Abstract:
A squeezable tail current source for use in a differential operational amplifier is disclosed that regulates the current through a main input differential pair while preventing output distortion and allowing high linearity. The squeezable tail current source includes a first transistor pair that replicates a main input transistor pair, wherein both the first transistor pair and the main input transistor pair receive a common voltage input at their respective gates. The squeezable tail current source also includes a second transistor pair, a bias transistor, a first current source, a folding transistor, and a second current source that biases the folding transistor. These components are configured such that current through the main input transistor pair is maintained as the voltage input varies. In addition, the second current source and the folding transistor isolate the bias transistor and the second transistor pair from a drain voltage of the first transistor pair, thereby causing the first transistor pair and the main input transistor pair to have a common drain bias, which prevents output distortion and allows high linearity to be achieved.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates generally to processors with an analog-to-digital conversion interface and a buffer to drive the input. More particularly, this invention relates to differential input buffers, such as used in programmable gain amplifiers of Asymmetric Digital Subscriber Line (ADSL) receivers.  
         [0003]     2. Related Art  
         [0004]     The Asymmetric Digital Subscriber Line (ADSL) is one of the new technologies used for high-speed Internet access. Data rates up to 9 Mb/s are currently available in the standard ADSL offering. An ADSL transmission system is specified in a frequency-division multiplexed scheme, with the downstream (i.e., central office to customer) utilizing a frequency range of approximately 160 kHz to 1.104 MHz and upstream (i.e., customer to central office) utilizing a frequency range of approximately 30 kHz to 138 kHz. In each frequency domain, the frequencies are divided into data bins with 4 kHz frequency spacing. For a discrete multi-tone system such as an ADSL system, the data rate is directly proportional to the signal-to-noise-distortion ratio (SNDR) available at the receive bins. The data rate increases with an increase in SNDR. High SNDR is achieved with large signal and low harmonic distortion and low noise.  
         [0005]     A single ADSL chip integrates many digital circuits with sensitive front-end analog circuitry. An ADSL receiver front-end consists of a programmable gain amplifier and a unit-gain input buffer for backend analog-to-digital conversion (ADC). Because the system is typically manufactured in a digital process, the supply voltage is scaled with the shrinking transistor geometry. For example, a 0.25 μm process uses a supply voltage of 2.5 volts while a 0.13 μm process can tolerate a supply voltage of only 1.2 volts. When a supply voltage is reduced, the signal swing that the unit-gain input buffer can handle is limited due to reduced headroom. This limitation causes a reduction in SNDR.  
         [0006]     It is therefore crucial to maximize the range of the signal swing that the receiver front-end circuit can process while maintaining low distortion performance (i.e., high linearity). Typically this is done with amplifiers that have one or more squeezable tail current sources. Linearity is important because harmonic distortion results in spillover from data bin to data bin that may corrupt the data spectrum. An ADSL system requires a high linearity performance in the neighborhood of 100 dB. To accomplish this, the squeezable tail current source must also be extremely linear, which requires a slightly different squeezable tail current source design.  
         [0007]     What is needed is a fully differential input buffer with a wide signal swing range that allows for high linearity performance independent of the input voltage.  
       SUMMARY OF THE INVENTION  
       [0008]     A squeezable tail current source for use in a differential operational amplifier is disclosed that regulates the current through a main input differential pair while preventing output distortion and allowing high linearity. According to an embodiment of the present invention, the squeezable tail current source includes a first transistor pair that replicates a main input transistor pair, wherein both the first transistor pair and the main input transistor pair receive a common voltage input at their respective gates. The squeezable tail current source also includes a second transistor pair, a bias transistor, a first current source, a folding transistor, and a second current source that biases the folding transistor. These components are configured such that current through the main input transistor pair is maintained as the voltage input varies. Current through the second transistor pair and the bias transistor is squeezed as the voltage input is decreased, thereby increasing a gate voltage of the bias transistor, and thereby causing current through both the first transistor pair and the main input transistor pair to remain nearly constant. In addition, the second current source and the folding transistor isolate the bias transistor and the second transistor pair from a drain voltage of the first transistor pair, thereby causing the first transistor pair and the main input transistor pair to have a common drain bias, which prevents output distortion and allows high linearity to be achieved.  
         [0009]     In one embodiment of the present invention, the first transistor pair, the second transistor pair, and the bias transistor are NMOS transistors, and the folding transistor is a PMOS transistor. In another embodiment of the present invention, the first transistor pair, the second transistor pair, and the bias transistor are PMOS transistors, and the folding transistor is an NMOS transistor.  
         [0010]     A differential input buffer that includes two main input differential pairs, and therefore two squeezable tail current sources is also disclosed, according to an embodiment of the present invention. The differential input buffer includes first and second main input transistor pairs. The differential input buffer further includes a first replica transistor pair that replicates the first main input transistor pair, wherein both the first replica transistor pair and first main input transistor pair receive a first common voltage input at their respective gates. The differential input buffer further includes a second replica transistor pair that replicates the second main input transistor pair, wherein both the second replica transistor pair and second main input transistor pair receive a second common voltage input at their respective gates. The differential input buffer further includes first and second tail transistor pairs, first and second bias transistors, first and second current sources, and first and second folding transistors. The differential input buffer further includes a first biasing current source that biases the first folding transistor, and a second biasing current source that biases the second folding transistor.  
         [0011]     In this differential input buffer embodiment, the first main input transistor pair, the first replica transistor pair, the first tail transistor pair, the first bias transistor, and the first current source are configured such that current through the first main input transistor pair is maintained as the first common voltage input to the first main input pair varies. Similarly, the second main input transistor pair, the second replica transistor pair, the second tail transistor pair, the second bias transistor, and the second current source are configured such that current through the second main input transistor pair is maintained as the voltage input to the second main input pair varies. In embodiments of the present invention, current through a tail transistor pair and its corresponding bias transistor is squeezed as the voltage input to the corresponding main input pair is decreased. This increases a gate voltage of the bias transistor and causes current through both the corresponding replica transistor pair and main transistor pair to remain nearly constant.  
         [0012]     In an embodiment of the present invention, the first biasing current source and the first folding transistor isolate the first bias transistor and the first tail transistor pair from a drain voltage of the first replica transistor pair, thereby causing the first replica transistor pair and the first main input transistor pair to have a common drain bias. Similarly, the second biasing current source and the second folding transistor isolate the second bias transistor and the second tail transistor pair from a drain voltage of the second replica transistor pair, thereby causing the second replica transistor pair and the second main input transistor pair to have a common drain bias. When the replica transistor pair and corresponding main input transistor pair have a common drain bias, output distortion is prevented and high linearity is achieved.  
         [0013]     In one embodiment of the present invention, the first main input transistor pair, the second main input transistor pair, the first replica transistor pair, the second replica transistor pair, the first tail transistor pair, the second tail transistor pair, the first bias transistor, and the second bias transistor are NMOS transistors. In this embodiment, the first folding transistor and the second folding transistor are PMOS transistors. In another embodiment of the present invention, the first main input transistor pair, the second main input transistor pair, the first replica transistor pair, the second replica transistor pair, the first tail transistor pair, the second tail transistor pair, the first bias transistor, and the second bias transistor are PMOS transistors. In this embodiment, the first folding transistor and the second folding transistor are NMOS transistors.  
         [0014]     In an embodiment of the present invention, the differential input buffer further includes first and second stages. The first stage includes an input coupled to drains of the first and second main input transistor pairs and also includes an output. The second stage includes an input coupled to the first stage output and also includes an output coupled to an output of the differential input buffer.  
         [0015]     A method of regulating current through a main input differential pair of a differential amplifier circuit, while maintaining high linearity, is also disclosed. According to an embodiment of the present invention, the method includes providing a voltage input to the main input differential pair. The method further includes mirroring a tail current to that of the current through the main input differential pair by using a squeezable tail current source that includes a current source, a first transistor pair, a bias transistor, and a second transistor pair. The method further includes isolating the bias transistor and the first transistor pair from a drain voltage of the second transistor pair, thereby causing the second transistor pair and the main input transistor pair to have a common drain bias. The method further includes squeezing the tail current as the voltage input is decreased, thereby increasing a gate voltage of the bias transistor and allowing current through the main input differential pair to remain nearly constant.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES  
       [0016]     The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention.  
         [0017]      FIG. 1  illustrates a conventional differential operational amplifier with a squeezable tail current source.  
         [0018]      FIG. 2  illustrates a portion of an operational amplifier, highlighting an exemplary conventional mirrored tail current source.  
         [0019]      FIG. 3  illustrates a portion of the operational amplifier of  FIG. 1 , highlighting an exemplary conventional squeezable tail current source.  
         [0020]      FIG. 4  illustrates a differential operational amplifier including a tail current source that provides high linearity, according to an embodiment of the present invention.  
         [0021]      FIG. 5  illustrates a portion of the operational amplifier of  FIG. 4 , highlighting the tail current source that provides high linearity, according to an embodiment of the present invention.  
         [0022]      FIG. 6  depicts a method of regulating current through a main input differential pair while maintaining high linearity, according to an embodiment of the present invention. 
     
    
       [0023]     The features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0024]     While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those skilled in the art with access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.  
         [0025]      FIG. 1  illustrates a conventional four-input differential operational amplifier with squeezable tail current. Differential operational amplifier  100  has four input terminals represented by transistors forming main input differential pairs  102  and  104 . Differential pair  102  has inputs vip′ and vin. Differential pair  104  has inpus vip and vin′. The transistor banks  106  are bias transistors. An output  108  of a first stage  109  of the amplifier is located at the drains of transistor pair  110 , which feed into an input of a second stage  112  of the amplifier. Ports vop and von are outputs of amplifier  100 . Connecting vop to vin′ and von to vip′ creates a unit-gain input buffer.  
         [0026]     The amplifier circuit  100  includes squeezable tail current sources to handle a large input swing. The portions of differential amplifier  100  represented by box  103  (encompassing transistors  114 ,  118 , and  122 ) and by box  105  (encompassing transistors  116 ,  120 , and  124 ) are referred to herein as squeezable tail current sources  103 ,  105 . As can be seen in  FIG. 1 , main input pair  102  is coupled to squeezable tail current source  103  and main input pair  104  is coupled to squeezable tail current source  105 . The squeezable tail current sources  103 ,  105  presented in  FIG. 1  are described in detail below. Although differential amplifier  100  includes more than one squeezable tail current source, only one squeezable tail current source ( 103 ) is described for simplicity of the description. Both squeezable tail current sources  103 ,  105  of differential amplifier  100  operate in a similar manner, as will be understood by those skilled in the art.  
         [0027]      FIG. 2  illustrates a conventional version  200  of a mirrored tail current source that is not shown in  FIG. 1 , but discussed here for explanatory purposes. The squeezable tail current source  203  operates as follows. To mirror current bias, a current source  217  and a bias transistor  218  are needed, and the current is mirrored to transistor pair  214 . As the input signal (vip′, vin) decreases, the tail current through transistor pair  214  is squeezed. The drain voltage (Vds) of transistor pair  214  decreases and the current through transistor pair  214  diminishes. When this happens, the common source voltage of main input pair  202  decreases, and the current through main input pair  202  decreases. To counter this problem, a replica differential pair  122  is used, as shown and described with reference to  FIGS. 1 and 3 .  FIG. 3  illustrates squeezable tail current source  103 , which is the squeezable tail current source  103  of differential operational amplifier  100  of  FIG. 1  that corresponds to main input pair  102 .  
         [0028]     As seen in  FIG. 1 , the input (vip′, vin) of replica pair  122  is tied to the same node as the input (vip′, vin) of the main input pair  102 , forming a duplicate of main input pair  102 . As the input signal (vip′, vin) is decreased, the common source voltage of replica pair  122  is decreased, just as it is for main input pair  102 . In order to maintain a constant current flowing through bias transistor  118 , the gate voltage (Vgs) of bias transistor  118  must increase. With the configuration shown in  FIGS. 1 and 3 , when the input (vip′, vin) is decreased, the current through bias transistor  118  is squeezed along with the current through transistor pair  114 . In this way, a self-adjusting loop is formed by the connection of the gate of bias transistor  118  to the drains of replica pair  122 . This loop adjusts the gate voltage (Vgs) of bias transistor  118  such that the gate voltage (Vgs) is increased as input (vip′, vin) is decreased. In other words, when input (vip′, vin) decreases, the drain voltage (Vds) of replica pair  122  increases,. which in turn increases the gate voltage (Vgs) of bias transistor  118 . This causes the current through replica pair  122  to remain nearly constant, and therefore the mirror tail current into the main input pair  102  remains nearly constant as well, not changing dramatically when the input (vip′, vin) swings up and down.  
         [0029]     A problem with tail current source  103 , however, is that the drain voltages (Vds) of main input pair  102  and replica pair  122  are different for most of the input signal. As can be seen in  FIGS. 1 and 3 , the drain of replica pair  122  is tied to the gate of transistor pair  114 , but the drain of main input pair  102  is not. Because of this difference, if the input (vip′, vin) is squeezed, the bias transistor  118  and transistor pair  114  will not have a common drain bias. The drain voltage of bias transistor  118  is different than the drain voltage of transistor pair  114  in this instance. Sub-micron devices have significant channel modulation effects. Differences in the drain voltage of the tail current source result in current variations (i.e., non-linear current) into main input pair  102  as the input signal (vip′, vin) changes, resulting in distortions. In other words, the drain voltage (Vds) of replica pair  122  is fixed by the gate voltage (Vgs) of bias transistor  118 . Therefore, differential pairs  102  and  122  operate at different operating points, which causes non-linearity of the tail current entering main input pair  102 , resulting in distortion in the output (vop, von).  
         [0030]     The distortion problem is more severe when the input signal (vip′, vin) is high. For a high input signal, replica pair  122  can enter triode region when the gate voltage is higher than its drain voltage by a threshold voltage. As main input pair  102  operates in the saturation region, the drain voltage of bias transistor  118  and the drain voltage of transistor pair  114  are so different that good linearity (e.g., in the 100 dB range) is not achievable.  
         [0031]     In order to rectify problems with the tail current source  103  described above, the feedback loop created from replica pair  122  and bias transistor  118  in  FIG. 3  is modified into a single-ended self-biased folded feedback loop, as shown in both  FIGS. 4 and 5 .  
         [0032]      FIG. 4  illustrates a differential operational amplifier  400  that includes squeezable tail current sources  432 ,  434  that allow high linearity, according to an embodiment of the present invention. The portions of differential amplifier  400  represented by box  432  (encompassing transistors  114 ,  118 ,  122 , and  440 ) and by box  434  (encompassing transistors  116 ,  120 ,  124 , and  442 ) are the squeezable tail current sources. As can be seen in  FIG. 4 , main input pair  102  is coupled to squeezable tail current source  432  and main input pair  104  is coupled to squeezable tail current source  434 . The squeezable tail current sources  432 ,  434  presented in  FIG. 4  are described in detail below. Although differential amplifier  400  includes more than one squeezable tail current source, only one squeezable tail current source ( 432 ) is described for simplicity of the description. Both squeezable tail current sources  432 ,  434  of differential amplifier  400  operate in a similar manner, as will be understood by those skilled in the art.  
         [0033]      FIG. 5  illustrates, according to an embodiment of the present invention, a single squeezable tail current source  432  as used with main input pair  102  of  FIG. 400 . In comparison, squeezable tail current source  432  is similar to squeezable tail current source  103 , but also includes a PMOS transistor  440  and a current source  444 . PMOS transistor  440  is a folding transistor that is biased by current source  444 . The feedback from the drain of replica pair  122  enters the source of folding transistor  440  rather than the gate of bias transistor  118 . In this way, bias transistor  118  and transistor pair  114  are isolated from the drain voltage of replica pair  122  by folding transistor  440  and current source  444 . As can be seen in  FIGS. 4 and 5 , folding transistor  440  is biased at the same voltage as transistor pair  110  of amplifier  400 . As a result, replica pair  122  and main input pair  102  operate in substantially the same manner, with a common drain bias. Even with a large input (vip′, vin), bias transistor  118  and transistor pair  114  also operate in the same bias condition (i.e., they have a common drain bias). With this arrangement, the current is duplicated and is not distorted by the input. Because no tail current distortion is generated, high linearity is readily achieved. The preferred linearity necessary for an ADSL system is 90 dB or higher, with a preferred range of 90 dB-120 dB. Those skilled in the art will appreciate that the replica input branch is scaled independently to meet different bandwidth requirements for settling.  
         [0034]     The squeezable tail current source  103  of the present invention is shown in  FIGS. 4 and 5  as an NMOS current source with PMOS folding transistor  440  and NMOS input pair  102 . Alternatively, a squeezable tail current source according to the present invention is possible as a complimentary PMOS current source with an NMOS folding transistor and a PMOS input pair (not shown).  
         [0035]     One advantage of the described invention is that although folding transistor  440  was added, there is still only a single-pole feedback loop. Therefore, there is no compensation issue and no compensation capacitor is needed for stability.  
         [0036]     A method, according to an embodiment of the present invention, of regulating current through a main input differential pair while maintaining high linearity is described in reference to  FIG. 6 . Method  600  begins at step  680  and immediately proceeds to step  682 . In step  682 , a voltage input is provided to a main input differential pair. For example, in  FIG. 5 , the voltage input is depicted as vip′ and vin at the gates of main input pair  102 . In step  684 , a tail current is mirrored to that of the current through the main input differential pair  102  by using a squeezable tail current source as depicted in  FIG. 5 . The squeezable tail current source of  FIG. 5  includes a current source ( 117 ), a first transistor pair (transistor pair  114 ), a bias transistor  118 , and a second transistor pair (replica pair  122 ) that are used to mirror the current. In step  686 , the bias transistor ( 118 ) and the first transistor pair (transistor pair  114 ) are isolated from a drain voltage of the second transistor pair (replica pair  122 ), thereby causing the second transistor pair (replica pair  122 ) and the main input pair  102  to have a common drain bias. In step  688 , the tail current is squeezed as the voltage input (vip′, vin) is decreased, thereby increasing a gate voltage of the bias transistor  118  and allowing current through the main input differential pair  102  to remain nearly constant. The method ends at step  690 .  
         [0037]     The invention described herein mainly addresses problems that occur when there is a low supply and a large swing. Linearity is not a problem when the supplied voltage is high. For example, if the supply is 5 volts, and there is only a 1-volt swing, linearity is not a problem because the swing is relatively small compared to the 5-volt supply, and a squeezable tail current is not needed. A tail current source like the one shown in  FIG. 2  is adequate in this example since transistor pair  214  does not enter the linear region. However, if the supply is 2.5 volts, and there is a 1-volt swing, linearity is a problem because the swing is relatively large compared to the 2.5-volt supply, which results in limited headroom for the tail current source to remain in the saturation region. One advantage of the described invention is that its operation is substantially independent of the input level. Whether the input is low or high, the tail current entering main input pair  102  remains nearly constant and no distortion is introduced. In addition, a large swing is representative of a large signal. If this signal can be enhanced with high linearity maintained, the better the signal-to-noise-distortion ratio (SNDR). As SNDR is increased, the faster the data rate that can be achieved.  
       CONCLUSION  
       [0038]     This disclosure presents a fully differential input buffer with a wide signal swing range that allows for high linearity performance. While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be understood by those skilled in the art that various changes in form and details can be made therein without departing from the spirit and scope of the invention as defined in the appended claims. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.