Abstract:
A class AB input differential amplifier employs a single loop output common mode feedback circuit (CMFC) to achieve high performance by controlling the common mode output voltage. The CMFC includes a small amplifier to compare the common mode voltage at the output with a desired voltage specified at the common mode output voltage pin. Having only one loop to control this voltage instead of two makes the design more reliable and easier to compensate since there is no need to worry about how multiple loops will interact.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to differential amplifiers, and more particularly to an amplifier with a class AB differential input stage that employs a single loop output common mode feedback circuit to achieve high performance as compared to amplifiers with class AB differential input stages that employ two loop solutions which have loop interplay. 
     2. Description of the Prior Art 
     Differential amplifiers are very useful devices when dealing with differential systems. They have several advantages such as very good common mode rejection ratio since everything that is common will get canceled at the differential output. Some applications of differential amplifiers include single ended to differential conversion, differential ADC driver, differential transmitter and receiver, and output level shifter. 
     Differential amplifiers are not too different from single ended amplifiers. Several single ended topologies can be converted to differential topologies. The most obvious difference between differential amplifiers and their single ended counterparts is that they have two outputs instead of one. With the addition of this output, another difference arises, which is the “output common mode feedback circuit” (CMFC). Extra circuitry is necessary to direct the outputs where to go DC wise. Since this signal is common to both outputs differentially, it will cancel, leaving at the differential output only the amplified differential input signal. 
     Implementing such a common mode circuit is challenging. FIG. 1 illustrates one implementation of a differential amplifier  100  that includes an emitter coupled pair input stage  102  with a folded cascode gain stage  104 . The CMFC includes an error amplifier  106  that compares the common mode voltage at the outputs to the voltage set on the V ocm  pin  108  (seen defaulted to mid rail in FIG.  1 ). The error amplifier  106  adjusts the common mode voltage by changing the voltage in the bases of transistors Q 5  and Q 6 . The feedback path of the error amplifier  106  in FIG. 1 is from the output of the error amplifier  106  to the bases of transistors Q 5  and Q 6 , to the outputs of the main amplifier through the buffers  110 ,  112 , and back into the positive input of the error amplifier  106 . 
     The class AB input stage in a differential configuration has several advantages over the class A single emitter coupled pair input stage illustrated in FIG.  1 . Some of these advantages include better slew rate and better THD performance, especially at high frequencies. It would be desirable to have a way of implementing the foregoing CMFC on this topology. FIG. 2 is a schematic diagram illustrating a differential class AB amplifier  200  without the CMFC. It can be seen this topology is quite different from the one shown in FIG.  1 . One of the primary differences between this kind of circuit and the circuit in FIG. 2 is that in FIG. 1 it is only necessary to manipulate the current of transistors Q 5  and Q 6  to control the output common mode voltage. In the circuit of FIG. 2, it is necessary to manipulate the top mirrors Q 5 , Q 6  and Q 13 , Q 14  as well as the bottom mirrors Q 7 , Q 8  and Q 15 , Q 16 . The conventional prior art solution includes the use of two loops to control the two sets of mirrors, and that way control the output common mode voltage. This conventional approach has been problematic however since it results in undesirable distortion associated with additional parasitics and mismatch between the loops, stability problems due to complexity of the multiple control loops and the interaction between them, added complexity, and circuitry that consumes additional die area. 
     In view of the foregoing, it would be desirable to have a way of implementing the CMFC on a class AB input stage in a differential configuration in a manner that does not result in additional distortion, or stability problems due to increased complexity of multiple control loops, and that does not undesirably increase the required die area. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an amplifier with a class AB differential input stage that employs a single loop output common mode feedback circuit (CMFC) to achieve high performance by controlling the common mode output voltage. The CMFC includes a small amplifier to compare the voltage at the outputs with a desired voltage. Having only one loop to control this voltage instead of two makes the design more reliable and easier to compensate since there is no need to worry about how multiple loops will interact. The distortion caused by the nonlinear parasitics associated with the CMFC are minimized by connecting the CMFC to a point at the input where the voltage does not change as much as in the output of the input stage, where the full voltage signal is seen. 
     According to one aspect of the invention, an amplifier with a class AB input stage in a differential configuration is implemented with common mode feedback circuitry having only one loop to control the output common mode voltage; 
     According to another aspect of the invention, an amplifier with a class AB input stage in a differential configuration is implemented with common mode feedback circuitry (CMFC) having only one loop to control the output common mode voltage such that distortion caused by the CMFC is minimized; 
     According to yet another aspect of the invention, an amplifier with a class AB input stage in a differential configuration is implemented with common mode feedback circuitry (CMFC) having only one loop to control the output common mode voltage such that interaction between multiple loops caused by a CMFC is eliminated; 
     According to still another aspect of the invention, an amplifier with a class AB input stage in a differential configuration is implemented with common mode feedback circuitry (CMFC) having only one loop to control the output common mode voltage such that the CMFC is less complex and requires less die area than known two loop CMFC implementations associated with class AB differential amplifiers. 
     According to still another aspect of the invention, an amplifier with a class AB input stage in a differential configuration is implemented with common mode feedback circuitry (CMFC) having only one loop to control the output common mode voltage such that the resultant class AB differential amplifier power consumption is reduced when compared with conventional class AB differential amplifier architectures that employ two loops to control the common mode output voltage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other aspects and features of the present invention and many of the attendant advantages of the present invention will be readily appreciated as the same become better understood by reference to the following detailed description when considered in connection with the accompanying drawings in which like reference numerals designate like parts throughout the figures thereof and wherein: 
     FIG. 1 is a schematic diagram that illustrates one implementation of an amplifier with a class A differential input stage that includes an emitter coupled pair input stage with a folded cascode gain stage; 
     FIG. 2 is a schematic diagram that illustrates a conventional amplifier with a differential class AB input stage without common mode feedback circuitry; 
     FIG. 3 is a schematic diagram illustrating the conventional amplifier shown in FIG. 2 modified with common mode feedback circuitry according to one embodiment of the present invention; and 
     FIG. 4 is a schematic diagram illustrating one implementation of a common mode feedback circuitry amplifier according to one embodiment of the present invention. 
     While the above-identified drawing figures set forth alternative embodiments, other embodiments of the present invention are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The embodiments presented herein regarding the present invention are best understood by again discussing the prior art with reference to FIGS. 1 and 2 before setting forth a detailed description of the preferred embodiments. Differential amplifiers are very useful devices when dealing with differential systems. They have several advantages such as very good common mode rejection ratio since everything that is common will get canceled at the differential output. Some applications of differential amplifiers include single ended to differential conversion, differential ADC driver, differential transmitter and receiver, and output level shifter. 
     Differential amplifiers are not too different from single ended amplifiers. Several single ended topologies can be converted to differential topologies. The most obvious difference between differential amplifiers and their single ended counterparts is that they have two outputs instead of one. With the addition of this output, another difference arises, which is the “output common mode feedback circuit” (CMFC). Extra circuitry is necessary to direct the outputs where to go DC wise. Since this signal is common to both outputs differentially, it will cancel, leaving at the differential output only the amplified differential input signal. 
     Implementing such a common mode circuit is challenging. FIG. 1 illustrates one implementation of an amplifier  100  having a class A input stage that includes an emitter coupled pair input stage  102  with a folded cascode gain stage  104 . The CMFC includes an error amplifier  106  that compares the common mode voltage at the outputs to the voltage set on the V ocm  pin  108  (seen defaulted to mid rail in FIG.  1 ). The error amplifier  106  adjusts the common mode voltage by changing the voltage in the bases of transistors Q 5  and Q 6 . The feedback path of the error amplifier  106  in FIG. 1 is from the output of the error amplifier  106  to the bases of transistors Q 5  and Q 6 , to the outputs of the main amplifier through the buffers  110 ,  112 , and back into the positive input of the error amplifier  106 . 
     The class AB input stage in a differential configuration has several advantages over the class A single emitter coupled pair input stage illustrated in FIG.  1 . Some of these advantages include better slew rate and better THD performance, especially at high frequencies. It would be desirable to have a way of implementing the foregoing CMFC on this topology. FIG. 2 is a schematic diagram illustrating an amplifier  200  having a differential class AB input stage without the CMFC. It can be seen this topology is quite different from the one shown in FIG.  1 . One of the primary differences between this kind of circuit and the circuit in FIG. 2 is that in FIG. 1 it is only necessary to manipulate the current of transistors Q 5  and Q 6  to control the output common mode voltage. In the circuit of FIG. 2, it is necessary to manipulate the top mirrors Q 5 , Q 6  and Q 13 , Q 14  as well as the bottom mirrors Q 7 , Q 8  and Q 15 , Q 16 . The conventional prior art solution includes the use of two loops to control the two sets of mirrors, and that way control the output common mode voltage. This conventional approach has been problematic however since it results in undesirable distortion associated with additional parasitics and mismatch between the loops, stability problems due to complexity of the multiple control loops and the interaction between them, added complexity, and circuitry that consumes additional die area. It is desirable to have a way of implementing the CMFC on a class AB input stage in a differential configuration in a manner that does not result in additional distortion, or stability problems due to increased complexity of multiple control loops, and that does not undesirably increase the required die area. 
     Looking now at FIG. 3, a schematic diagram illustrates an amplifier  300  having differential class AB input stage with common mode feedback circuitry (CMFC)  302  according to one embodiment of the present invention. The common mode output voltage in amplifier  300  is set by introducing an offset of current into the current mirrors formed by transistors Q 5  to Q 8  and Q 13  to Q 16  (4 mirrors in total). This technique creates a DC offset voltage in the outputs that will be equal to the common mode output voltage. It is necessary to be able to manipulate the top mirrors formed by transistors Q 5 , Q 6 , Q 13 , and Q 14  as well as the bottom mirrors formed by transistors Q 7 , Q 8 , Q 15  and Q 16  as stated herein before. This task is implemented using a single loop by introducing standing currents  304 ,  306  at the top to the collector nodes of transistors Q 3  and Q  11 , and then connecting the CMFC  302  at the bottom to the collector nodes of transistors Q 4  and Q 12 . The DC currents of the CMFC  302  and the standing currents  304 ,  306  are the same, so in steady state, everything is balanced. When there is a change in the output or in the V ocm  pin  308 , the currents in the CMFC  302  will change causing the currents that get fed to the bottom mirrors formed by transistors Q 7 , Q 8 , Q 15  and Q 16  to change. This current will increase or decrease creating a difference between the standing currents  304 ,  306  that get fed on the top and the DC currents of the CMFC that get fed on the bottom. This action creates an offset that will change the common mode output voltage until it settles to what is set at the V ocm  pin  308 . 
     FIG. 4 is a schematic diagram illustrating one implementation of the common mode feedback circuitry (CMFC) amplifier  400  seen in FIG. 3 according to one embodiment of the present invention. The tail current  402  is two times as large as the standing current ( 304  or  306 ) seen in FIG.  3 . This means that when there is a change in to V own    308 , the currents from transistors Q 17  and Q 18  can get as large as  21 . This feature allows a symmetrical bipolar common output range. 
     The present method then, represents a significant advancement in the art of high performance class AB differential amplifiers. This invention has been described in considerable detail in order to provide those skilled in the equalizer art with the information needed to apply the novel principles and to construct and use such specialized components as are required. In view of the foregoing descriptions, it should be apparent that the present invention represents a significant departure from the prior art in construction and operation. However, while particular embodiments of the present invention have been described herein in detail, it is to be understood that various alterations, modifications and substitutions can be made therein without departing in any way from the spirit and scope of the present invention, as defined in the claims which follow.