Abstract:
The common-mode feedback circuit generates currents representing the output voltages of a fully differential amplifier, and sums these current to produce a summation current. Based on the comparison of the summation current to a reference current, the common-mode feedback circuit generates a feedback voltage for stabilizing the fully differential amplifier.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a common-mode feedback circuit. 
     2. Description of the Prior Art 
     The purpose of a common-mode feedback circuit is to stabilize an associated fully differential amplifier. A stable fully differential amplifier can only be achieved when the bandwidth of the common-mode feedback circuit is greater than the bandwidth of the fully differential amplifier. Also, the common-mode feedback circuit needs to be stable as well. Instability within a common-mode feedback circuit is caused by, for example, high impedance nodes. Traditionally, capacitors are used to compensate for high impedance nodes, but the addition of capacitors decreases the bandwidth of the common-mode feedback circuit, and, thus, places a restriction on the bandwidth of the fully differential amplifier. 
     SUMMARY OF THE INVENTION 
     A common-mode feedback circuit according to the present invention includes a converting circuit converting the output voltages of a fully differential amplifier into currents, and a summation circuit summing the currents to produce a summation current. The summation current is then compared by a comparison circuit to reference current. A feedback circuit generates a feedback voltage for controlling the fully differential amplifier based on the results of the comparison. Advantageously, the common mode feedback circuit according to the present invention does not include any high impedance nodes or suffer from the problems and disadvantages associated therewith. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, wherein like reference numerals designate corresponding parts in the various drawings, and wherein: 
     FIG. 1 illustrates an embodiment of the common-mode feedback circuit according to the present invention applied to a conventional fully differential amplifier; and 
     FIG. 2 illustrates another embodiment of the common-mode feedback circuit according to the present invention applied to the conventional fully differential amplifier. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates an embodiment of the common-mode feedback circuit according to the present invention applied to the fully differential amplifier  10 . Because the fully differential amplifier  10  in FIG. 1 is well-known, a description of the structure and operation of the fully differential amplifier  10  will be omitted. 
     The common-mode feedback circuit  100  includes first and second bipolar transistors  102  and  104 , which receive the output voltages of the fully differential amplifier  10  at their gates, respectively. The first bipolar transistor  102  is connected in series with a first N-MOS transistor  106  between the power source voltage VDD and ground. The second bipolar transistor  104  is connected in series with a second N-MOS transistor  108  between the power source voltage VDD and ground. The gates of the first and second N-MOS transistors  106  and  108  are connected to the fourth fixed bias. 
     A first and second resister  110  and  112  are connected in series between the emitters of the first and second bipolar transistors  102  and  104 . A first constant current source  114  is connected between the junction of the first and second resisters  110  and  112  and ground. Hereinafter, the junction between the first and second resistors  110  and  112  will be referred to as node  122 . 
     As further shown in FIG. 1, a second constant current source  116  is connected in series with a reference bipolar transistor  118  between the power source voltage VDD and the node  122 . A feedback P-MOS transistor  120  is connected in parallel to the second constant current source  116  and the reference bipolar transistor  118 . The gate of the reference bipolar transistor  118  receives a reference voltage Vref which places the reference bipolar transistor  118  in the active state. The gate of the feedback P-MOS transistor  120  is connected to the junction between the second constant current source  116  and the reference bipolar transistor  118 . Furthermore, a feedback path  124  supplies the voltage at the gate of the feedback P-MOS transistor  120  to the gates of the first and second resistive P-MOS transistors  18  and  26  in the fully differential amplifier  10 . 
     The operation of the common-mode feedback circuit  100  will now be described with respect to an increase in the output voltages of the fully differential amplifier  10 . As the output voltages of the fully differential amplifier  10  increase, more current flows through the first and second bipolar transistors  102  and  104 . As a result, the currents flowing through the first and second resisters  110  and  112  to the node  122  increases. 
     The current flowing from the node  122  to ground is fixed by the first constant current source  114 . The current flowing to the node  122  via the reference bipolar transistor  118  is substantially fixed by the application of the reference voltage Vref to the gate of the reference bipolar transistor  118  and the provision of the second constant current source  116  except for a negligible base current in the reference bipolar transistor  118 . 
     Accordingly, any difference between (1) the current flowing through the reference bipolar transistor  118  to the node  122  and (2) the current through the first and second resisters  110  and  112  to the node  122  affects the current flowing through of the feedback P-MOS transistor  120  to the node  122 . Consequently, the current flowing through the feedback P-MOS transistor  120  decreases by the same amount of increase in total current through the first and second resistors  110  and  112 . With a decrease in the current flowing through the feedback P-MOS transistor  120 , the voltage at the gate of the feedback P-MOS transistor  120  increases. The feedback path  124  supplies this increased voltage to the first and second resistive P-MOS transistors  18  and  26  of the fully differential amplifier  10 . As a result, less current flows through the second and fourth resistive P-MOS transistors  18  and  26 , and the output voltages from the fully differential amplifier  10  decrease. 
     While the operation of the common-mode feedback circuit  100  has been described respect to an increase in the output voltages of the fully differential amplifier  10 , it is to be understood that the common-mode feedback circuit  100  operates in a similar, but opposite, manner when the output voltages of the fully differential amplifier  10  decrease. Both increases and decreases in the output voltages of the differential amplifier  10  are made with respect to the reference voltage Vref. Namely, the common-mode feedback circuit  100  serves to stabilize the output voltages around the reference voltage Vref. 
     Unlike conventional common-mode feedback circuits, the common-mode feedback circuit  100  does not rely upon a comparison of voltages to generate the feedback voltage. Instead, the common-mode feedback circuit  100  is a current-mode common-mode feedback circuit that generates a feedback voltage based on the comparison of currents representing the output voltages of fully differential amplifier with a reference current. The reference current in the common-mode feedback circuit  100  corresponds to the reference voltage Vref; and therefore, the common-mode feedback circuit  100  stabilizes the output voltages of the fully differential amplifier  10  about this reference voltage Vref. Also, in contrast to conventional common-mode feedback circuits, the common mode feedback circuit  100  does not include any high impedance nodes, or suffer from the problems and disadvantages related thereto. 
     Furthermore, it should be noted in that at low frequencies the gain of the feedback transistor  120  controls the input impedance at the emitter of the reference bipolar transistor  118 . However, as the frequency increases, the gain of the feedback transistor  120  decreases and the effect of this gain on the input impedance at the emitter of the reference bipolar transistor  118  decreases. At higher frequencies, the input impedance at the emitter of the reference bipolar transistor  118  is determined by the transconductance of the reference bipolar transistor  118  and the parasitic capacitance with respect thereto. The two poles associated with the emitter of the reference bipolar transistor  118  and the gate of the feedback P-MOS transistor  120  interact with each other, and a complex-pole pair may be created, which is accompanied by undesirable peaking in the frequency response. To avoid this complex-pole pair, the transconductance of the reference bipolar transistor  118  should be larger than that of the feedback P-MOS transistor  120 . This is easily accomplished in the BiCMOS (Bipolar-CMOS) implementation discussed above with respect to FIG. 1, because higher transconductances can be achieved with bipolar transistors than with their MOS counterparts. It should be understood however, that implementations of the present invention can be made using any other silicon technologies as long as the above rule is maintained. 
     While the embodiment of the present invention discussed above with respect to FIG. 1 operates well when the power source voltage VDD is greater than or equal to 3 volts, the common-mode feedback circuit of FIG. 1 is not applicable to low-power fully differential amplifiers. FIG. 2 illustrates another embodiment of the common-mode feedback circuit according to the present invention, which is applicable to low-power fully differential amplifiers. For ease of description, however, the common-mode feedback circuit  200  of FIG. 2 has been shown applied to the fully differential amplifier  10 . 
     As shown in FIG. 2, the output voltages of the fully differential amplifier  10  are respectfully connected to the gates of a first P-MOS transistor  202  and a second P-MOS transistor  204  in the common-mode feedback circuit  200 . The first P-MOS transistor  202  is connected in series with a third P-MOS transistor  206  between the power source voltage VDD and ground. The second P-MOS transistor  204  is also connected in series with a fourth P-MOS transistor  208  between the power source voltage VDD and ground. The gates of the third and fourth P-MOS transistors  206  and  208  are connected to the first fixed bias of the fully differential amplifier  10 . 
     A first and second resister  210  and  212  are connected in series between the sources of the first and second P-MOS transistors  202  and  204 . As further shown in FIG. 2, a feedback P-MOS transistor  214 , a reference P-MOS transistor  216 , and a constant current source  218  are connected in series between the power source voltage VDD and ground. The source of the reference P-MOS transistor  216  is connected to the junction between the first and second resisters  210  and  212 . Hereinafter the junction between the first and second resisters  210  and  212  and the junction between the feedback P-MOS transistor  214  and the reference P-MOS transistor  216  are collectively referred to as node  222 . 
     The gate of the reference P-MOS transistor  216  is connected to a reference voltage Vref, while the gate of the feedback P-MOS transistor  214  is connected to the drain of the reference P-MOS transistor  216 . A feedback path  220  also connects the gate of the feedback P-MOS transistor  214  to the gates of the first and second resistive P-MOS transistors  18  and  26  in the fully differential amplifier  10 . 
     The operation of the common-mode feedback circuit  200  will now be described with respect to an increase in the output voltages of the fully differential amplifier  10 . As the output voltages of the filly differential amplifier  10  increase, more current flows through the first and second P-MOS transistors  202  and  204 . As a result, the currents flowing through the first and second resisters  210  and  212  increases. 
     The current flow from the node  222  to ground is fixed by the application of the reference voltage Vref to the reference P-MOS transistor  216  and the constant current source  218 . Accordingly, any difference between (1) the current flowing from the node  222  through the reference P-MOS transistor  216  and (2) the current flowing to the node  222  from the first and second resistors  210  and  212  affects the current flowing through the feedback P-MOS transistor  214 . Consequently, the current flowing through the feedback P-MOS transistor  214  decreases by the same amount of increase in total current through the first and second resistors  210  and  212 . With a decrease in the current flowing through the feedback P-MOS transistor  214 , the voltage at the gate of the feedback P-MOS transistor  214  increases. The feedback path  220  supplies this increased voltage to the gates of the first and second resistive P-MOS transistors  18  and  26  in the fully differential amplifier  10 . As a result, less current flows through the first and second resistive P-MOS transistors  18  and  26 , and the output voltages from the fully differential amplifier  10  decrease. 
     Unlike conventional common-mode feedback circuits, the common-mode feedback circuit  200  does not rely upon a comparison of voltages to generate the feedback voltage. Instead, the common-mode feedback circuit  200  is a current-mode common-mode feedback circuit that generates a feedback voltage based on the comparison of currents representing the output voltages of the fully differential amplifier with a reference current. The reference current in the common-mode feedback circuit  200  corresponds to the reference voltage Vref; and therefore, the common-mode feedback circuit  200  stabilizes the output voltages of the fully differential amplifier  10  about this reference voltage Vref. Also, in contrast to conventional common-mode feedback circuits, the common mode feedback circuit  200  does not include any high impedance nodes, or suffer from the problems and disadvantages related thereto. Furthermore, the common-mode feedback circuit  200  operates even when powered at low voltage levels. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications are intended to be included within the scope of the following claims.