Patent Publication Number: US-2021194446-A1

Title: Operational transconductance amplifier and a method for converting a differential input voltage to a differential output current

Description:
BACKGROUND OF THE INVENTION 
     Operational amplifier that require a responds to large input signals usually use some slew rate boosting techniques. Most operational amplifiers uses a differential input stage constructed with an input pair transistors and a tail current. The current of a differential input stages is defined by its tail current. In the event of a step response, the maximum current the differential pair can drive is limited by the tail current. Common methods to mitigate this challenge is known as current boosting or slew rate enhancement. There are many different slew rate boosting techniques. In general they are divided in to two groups. 
     Techniques which enhance the output driving current (such as class AB output stage) and ones which boost the input differential pair tail current. Most of the techniques to boost tail current are based on a feedback current branches or voltage nodes further along the signal path of the amplifier. Such techniques defines the amplifier architecture. 
     SUMMARY 
     According to an embodiment of the invention there may be provided an operational transconductance amplifier and a method. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings in which: 
         FIG. 1  is example of an operational transconductance amplifier; 
         FIG. 2  is example of an operational transconductance amplifier; 
         FIG. 3  is example of a method; and 
         FIG. 4  is an example of a comparison of relationships between differential input voltage and tail current. 
     
    
    
     It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, and components have not been described in detail so as not to obscure the present invention. 
     The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings. 
     Because the illustrated embodiments of the present invention may for the most part, be implemented using electronic components and circuits known to those skilled in the art, details will not be explained in any greater extent than that considered necessary as illustrated above, for the understanding and appreciation of the underlying concepts of the present invention and in order not to obfuscate or distract from the teachings of the present invention. 
       FIG. 1  illustrates an example of operational transconductance amplifier  10 . 
     The operational transconductance amplifier is configured to drive large current with respect to its input voltage and it may be limited only by the dimensions of the differential input pair. 
     The suggested operational transconductance amplifier may provide a simple input stage architecture which is insensitive to the type of load it drives or the amplifier architecture. In addition, the operational transconductance amplifier doesn&#39;t interfere a differential pair operation, doesn&#39;t change its characteristics (including voltage swing, noise or offset). 
     The operational transconductance amplifier  10  may include: 
     A differential input pair  30  that includes a first transistor (M 1   31 ) and a second transistor (M 2   32 ) that are coupled to each other at a certain node (at node  90 ) together with a current tail ( 40 ). The differential input pair is configured to convert a differential input voltage (which equals the difference between first input voltage VIN 1   21  and second input voltage VIN 2   22 ) to a first output current I 1   11  and a second output current I 2   12 . 
     A current source that is coupled to the certain node and may include a fixed tail current source (ITAIL  40 ) and adjustable current sources (third transistor M 3   33  and fourth transistor M 4   34 ). The combination of the fixed tail current from the fixed current source and the currents outputted from the adjustable current sources is a boosted tail current which is driven to the output current (IL  11  I 2   12 ) according to the input voltages. 
     In contrary to a common differential pair (in which the maximum output current in either of the current outputs is limited to the fixed current tail current)—the suggested operational transconductance amplifier provides a much higher output current. 
     Feedback unit ( 70 ) that is coupled to the certain node and is configured to (a) receive the differential input voltage, (b) maintain a voltage (VCM  91 ) of the certain node  90  substantially fixed regardless of changes in the differential input voltage, and (c) control the adjustable current sources (third transistor M 3   33  and fourth transistor M 4   34 ) based on the differential input voltage. 
     The feedback unit  70  may include first feedback sub-unit  71  and second feedback sub-unit  72 . 
     First feedback sub-unit  71  is configured to (a) receive a first input voltage (VIN 1   21 ), and (b) control a first adjustable current source (M 3   33 ) of the adjustable current sources, based on the first input voltage. 
     Second feedback sub-unit  72  is configured to (a) receive a second input voltage (VIN 2   22 ), and (b) control a second adjustable current source (M 4   34 ) of the adjustable current sources, based on the second input voltage. 
     First feedback sub-unit  71  is configured to maintain the voltage (VCM  91 ) of the certain node substantially fixed regardless of an increase in the second input voltage (VIN 2   22 ). 
     Second feedback sub-unit  72  is configured to maintain the voltage (VCM  91 ) of the certain node substantially fixed regardless of an increase in the first input voltage (VIN 1   21 ). 
     Each one of the first feedback sub-unit and the second feedback sub-unit could be comprises a folded cascode amplifier. 
       FIG. 1  illustrates the first feedback sub-unit  71  as including a first amplifier  61  and a first voltage source  51  that virtually adds a threshold voltage Vth to VCM  91  (voltage of node  90 ). The first voltage source is coupled to a non-inventing input of the first amplifier  61 . An inverting input of the first amplifier is fed by the first input voltage VIN 1   21 . 
       FIG. 1  illustrates the second feedback sub-unit  72  as including second amplifier  62  and a second voltage source  52  that virtually adds a threshold voltage Vth the VCM  91  of node  90 . The second voltage source  52  is coupled to a non-inventing input of the second amplifier  62 . An inverting input of the second amplifier  62  is fed by the second input voltage VIN 2   22 . 
     The first and second amplifiers ( 61  and  62 ) are configured to detect changes in the first and second input voltages (minus Vth) in reference to the VCM  91  (of node  90 ). 
     When one of the first and second input voltages drops below (VCM  91 +VTN) the corresponding amplifier drives its output high turning on the respective adjustable current source out of third and fourth transistors (M 3   33  or M 4   34 ) so as to driver additional tail current to the outputs of M 1   31  or M 2   32  (for outputting first output current I 1  and second output current, I 2 ). 
     The selective operation of the adjustable current sources creates a low impedance on node  90  enabling first and second transistors M 1   31  and M 2   32  to behave as a common source NMOS with no limit current. This allows a user to connect any load to the terminal over which the first and second output currents (I 1  and I 2 ) are supplied. 
       FIG. 2  illustrates an example of operational transconductance amplifier  10 ′. 
     The first feedback sub-unit  71  includes seventh transistor M 7   37  and serially coupled elements that include (a) first additional fixed current source I 3   43 , (b) fifth transistor M 5   35 . and (c) second additional fixed current source I 5   45 . 
     The gates of the seventh transistor M 7   37  and the first transistor M 1   31  receive the first input voltage VIN 1   21 . 
     Fifth transistor M 5   35  and eighth transistor M 8   38  may act as cascode stages. 
     The second feedback sub-unit  72  includes eighth transistor M 8   38  and serially coupled elements that include (a) third additional fixed current source I 4   44 , (b) sixth transistor M 6   36 , and (c) fourth additional fixed current source I 6   46 . 
     The gates of the eighth transistor M 8   38  and the second transistor M 2   32  receive the second input voltage VIN 2   22 . 
     Seventh transistor M 7   37  and eighth transistor M 8   38  are in a folded cascade connection which allows the wide input swing option. With this architecture, each one of the first and second input voltages can swing between VTH+VDS (drain source voltage) as in a standard differential N-type pair to as high as VDD−VDS (VDD being the voltage supply). To decrease offset and noise of this topology the value of the tail current can be adjust to place the amplifiers at saturation level up to certain differential input level. 
     This method and device allows to be used in any amplifier design that requires a differential input stage. 
     The suggested operational transconductance amplifier improves the driving capability of the differential pair with no effect to its other parameters such as input voltages, noise and offset. This concept can be implemented also for a PMOS type differential pair. 
       FIG. 3  is an example of method  100 . 
     Method  100  is for converting a differential input voltage to an amplifier tail current. 
     Method  100  may start by steps  110 ,  120  and  130 . 
     Step  110  may include receiving, by a first differential pair, the differential input voltage. The differential input voltage is a difference between a first input voltage and a second input voltage. The first differential pair may include a first transistor and a second transistor that are coupled to each other at a certain node. 
     Step  110  may be followed by step  115  of converting, by the first differential pair, the differential input voltage to two output currents. 
     Step  120  may include feeding, by a current source that is coupled to the certain node, the first differential pair with a boosted tail current. The current source may include an adjustable current sources and may include a fixed tail current source. The boosted tail current is a sum of currents outputted by the first tail current source and the adjustable current sources. 
     Step  130  may include receiving, by a feedback unit that is coupled to the certain node, the differential input voltage. 
     Step  130  may be followed by step  135 . 
     Step  135  may include maintaining, by the feedback unit, a voltage of the certain node substantially fixed regardless of changes in the differential input voltage. This may be achieved by controlling, by the feedback unit, the adjustable current sources based on the differential input voltage. 
     The feedback unit may include a first feedback sub-unit and a second feedback sub-unit. 
     Step  130  may include receiving, by the first feedback sub-unit, the first input voltage, and receiving, by the second feedback sub-unit, the second input voltage. 
     Step  135  may include at least one of the following: 
     a. Controlling, by the first feedback sub-unit, a first adjustable current source of the adjustable current sources, based on the first input voltage.
 
b. Controlling, by the second feedback sub-unit, a second adjustable current source of the adjustable current sources, based on the second input voltage.
 
c. Maintaining, by the first feedback sub-unit, the voltage of the certain node substantially fixed regardless of an increase in the second input voltage.
 
d. Maintaining, by the second feedback sub-unit, the voltage of the certain node substantially fixed regardless of an increase in the first input voltage.
 
     Each one of the first feedback sub-unit and the second feedback sub-unit may include a folded cascode amplifier or any other eligible amplifier. 
       FIG. 4  is an example of a comparison of relationships between differential input voltage and output currents I 1  and I 2 . 
     Curves  201  and  202  represent the relationship between I 1  and I 2  (respectively) and the input differential voltage of a prior art circuit. 
     Curves  211  and  212  represent the relationship between I 1  and I 2  (respectively) and the input differential voltage of the operational transconductance amplifier of  FIGS. 1-2 . 
     Any reference to any of the terms “comprise”, “comprises”, “comprising” “including”, “may include” and “includes” may be applied to any of the terms “consists”, “consisting”, “consisting essentially of”. For example—any of the rectifying circuits meaning illustrated in any figure may include more components that those illustrated in the figure, only the components illustrated in the figure or substantially only the components illustrated in the figure. 
     In the foregoing specification, the invention has been described with reference to specific examples of embodiments of the invention. It will, however, be evident that various modifications and changes may be made therein without departing from the broader spirit and scope of the invention as set forth in the appended claims. Moreover, the terms “front,” “back,” “top,” “bottom,” “over,” “under” and the like in the description and in the claims, if any, are used for descriptive purposes and not necessarily for describing permanent relative positions. It is understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in other orientations than those illustrated or otherwise described herein. 
     Those skilled in the art will recognize that the boundaries between logic blocks are merely illustrative and that alternative embodiments may merge logic blocks or circuit elements or impose an alternate decomposition of functionality upon various logic blocks or circuit elements. Thus, it is to be understood that the architectures depicted herein are merely exemplary, and that in fact many other architectures can be implemented which achieve the same functionality. Any arrangement of components to achieve the same functionality is effectively “associated” such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality can be seen as “associated with” each other such that the desired functionality is achieved, irrespective of architectures or intermedial components. Likewise, any two components so associated can also be viewed as being “operably connected,” or “operably coupled,” to each other to achieve the desired functionality. 
     Furthermore, those skilled in the art will recognize that boundaries between the above described operations merely illustrative. The multiple operations may be combined into a single operation, a single operation may be distributed in additional operations and operations may be executed at least partially overlapping in time. Moreover, alternative embodiments may include multiple instances of a particular operation, and the order of operations may be altered in various other embodiments. 
     Also for example, in one embodiment, the illustrated examples may be implemented as circuitry located on a single integrated circuit or within a same device. Alternatively, the examples may be implemented as any number of separate integrated circuits or separate devices interconnected with each other in a suitable manner. 
     However, other modifications, variations and alternatives are also possible. The specifications and drawings are, accordingly, to be regarded in an illustrative rather than in a restrictive sense. 
     In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word ‘comprising’ does not exclude the presence of other elements or steps then those listed in a claim. Furthermore, the terms “a” or “an,” as used herein, are defined as one or more than one. Also, the use of introductory phrases such as “at least one” and “one or more” in the claims should not be construed to imply that the introduction of another claim element by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim element to inventions containing only one such element, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an.” The same holds true for the use of definite articles. Unless stated otherwise, terms such as “first” and “second” are used to arbitrarily distinguish between the elements such terms describe. Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements. 
     While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those of ordinary skill in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.