Abstract:
An operational amplifier and a method for amplifying a signal. Embodiments provide a convenient and effective mechanism for reducing die area, design time and design verification time by sharing compensation components between the common-mode and differential feedback networks of the operational amplifier. As such, fewer compensation components are required, thereby reducing component die area. Additionally, given that the compensation components are shared between the common-mode and differential feedback networks, the feedback networks can be stabilized together with fewer compensation components to specify and verify, thereby reducing design and design verification time. Further, embodiments provide a compensation component coupling which does not couple directly to virtual ground, thereby reducing the noise of the operational amplifier.

Description:
BACKGROUND OF THE INVENTION 
     Fully differential operational amplifiers (op amps) require stable differential and common-mode operation. As such, modern op amps are designed with separate differential and common-mode feedback compensation networks to stabilize both differential and common-mode operation of the op amp. The compensation in each compensation network is provided by capacitors, where the capacitors either perform differential or common-mode compensation. 
       FIG. 1  shows conventional operational amplifier  100 . Op amp  100  operates between +V CC  and −V CC  rails, where differential signals fed to differential inputs V ip  and V in  are amplified and output at differential outputs V op  and V on  during differential operation. Amplification of the differential signals is provided by flowing current from the +V CC  rail, through the individual cascoded transistor pairs (e.g., pmos transistor  110  and nmos transistor  115 , and pmos transistor  120  and nmos transistor  125 ) and current sink  130 , and into the −V cc  rail. Differential feedback is provided by differential feedback loop  140 , which feeds back voltages from the differential outputs to the differential inputs via differential feedback circuit  145 . 
     Common-mode feedback loop  150  is used to set the operating point of op amp  100  during common-mode operation. Resistors  151  and  152  provide a voltage divider for generating an average voltage (e.g., an average of the differential outputs) at common-mode output V ocm , where the average voltage is fed back to the common-mode feedback circuit  155  for comparison with a reference voltage V ref  also fed to circuit  155 . In response to the comparison, circuit  155  will generate a common-mode control signal fed to common-mode input V icm . The voltage at V icm  is used to adjust the bias of pmos transistors  110  and  120 , thereby shifting the voltage at the differential outputs to set the operating point of op amp  100 . 
     As shown in  FIG. 1 , differential feedback loop  140  and separate common-mode feedback loop  150  utilize separate compensation capacitors to stabilize both modes of operation. For example, differential feedback loop  140  uses compensation capacitors  146  and  147  to stabilize differential operation, while common-mode feedback loop  150  uses compensation capacitor  158  to stabilize common-mode operation. Given the large area required on integrated circuit dies to implement capacitors, the compensation networks of op amp  100  require large die area, thereby limiting use in integrated circuits with smaller allotted die areas. Additionally, the separate differential and common-mode compensation networks of op amp  100  increase the design and design verification time as both networks must be individually optimized. Further, given that both compensation networks couple to virtual ground as shown in  FIG. 1 , the noise of op amp  100  is increased, thereby limiting use in designs requiring low noise. 
     SUMMARY OF THE INVENTION 
     Accordingly, a need exists for an operational amplifier with compensation components occupying a reduced die area. Additionally, a need exists for an operational amplifier with compensation networks requiring reduced design and design verification time. Further, a need exists for an operational amplifier with compensation components producing lower noise. Embodiments of the present invention provide novel solutions to these needs and others as described below. 
     Embodiments of the present invention are directed to an operational amplifier and a method for amplifying a signal. More specifically, embodiments provide a convenient and effective mechanism for reducing die area, design time and design verification time by sharing compensation components between the common-mode and differential feedback networks of the operational amplifier. As such, fewer compensation components are required, thereby reducing component die area. Additionally, given that the compensation components are shared between the common-mode and differential feedback networks, the feedback networks can be stabilized together with fewer compensation components to specify and verify, thereby reducing design and design verification time. Further, embodiments provide a compensation component coupling which does not couple directly to virtual ground, thereby reducing the noise of the operational amplifier. 
     In one embodiment, an operational amplifier circuit includes amplifier circuitry for amplifying a differential signal, wherein the amplifier circuitry is operable to generate a differential output signal. A differential feedback network is coupled to the amplifier circuitry and operable to provide differential feedback compensation therein, the differential feedback network including compensation elements. The operational amplifier circuitry also includes a common-mode feedback network coupled to the amplifier circuitry and operable to provide common-mode feedback compensation therein, the common-mode feedback network sharing the compensation elements of the differential feedback network. 
     In another embodiment, an operational amplifier includes amplifier circuitry, a common-mode feedback loop coupled to the amplifier circuitry and for controlling common-mode operation of the operational amplifier, the common-mode operation comprising an adjustment of an operating point of the operational amplifier. A differential feedback loop is coupled to the amplifier circuitry and for controlling differential operation of the operational amplifier, the differential operation comprising an amplification of a differential signal input to the operational amplifier. The common-mode and differential feedback loops are coupled to at least one common compensation component operable to improve stability of both the common-mode and differential operation of the operational amplifier. 
     And in yet another embodiment, a method for amplifying a differential signal includes controlling differential operation of an operational amplifier using a differential feedback loop, the differential operation including an amplification of the differential signal input to the operational amplifier. Common-mode operation of the operational amplifier is controlled using a common-mode feedback loop, the common-mode operation including an adjustment of an operating point of the operational amplifier. The differential and common-mode operation of the operational amplifier is stabilized using shared compensation components coupled to the differential and common-mode feedback loops. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements. 
         FIG. 1  shows a conventional operational amplifier. 
         FIG. 2  shows an exemplary operational amplifier that shares compensation components between the differential and common-mode feedback networks in accordance with a first embodiment of the present invention. 
         FIG. 3  shows an exemplary operational amplifier that shares compensation components between the differential and common-mode feedback networks in accordance with a second embodiment of the present invention. 
         FIG. 4  shows a process for amplifying a signal in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings. While the present invention will be discussed in conjunction with the following embodiments, it will be understood that they are not intended to limit the present invention to these embodiments alone. On the contrary, the present invention is intended to cover alternatives, modifications, and equivalents which may be included with the spirit and scope of the present invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, embodiments of the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, components, and circuits have not been described in detail so as not to unnecessarily obscure aspects of the present invention. 
     Embodiments of the Invention 
       FIG. 2  shows exemplary operational amplifier (op amp)  200  that shares compensation components between the differential and common-mode feedback networks in accordance with a first embodiment of the present invention. As shown in  FIG. 2 , operational amplifier  200  comprises amplifier circuitry for amplifying differential signals (e.g., analog, digital, etc.) fed to differential inputs V ip  and V in , where the amplified signal may be generated at differential outputs V op  and V on . The amplification of differential signals may comprise a differential operating mode of op amp  200  requiring stabilization, where differential feedback loop  240  (e.g., comprising differential feedback circuit  245 ) provides differential feedback compensation to stabilize op amp  200  during differential operation. In one embodiment, circuit  245  may implement a feedback factor β such that 
     
       
         
           
             β 
             = 
             
               
                 
                   
                     V 
                     ip 
                   
                   - 
                   
                     V 
                     in 
                   
                 
                 
                   
                     V 
                     op 
                   
                   - 
                   
                     V 
                     on 
                   
                 
               
               . 
             
           
         
       
     
     Compensation is provided by compensation components  220  and  222 , which are coupled in series across the differential outputs V op  and V on . In one embodiment, compensation components  220  and  222  may comprise capacitors (e.g., non-polarized, polarized, etc.), a combination of capacitors and resistors, or other components (e.g., passive, active, etc.) or combinations of components. 
     Common-mode feedback loop  250  is used to set the operating point of op amp  200  during common-mode operation. An operating point (e.g., an average of the differential outputs) of op amp  200  may be fed to the common-mode output V ocm  such that common-mode feedback circuit  255  (e.g., comprising a comparator, etc.) may compare the operating point with a reference voltage V ref  also fed to circuit  255 . Vref may be in internally generated by op amp  200 , externally generated and fed to op amp  200 , etc. In response to the comparison, circuit  255  will generate a common-mode control signal fed to amplifier circuitry  210  via common-mode input V icm . The voltage at V icm  may then adjust an operating point of op amp  200  (e.g., by adjusting the bias of transistors within amplifier circuitry  210 , etc.), which may comprise a common-mode operating mode of op amp  200  requiring stabilization. Common-mode feedback loop  250  provides common-mode feedback compensation to stabilize op amp  200  during common-mode operation, where compensation is provided by compensation components  220  and  222  coupled to common-mode input V icm . 
     As shown in  FIG. 2 , the common-mode and differential feedback networks (e.g., loops  240  and  250 ) share compensation components (e.g.,  220  and  222 ), thereby reducing the number of components required to stabilize the two operation modes in comparison to conventional solutions (e.g., requiring at least three components as shown in  FIG. 1 ). As such, the die area required to implement op amp  200 , and more specifically compensation components  220  and  222 , is reduced from that of op amp  100  as shown in  FIG. 1 . Additionally, sharing of the components between feedback networks allows them to be stabilized in a single operation with fewer components to account for, thereby reducing design and design verification time. Further, given that the compensation configuration as depicted in  FIG. 2  does not require direct coupling to virtual ground (e.g., as compared with capacitors  146 ,  147  and  158  of  FIG. 1 ), op amp  200  is able to operate with lower noise than conventional solutions. 
     Where compensation components  220  and  222  are capacitors coupled in series, their values may be calculated using equations, modeling or a combination of the two. For example, the compensation capacitor values (C C ) may be estimated by the equation 
                 C   c     =       g   m       G   bw         ,         
where g m  is the transconductance of a coupled input device (e.g., transistor, etc.) and G bw  is the gain bandwidth product of the op amp (e.g.,  200 ). In other embodiments, other equations may be used (e.g., to account for changes in amplifier circuitry  210 , circuit  245 , circuit  255 , etc.). The compensation components may then be modeled using the estimated values as a starting point to more accurately determine their values (e.g., taking into account second, third, etc. order effects on G bw ).
 
     Amplifier circuitry  210  may comprise at least one input stage, gain stage, bias stage and output stage for amplifying signals input to circuitry  210 . Additionally, it should be appreciated that circuitry  210  may comprise additional active and/or passive circuitry for interfacing the staged circuitry, where such additional circuitry may comprise current mirrors, current sources/sinks, voltage dividers, etc. 
     Although  FIG. 2  depicts exemplary op amp  200  with specific inputs and/or outputs to amplifier circuitry  200 , it should be appreciated that other inputs and/or outputs (e.g., for external compensation pins, balance, etc.) may be used in other embodiments in addition to or in place of those depicted in  FIG. 2 . Additionally, although only two compensation components (e.g.,  220  and  222 ) are depicted in  FIG. 2 , it should be appreciated that a larger or smaller number may be used to compensate the common-mode and/or differential feedback networks in other embodiments. Further, although circuits  245  and  255  are depicted as single units in  FIG. 2 , it should be appreciated that circuits  245  and/or  255  may be implemented using more than one circuit in other embodiments. And in another embodiment, circuits  245  and  255  may share at least one non-compensation component (e.g., in addition to at least one compensation component). 
       FIG. 3  shows exemplary operational amplifier  300  that shares compensation components between the differential and common-mode feedback networks in accordance with a second embodiment of the present invention. As shown in  FIG. 3 , op amp  300  may perform both differential operation (e.g., amplifying a differential signal fed to differential inputs V ip  and V in  to generate an amplified signal at differential outputs V op  and V on ) and common-mode operation (e.g., setting the operating point of op amp  300 ) similar to that of op amp  200  discussed above. Amplification of the differential signals is provided by flowing current from the +V CC  rail, through the individual cascoded transistor pairs dedicated to a respective differential input (e.g., pmos transistor  110  and nmos transistor  115  dedicated to V ip , and pmos transistor  120  and nmos transistor  125  dedicated to V in ) and current sink  130 , and into the −V CC  rail. 
     Differential feedback loop  340  provides differential feedback compensation to stabilize op amp  200  during differential operation. Similar to differential feedback loop  240  of  FIG. 2 , differential feedback loop  340  feeds back signals from the differential outputs (e.g., V op  and V on ) to the differential inputs (e.g., V ip  and V in ) via differential feedback circuit  345 . In one embodiment, circuit  345  may comprise identical circuitry of circuit  245  of  FIG. 2 . In another embodiment, the circuitry of circuit  345  may differ from that of circuit  245  to accommodate for other circuitry changes of op amp  300  with respect to op amp  200  of  FIG. 2 . Additionally, compensation is provided by compensation components  220  and  222  similar to the compensation configuration discussed above with respect to  FIG. 2 . 
     Common-mode feedback loop  350  provides common-mode feedback compensation to stabilize op amp  300  during common-mode operation, where compensation is provided by compensation components  220  and  222  coupled to common-mode input V icm  (e.g., as discussed above with respect to op amp  200  of  FIG. 2 ). Similar to common-mode feedback loop  250  of  FIG. 2 , common-mode feedback circuit  355  compares an operating point of op amp  300  with a reference voltage V ref  to generate a common-mode control signal fed to V icm  for adjusting the operating point (e.g., by changing the bias of transistors  110  and  120 ) of op amp  300 . In one embodiment, circuit  355  may comprise identical circuitry of circuit  255  of  FIG. 2 . In another embodiment, the circuitry of circuit  355  may differ from that of circuit  255  to accommodate for other circuitry changes of op amp  300  with respect to op amp  200  of  FIG. 2 . 
     The operating point may be sensed using resistive components  151  and  152  coupled in series to form a voltage divider, where a divided voltage is generated at the common node shared by both resistive components  151  and  152 . Resistive components may comprise any resistive component (e.g., a resistor, transistor, etc.), voltage dividing component, voltage generating component (e.g., a diode with substantially constant voltage drop, etc.), or the like. 
     As shown in  FIG. 3 , the common-mode and differential feedback networks (e.g., loops  340  and  350 ) share compensation components (e.g.,  220  and  222 ), thereby providing the same advantages (e.g., smaller die area, reduced design and design verification time, lower noise, etc.) over conventional solutions as discussed above with respect to op amp  200  of  FIG. 2 . Additionally, the values of components  220  and  222  may be calculated as discussed above with respect to  FIG. 2 . 
     Although  FIG. 3  depicts exemplary op amp  300  with specific inputs and/or outputs, it should be appreciated that other inputs and/or outputs (e.g., for external compensation pins, balance, etc.) may be used in other embodiments in addition to or in place of those depicted in  FIG. 3 . Additionally, although only two compensation components (e.g.,  220  and  222 ) are depicted in  FIG. 3 , it should be appreciated that a larger or smaller number may be used to compensate the common-mode and/or differential feedback networks in other embodiments. Further, although circuits  345  and  355  are depicted as single units in  FIG. 3 , it should be appreciated that circuits  345  and/or  355  may be implemented using more than one circuit in other embodiments. And in another embodiment, circuits  345  and  355  may share at least one non-compensation component (e.g., in addition to at least one compensation component). Additionally, although  FIG. 3  depicts transistors  110 - 125  as pmos and nmos transistors, it should be appreciated that other types of transistors may be used in other embodiments. Further, although  FIG. 3  depicts op amp  300  with specific circuitry to amplify differential signals and set an operating point of the op amp, it should be appreciated that other designs may be used in other embodiments. 
       FIG. 4  shows process  400  for amplifying a signal in accordance with one embodiment of the present invention. As shown in  FIG. 4 , step  410  involves controlling differential operation of an operational amplifier (e.g.,  200 ,  300 , etc.) using a differential feedback loop (e.g.,  240 ,  340 , etc.). The differential operation may comprise amplifying differential signals fed to the op amp (e.g., to differential inputs) to generate amplified differential signals (e.g., at differential outputs). Additionally, the differential feedback loop may comprise one or more differential feedback circuits for controlling the amplification (e.g., by implementing a feedback factor, etc.) and providing feedback compensation for the control system. 
     Step  420  involves controlling common-mode operation of an operational amplifier (e.g.,  200 ,  300 , etc.) using a common-mode feedback loop (e.g.,  250 ,  350 , etc.). The common-mode operation may comprises adjusting and/or setting an operating point (e.g., an average of differential outputs) of the op amp. Additionally, the common-mode feedback loop may comprise one or more common-mode feedback circuits for controlling the operation point adjustment and/or setting (e.g., to generate a common-mode control signal in response to a comparison of a current operating point with a reference voltage) and providing feedback compensation for the control system. 
     Step  430  involves stabilizing the differential and common-mode operation of the operational amplifier (e.g.,  200 ,  300 , etc.) using shared compensation components (e.g.,  220  and  222  of  FIGS. 2 and 3 ) coupled to the differential and common-mode feedback loops (e.g.,  245 / 345  and  255 / 355 , respectively). In one embodiment, the compensation components (e.g.,  220  and/or  222 ) may comprise capacitors (e.g., non-polarized, polarized, etc.), a combination of capacitors and resistors, or other components (e.g., passive, active, etc.) or combinations of components. Sharing of the compensation components between differential and common-mode feedback networks enables the use of fewer parts over conventional solutions as discussed above with respect to  FIG. 2 , thereby reducing the die area needed to implement the op amp. Additionally, sharing of the components between feedback networks allows them to be stabilized in a single operation with fewer components to account for, thereby reducing design and design verification time. Further, sharing compensation components enables a component coupling that avoids direct connection to a ground (e.g., virtual ground, etc.), thereby reducing the noise of the op amp over conventional solutions. 
     In the foregoing specification, embodiments of the invention have been described with reference to numerous specific details that may vary from implementation to implementation. Thus, the sole and exclusive indicator of what is, and is intended by the applicant to be, the invention is the set of claims that issue from this application, in the specific form in which such claims issue, including any subsequent correction. Hence, no limitation, element, property, feature, advantage, or attribute that is not expressly recited in a claim should limit the scope of such claim in any way. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.