Abstract:
An apparatus for an improved operational amplifier. The disclosed improved operational amplifier comprises an operational amplifier, a first feedback circuit, and one or more secondary feedback circuits. The operational amplifier include a plurality of serially coupled gain stages and is configured so that an output of each gain stage drives an input of a next gain stage and an output of a last gain stage drives a load external to the improved operational amplifier. The first feedback circuit is coupled between an output of a designated gain stage and an output of a previous gain stage to provide a first feedback to the previous gain stage. Each secondary feedback circuit provides an additional feedback to the output of the previous gain stage.

Description:
RELATED APPLICATION 
     The present invention claims priority of provisional patent application No. 61/264,149 filed Nov. 24, 2009, the contents of which are incorporated herein in their entirety. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present teaching relates to method and system for analog circuits. More specifically, the present teaching relates to method and system for operational amplifiers and systems incorporating the same. 
     2. Discussion of Technical Background 
     Operational amplifiers (op-amp) are widely used to drive a load which often corresponds to a capacitive load. A typical op-amp with a Miller feedback is shown as  100  in  FIG. 1  (Prior Art). Circuit  100  includes two gain stages  115  and  125  in series (the output of the first gain stage drives the input of the second gain stage) and a Miller feedback element  150 . The first gain stage  115  comprises a gm stage ( 110 ) with parasitic loads ( 120  and  121 ). The first gain stage has a positive voltage gain at zero frequency (DC). A gm stage is a functional block where the output current is a function (normally a linear function) of input voltage difference. The second gain stage  125  comprises a gm stage ( 130 ) with an external load ( 104 ). The Miller feedback  150  is a capacitive element  160  (sometimes in series with a resistive element  155 ) from the output of the second gain stage to the output of the first gain stage. The second gain stage  125  has a negative voltage gain at zero frequency (DC). 
     Specifically, the first gain stage has its positive input connected to the op-amp positive input ( 180 ) and its negative input connected to the op-amp negative input ( 190 ). At the output of gain stage  115  (VA), a resistor  120  and a capacitor  121  represent the load at that node (including parasitic and next stage load). The second gain stage  125  has its input connected to the first gain stage output (VA). The Miller feedback circuit ( 150 ) connects VOUT which is the output of the second gain stage ( 125 ) and VA which is the output of the first gain stage ( 115 ). Alternatively, the input of the second gain stage ( 125 ) may be generated by a buffer (not shown) situated between the output of first gain stage  115  and the input of second gain stage  125 . 
     At the second gain stage, the output of amplifier (VOUT) is connected to the external load (represented by load resistor  135  and a load capacitor  140 , which are connected in parallel). The output of the amplifier (VOUT) is also fed back to node VA through the Miller feedback element  150 . The Miller feedback element  150  comprises a serially connected resistor  155  and a capacitor  160 . 
     To drive a larger capacitive load, traditionally, a resistor  170  is introduced which is connected in series, with the load capacitor. However, this solution may cause voltage swing problem with degradation of gain bandwidth product and slew rate depending on the load. Therefore, there is a need for an operational amplifier that can drive a larger capacitive load but avoid voltage swing problem while maintaining a good slew rate and a good gain bandwidth product. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The inventions claimed and/or described herein are further described in terms of exemplary embodiments. These exemplary embodiments are described in detail with reference to the drawings. These embodiments are non-limiting exemplary embodiments, in which like reference numerals represent similar structures throughout the several views of the drawings, and wherein: 
         FIG. 1  (Prior Art) illustrates a conventional two stage operational amplifier; 
         FIG. 2  depicts an exemplary circuit for an improved operational amplifier, according to an embodiment of the present invention; 
         FIG. 3  depicts a different exemplary circuit for an improved operational amplifier, according to an embodiment of the present invention; 
         FIG. 4  shows plots comparing the gain and phase observed at the output of an operational amplifier with or without the improvement in accordance with the present invention; 
         FIG. 5  shows plots comparing the gain and phase observed at a feedback point (VA) of an operational amplifier with or without the improvement in accordance with the present invention; 
     
    
    
     DETAILED DESCRIPTION 
     The present invention discloses a method for an improved operational amplifier which can be used to drive a larger capacitive load without a series isolation resistor while maintaining good slew rate and gain bandwidth product. 
       FIG. 2  depicts an exemplary op-amp circuit  200  with parallel Miller feedback for improved ability to drive a larger capacitive load, according to an embodiment of the present teaching. The circuit  200  comprises a Miller compensated operational amplifier  201  and a secondary Miller feedback circuit  202 . The Miller compensated op-amp  201  is constructed similarly as a traditional Miller compensated op-amp as shown in  FIG. 1 . Specifically, the Miller compensated op-amp  201  includes two gain stages  215  and  225  in series (the output of the first gain stage  215  drives the input of the second gain stage  225 ) and a Miller feedback element  250 . The second gain stage  225  and a first Miller feedback circuit  250  form a primary Miller feedback circuit  203 . 
     The secondary Miller feedback circuit  202  comprises a gain stage  275  (gm stage  235  and an internal load  245 ) and a Miller feedback element ( 265 ). The input of the secondary Miller feedback stage is the same as the primary Miller feedback  203  and the feedback is from the output of the gain stage  275  to the output of the first gain stage  215  of the op-amp. The secondary Miller feedback is in parallel to the primary Miller feedback  203 . 
     As discussed previously, the DC voltage gain of the first gain stage  215  is normally high because of the high parasitic resistance of circuit  220 . The DC voltage gain of the second gain stage  225  depends on the external load resistance  235 . When the load capacitance  240  is large and load resistance  235  is high, the effective gain of the second stage is high at low frequencies and low at high frequencies. This is because the effective load impedance is low at high frequencies. The secondary Miller feedback gain stage  275  is designed to have a DC voltage gain much less than the DC gain of the second gain stage  225 . Resistor  245  is designed to make the secondary Miller feedback gain stage  275  have a certain DC gain. 
     The gm of the gain stage  235  of the secondary Miller feedback circuit  202  can be much less than that ( 230 ) of the second gain stage  225 , which makes it consume little current compared to the second gain stage  225  in the primary Miller feedback circuit  203 . The capacitance of the Miller feedback element  265  of the secondary Miller feedback circuit  202  can be designed to have a value comparable to that of capacitor  260  in the primary Miller feedback circuit  203 . The secondary Miller feedback circuit  202  may also have a residual capacitor  280  with a small capacitance. 
     In operation, because the DC gain of the secondary Miller feedback circuit  202  is low compared to that of the second gain stage  225  of the primary Miller feedback circuit  203 , the gain bandwidth and slew rate is not noticeably affected by the addition of the secondary Miller feedback circuit  202 . Thus, the added secondary Miller feedback circuit does not affect the operational amplifier at lower frequencies. That is, the secondary Miller feedback circuit  202  is only effective at high frequencies when the AC gain of the second gain stage ( 225 ) is higher than the AC gain of the secondary Miller feedback circuit  202 . When there is a high capacitive load, the secondary Miller feedback circuit  202  is effective at higher frequencies due to the relatively lower effective gain of the second stage in the primary Miller feedback circuit  203  when compared to the secondary Miller feedback circuit  202 . The underlying effect of having two Miller feedback circuits is that the second Miller compensation is to split the non-dominant pole of the operational amplifier with its own pole and insert a zero to stabilize the operational amplifier. 
     As discussed herein, the addition of the secondary Miller feedback circuit  202  does not affect the performance of the operational amplifier at lower frequencies. The added secondary Miller feedback circuit  202  compensates for the degradation of the primary Miller feedback circuit at higher frequencies. Thus, the primary and the secondary Miller feedback circuits play roles in different ranges of frequencies so that, in combination, the improved operational amplifier  200  provides a better performance across a wider range of frequencies without having a noticeable impact on its slew rate and gain bandwidth product. In addition, since the secondary Miller feedback circuit  202  consumes much lower current as compared with the primary Miller feedback circuit  203  and has few additional components, the cost of adding the secondary Miller feedback circuit  202  is negligible. 
     As illustrated in  FIG. 2 , the output of the first gain stage  215  and the feedback from both the primary and secondary Miller feedback circuits  203  and  202  are connected together at VA. There is no buffering between the first gain stage  215  and the second gain stage  225 . In practice, an implementation may include a buffer between the first gain stage  215  and the second gain stage  225 . In such implementations, a buffer can be deployed between the first gain stage  215  and the second gain stage  225  with the feedbacks from the primary and secondary Miller feedback circuits ( 203  and  202 ) being connected to the output of the first gain stage. 
       FIG. 3  depicts a different exemplary circuit  300  for an improved operational amplifier, according to an embodiment of the present teaching. Circuit  300  is constructed similar to circuit  200 , comprising a traditional operational amplifier  301 , and a secondary Miller feedback circuit  302 . The difference between circuit  200  and circuit  300  is the addition of a buffer  325 , which is inserted between the output of the first gain stage and input of the second gain stage. However, it is understood that such a buffer is not topologically necessary in realizing an improved operational amplifier as disclosed herein. In addition, although the exemplary circuits as illustrated in  FIGS. 2 and 3  are two-stage Miller compensated with two feedback circuits, the present teaching should not be limited to what is illustrated herein. As a person skilled in the art would appreciate, the present teaching is also applicable to an operational amplifier with multiple stages and multiple feedbacks. 
       FIG. 4  shows plots of the gain and phase observed at the output of an operational amplifier with or without the parallel Miller feedback improvement. Specifically, the plots represent the observed gain and phase at VOUT with and without the improvements as described herein. In  FIG. 4 , the X-axis represents frequency and Y-axis represents gain (in dB) or phase (in degrees). There are two groups of plotted curves. The top group having two curves represents the gains observed at VOUT, with and without the secondary Miller feedback circuit, respectively. Similarly, the bottom group of two curves represents the observed phase values observed at VOUT, with and without the secondary Miller feedback circuit, respectively. Within each group, the curve with rectangles is plotted based on observations made from a circuit with a secondary Miller feedback circuit. The curve with triangles is plotted based on observations made from a circuit without the secondary Miller feedback circuit. 
     As can be seen from the four curves shown in  FIG. 4 , at frequencies lower than 1 kHz, the curves are almost identical with or without the parallel Miller feedback implementation. This evidences that the addition of the parallel Miller feedback circuit has no visible impact to the operational amplifier at lower frequencies. It can also be seen that in the frequency range between 1 kHz and 10 MHz, the performances of operational amplifiers with and without the parallel Miller feedback circuit differ. Specifically, the gain observed at VOUT is higher without the parallel Miller feedback implementation and lower with the parallel Miller feedback implementation. As to the phase, the phase value of the op-amp with parallel feedback implementation is higher than the phase value observed from the op-amp without parallel feedback implementation at the 0 dB gain points. So the op-amp with parallel Miller implementation is more stable than the one without. For the parameters in the example, it can be observed that with the parallel Miller feedback implementation, the phase margin is improved from 8 to 41, and the gain margin is improved from 22 to 38. 
       FIG. 5  shows plots comparing the gain and phase observed at the feedback point VA of op-amps with or without the parallel Miller feedback improvement. The curves are plotted based on similar observations as those in  FIG. 4  except that the observations here are made at VA rather than at VOUT. The top group has two curves representing the phase observed at VA with and without the parallel Miller feedback circuit, respectively. Similarly, the bottom group of two curves represents the observed gain values at VA with and without the parallel Miller feedback circuit, respectively. Within each group, the curve with rectangles is plotted based on observations made from a circuit with the parallel Miller feedback implementation. The curve with triangles is plotted based on observations made from a circuit without the parallel Miller feedback implementation. The plot representing the phase values from a circuit with parallel Miller feedback implementation clearly demonstrates the additional pole-zero split. 
     While the inventions have been described with reference to the certain illustrated embodiments, the words that have been used herein are words of description, rather than words of limitation. Changes may be made, within the purview of the appended claims, without departing from the scope and spirit of the invention in its aspects. Although the inventions have been described herein with reference to particular structures, acts, and materials, the invention is not to be limited to the particulars disclosed, but rather can be embodied in a wide variety of forms, some of which may be quite different from those of the disclosed embodiments, and extends to all equivalent structures, acts, and, materials, such as are within the scope of the appended claims.