:1. Thus, there is six times as much current in P-channel transistor  402  as there is in P-channel transistor  502 . This current is then sent to the output sink network  302  to provide the above mentioned constant current product of transistors  402  and  404 , as described in greater detail subsequently.  
         [0034]    The output sink network  302  includes a loop of V GS  voltages. Beginning on the left side of FIG. 5, N-channel transistor  508  is coupled in a diode connection providing one V GS , and diode  510  provides a diode change to node n 6 . Both N-channel transistor  508  and diode  510  have a current I. Thus, node n 6  is essentially a bias node having one V GS  and one diode drop. Then from the gate of N-channel transistor  512  at node n 6  to its source there is a one V GS  drop. N-channel transistor  514  provides one V GS  UP from its source to its gate to node n 13 . Then back down one diode drop from diode  516 . Finally, N-channel transistor  404  provides one V GS  drop.  
         [0035]    Thus, going through the loop of V GS  voltages, there is the V GS  for N-channel transistor  508 , plus the diode drop of diode  510 , minus a V GS  of P-channel transistor  402 , plus the V GS  of N-channel transistor  514 , minus the diode drop of diode  516 , minus the V GS  of N-channel transistor  404 , all of which is equal to zero as set forth in the following equations:  
         ( I   P/3   −I )/( W/L   512 )= I   D512   (1)  
           I   D   =I   D0 ( W/L ) exp ( V   gs   /nV   T ) exp (− V   S   /V   T )− exp (− V   d   /V   T )  (2)  
           nV   T ln( I /( I   D0 ( W/L   508 ))+ V   T ln( I/I   S )− nV   T ln(( I   P/3   −I )/( I   D0 ( W/L   512 )))+ nV   T ln(2 I /( I   D0 ( W/L   514 )))− V   T ln(2 I/ 2 I   S )− nV   T ln( I   N /( I   D0 ( W/L   404 )))=0  (3)  
         2 I   2 /(( W/L   508 )( W/L   514 ))=(( I   P/3   −I )( I   N ))/(( W/L   512 )( W/L   404 ))  (4)  
           K   1 (( I   P )( I   N ))/( K   2 )→push-pull action  
         [0036]    For quiescent point, I P =I N =I Q    
         [0037]    The above equations assume all MOSFETS operate in the sub threshold region. To calculate the quiescent current IQ the following equation can be used:  
         (2 I   2 )/(( W/L   508 )( W/L   514 ))=((1/3)( I   Q   2 )−( I   Q )( I ))/(( W/L   512 )( W/L   404 ))  (5)  
         →(1/3)( I   Q   2 )−( I   Q )( I )−(2 I   2 )((( W/L   512 )( W/L   404 ))/(( W/L   508 )( W/L   514 )))=0,  
         [0038]    which can be solved using the quadratic equation.  
         [0039]    Similar equations can be derived for the MOSFETS operating in saturation. Essentially, in saturation:  
           I   N   +I   P   =K,   (1)  
         [0040]    wherein K is a constant value.  
         [0041]    As can be seen in the above equations, diodes  510  and  516  cancel each other out. Their primary purpose is to create a voltage at the source of N-channel transistors  512  and  514  at node n 10 , which creates a current so the current sources can operate. In an alternate embodiment, diodes  510  and  516  mat be replaced by resistors, which perform essentially the same function.  
         [0042]    Referring to equation (3) above, two times I 2 , which is set by N-channel transistors  506  and  504 , divided by the size of N-channel transistors  508  and  514  is equal to I P , which is the current in P-channel transistor  402 , multiplied by I N , which is the current in N-channel transistor  404 , divided by three, which is derived from the ratio of transistors  402  and  502  and  520  and  522 , times the size of transistors  402  and  404 . Thus, a symmetrical rail-to-rail push-pull output is achieved.  
         [0043]    In use, output stage  500  is connected to the output of the input stage, as shown in FIG. 2, and the P-channel devices are controlled directly. The output sink network  302  determines how to bias output P-channel transistor  402  such that a push-pull output is achieved.  
         [0044]    In the present invention, there is no more than one V GS  and two V Dsat  from either rail. Thus, the present invention will operate at less than one volt.  
         [0045]    In addition, unlike conventional output stages, the present invention is able to drive the gate voltage of N-channel transistor  404  to nearly V CC . For example, if transistor  402  is turned off, so the gate voltage of transistor  402  is close to V CC , the current in transistor  502  is reduced. But, transistor  504  is biased at  21 , while transistor  518  is biased at I. Thus, the voltage at the gate of transistor  404  will increase to within a saturation voltage of transistor  504  and the diode drop of diode  516 . Thus, when the output is to be driven very hard, where the amplifier is open loop (i.e. the differential input voltage is large), the voltage at the gate of transistor  404  will increase dramatically, thus providing a very good output drive.  
         [0046]    It should again be noted that although output stage  500  has been described with an output sink network to control NMOS  404 , an alternative approach is to use an output source network similar to circuit  302  to drive PMOS  402 , and drive NMOS  404  directly from input V IN .  
         [0047]    While the present invention has been described in terms of several preferred embodiments, there are many alterations, permutations, and equivalents which may fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and apparatuses of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and equivalents as fall within the true spirit and scope of the present invention.
An output stage suitable for low voltage operation and capable of providing an essentially symmetrical rail-to-rail output voltage including a first field effect device having a first drain, a first gate, and a first source coupled to a power supply V CC. The output stage further includes a second field effect device complimentary to the first field effect device, having a second drain, a second gate, and a second source coupled to a power supply having a nominal voltage of V EE. Further, the second drain is coupled to the first drain. Further, the output stage is an output sink network coupled to the second field effect device. The output sink network drives the second field effect device such that a current is produced in the first field effect device and a current in the second field effect device is essentially equal to a predetermined constant during operation of the output stage.