Abstract:
A linearity-improved differential amplification circuit is provided, A linearity-improved differential amplification circuit comprises a main differential amplification unit differentially amplifying a first and a second input signals, a main bias unit biasing the main differential amplification unit, a first current source coupled in series between a power supply voltage terminal and the main bias unit and an auxiliary differential amplification unit differentially amplifying the first and the second input signal and coupled to the main differential amplification unit.

Description:
This non-provisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 10-2005-0078532 filed in Korea on Aug. 26, 2005, the entire contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a differential amplification circuit, and more particularly, to a differential amplification circuit extending an operation region and improving a linearity. 
   2. Description of the Background Ar 
   A radio frequency (RF) circuit configured with a single ended circuit often has disadvantages such as a signal coupling event and an even order distortion event in a highly integrated circuit like a system-on-a-chip (SoC). 
   Hence, a differential circuit is generally employed to overcome the above disadvantages. 
   The typical differential circuit is used more frequently in a highly integrated circuit (e.g., SoC) than in the aforementioned single ended circuit because the differential circuit has a high common mode rejection ratio (CMRR) and second-order intercept point (IIP2). 
   However, when the typical differential circuit uses a fully differential amplifier (FDA), an operation region may be reduced due to a voltage headroom limitation associated with the FDA. 
   Another type of FDA was introduced by Nokia Corporation to overcome the voltage headroom limitation 
   The other type of FDA was taught in an article, entitled “Cancellation of Second-Order Intermodulation Distortion and Enhancement of IIP2 in Common-Source and Common-Emitter RE Transconductors” (IEEE, Vol, 52, NO. 2, February, 2005), 
     FIG. 1  illustrates the other type of FDA introduced by Nokia Corporation. 
   The other type of FDA includes first to fourth transistors MN 1 , MN 2 , MN b1 , and MN b2 , a current source Isb, first and second bias resistors R B  and R B , and first and second capacitors C 1  and C 2 . 
   The first and second transistors MN 1  and MN 2  are parts of an amplification circuit, wherein the first and second transistors MN 1  and MN 2  are configured as a differential pair that amplifies a difference between input voltages Vin+ and Vin−. 
   The first and second transistors MN 1  and MN 2  are biased by the first and second bias resistors R B  and R B , and the current source I sb . The first and second bias resistors R B  and R B  have the same resistance level. 
   The first and second capacitors C 1  and C 2  are configured in a direct current (DC)-blocking circuit that removes a DC component from the input voltages Vin+ and Vin−. The third and fourth transistors MN b1  and MN b2  are configured as a bias circuit. 
   The first and second transistors MN 1  and MN 2 , which are the differential pair of the amplification circuit, are configured to common source circuits. Due to this configuration, the first and second transistors MN 1  and MN 2  can reduce second-order intermodulation (IM2) distortion and enhance IIP2. 
   A method of reducing the IM2 distortion and enhancing the IIP2 by the configuration of the amplification circuit with the differential circuit is described in the aforementioned article, and thus, detailed description thereof will be omitted. 
   The above differential circuit allows a sufficient level of voltage headroom, and thus, the operation region can be enlarged, 
   However, the other type of FDA may not improve the linearity since the improvement on the linearity of the entire differential circuit usually depends on the improvement on the linearity of the first and second transistors MN 1  and MN 2  of the amplification circuit. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention is directed to provide a differential amplification circuit that can overcome a voltage headroom limitation, enhance a CMRR or IIP2, and improve third-order intercept point (IIP3). 
   The present invention is also directed to provide a differential amplification circuit that can improve the linearity thereof. 
   According to an embodiment of the present invention, a differential amplification circuit with improved linearity comprises a main differential amplification unit differentially amplifying a first and a second input signals; a main bias unit biasing the main differential amplification unit; a first current source coupled in series between a power supply voltage terminal and the main bias unit; and an auxiliary differential amplification unit differentially amplifying the first and the second input signal and coupled to the main differential amplification unit, 
   Consistent with the embodiment of the present invention, the differential amplification circuit further comprises a first load and a second load coupled between the power supply voltage terminal and the main differential amplification unit 
   Consistent with the embodiment of the present invention, the main differential amplification unit comprises a first transistor and a second transistor, each comprising first to third terminals wherein the first transistor and the second transistors are configured with a common-source circuit, each. 
   Consistent with the embodiment of the present invention, the auxiliary differential amplification unit comprises a third transistor and a fourth transistor, each comprising first to third terminals, wherein the third transistor and the fourth transistor are configured with a common-source circuit, each, 
   Consistent with the embodiment of the present invention, the first transistor and the third transistor are coupled together; and the second transistor and the fourth transistor are coupled together. 
   Consistent with the embodiment of the present invention, the first transistor and the third transistor have a different transconductance characteristic; and the second transistor and the fourth transistor has a different transconductance characteristic. 
   Consistent with the embodiment of the present invention, the auxiliary differential amplification unit comprises one or more than one transistor coupled in parallel. 
   Consistent with the embodiment of the present invention, the main bias unit comprises a fifth transistor and a sixth transistor, each comprising first to third terminals, wherein the fifth transistor and the sixth transistor are configured with a common-source circuit, each, and the first transistor and the fifth transistor are coupled together; and the second transistor and the sixth transistor are coupled together. 
   Consistent with the embodiment of the present invention, the differential amplification circuit may further comprise a seventh transistor configured with a common-source circuit; and a second current source coupled in series between the power supply voltage terminal and the seventh transistor, wherein the auxiliary bias unit biases the auxiliary differential amplification unit. 
   Consistent with the embodiment of the present invention, the first to seventh transistors are one of metal-oxide semiconductor field effect transistors (MOSFETs) and bipolar junction transistors (BTJs). 
   Detailed description of various embodiments of the present invention will be provided herein below with reference to the accompanying drawings, 
   Various features and advantages of the present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention on an amplification circuit with improved linearity and a frequency converter using the same are shown. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art. Also, the invention is defined within the scope of the appended claims. Like reference numerals denote like elements even in different drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in detail with reference to the following drawings in which like numerals refer to like elements. 
       FIG. 1  illustrates a simplified diagram of a typical differential amplification circuit introduced by Nokia Corporation; 
       FIG. 2  illustrates a simplified diagram of a linearity-improved differential amplification circuit according to an embodiment of the present invention; 
       FIG. 3  illustrates a simplified diagram of a linearity-improved differential amplification circuit according to another embodiment of the present invention; and 
       FIG. 4  illustrates a graph of a linearity characteristic exhibited by a linearity improved differential amplification circuit according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF EMBODIMENTS 
   Embodiments of the present invention will be described in a more detailed manner with reference to the drawings. 
     FIG. 2  illustrates a simplified diagram of a linearity-improved amplification circuit according to an embodiment of the present invention. 
   The differential amplification circuit comprises a main differential amplification unit  210 , an auxiliary differential amplification unit  220 , a main bias unit  230 , and a current source I sb1 . 
   The main differential amplification unit  210  comprises a first transistor MN 21a , a second transistor MN 22a , a first capacitor C 21a , a second capacitor C 22a  a first load terminal R 21 , and a second load terminal R 22 . 
   The auxiliary differential amplification unit  220  comprises a third transistor MN 21b , a fourth transistor MN 22b , and a third capacitor C 21b , and a fourth capacitor C 22b . 
   The main bias unit  230  comprises a fifth transistor MN b1 , a sixth transistor MN b2 , a first bias resistor R b1 , and a second bias resistor R b2 . 
   A gate terminal of the first transistor MN 21a  is coupled to a node {circle around ( 2 )}. A drain terminal of the first transistor MN 21a  is coupled to a node {circle around ( 4 )}, and a source terminal of the first transistor M 21a  is coupled to a ground terminal. 
   A gate terminal of the second transistor MN 22a  is coupled to a node {circle around ( 3 )}. A drain terminal of the second transistor MN 22a  is coupled to a node {circle around ( 5 )}, and a source terminal of the second transistor MN 22a  is coupled to the ground terminal. 
   A gate terminal of the third transistor MN 21b  is coupled to one common terminal between a first bias terminal V bias1  and the third capacitor C 21b . A drain terminal of the third transistor MN 21b  is coupled to the node {circle around ( 4 )}, and a source terminal of the third transistor MN 21b  is coupled to the ground terminal. 
   A gate terminal of the fourth transistor MN 22b  is coupled to one common terminal between a second bias terminal and the fourth capacitor C 22b . A drain terminal of the fourth transistor MN 22b  is coupled to the node {circle around ( 5 )}, and a source terminal of the fourth transistor MN 22b  is coupled to the ground terminal. 
   A drain terminal of the fifth transistor MN b1  is coupled to a node {circle around ( 1 )}, and a gate terminal of the fifth transistor MN b1  is coupled to the node {circle around ( 2 )}. A source terminal of the fifth transistor MN b1  is coupled to the ground terminal. 
   A drain terminal of the sixth transistor MN b2  is coupled to the node {circle around ( 1 )}, and a gate terminal of the sixth transistor MN b2  is coupled to the node {circle around ( 3 )}. A source terminal of the sixth transistor MN b2  is coupled to the ground terminal. 
   One terminal of the first bias resistor R b1  is coupled to the node {circle around ( 1 )}, and the other terminal of the first bias resistor R b1  is coupled to the node {circle around ( 2 )}. 
   One terminal of the second bias resistor R b2  is coupled to the node {circle around ( 1 )}, and the other terminal of the second bias resistor R b2  is coupled to the node {circle around ( 3 )}. 
   An output terminal of the current source I sb1  is coupled to the node {circle around ( 1 )}, and an input terminal of the current source I sb1  is supplied with a power supply voltage V DD . 
   One terminal of the first capacitor C 21a  is coupled to the node {circle around ( 2 )}, and the other terminal of the first capacitor C 21a  is supplied with a first input voltage Vin+. 
   One terminal of the second capacitor C 22a  is coupled to the node {circle around ( 3 )}, and the other terminal of the second capacitor C 22a  is supplied with a second input voltage Vin−. 
   The first input voltage Vin+ is supplied to the other terminal of the first capacitor C 21a  and the other terminal of the third capacitor C 21b . The second input voltage Vin− is supplied to the other terminal of the second capacitor C 22a  and the other terminal of the fourth capacitor C 22b . 
   One end of the first load terminal R 21  is coupled to the node {circle around ( 4 )}, and the other end of the first load terminal R 21  is supplied with the power supply voltage V DD . 
   One end of the second load terminal R 22  is coupled to the node {circle around ( 5 )}, and the other end of the second load terminal R 22  is supplied with the power supply voltage V DD . 
   A first output terminal Vout+ is coupled to the node {circle around ( 4 )}, and a second output terminal Vout− is coupled to the node {circle around ( 5 )}. 
   When the first input voltage Vin+ and the second input voltage Vin− are supplied, the first to fourth capacitors C 21a , C 22a , C 21b , and C 22b  block DC components of the first and second input voltages Vin+ and Vin−. Herein, the first to fourth capacitors C 21a , C 22a , C 21b , and C 22b  serve as a DC-blocking circuit. 
   The first transistor MN 21a  is biased due to the current source I sb1  supplied to the first bias resistor R b1  from a power supply voltage V DD  terminal that is coupled to the fifth transistor MN b1 . 
   The second transistor MN 22a  is biased due to the current source I sb1  supplied to the second bias resistor R b1  from the power supply voltage V DD  terminal that is coupled to the sixth transistor MN b2 . 
   Due to the above circuit configuration, when the first and second input voltages Vin+ and Vin− that do not have the DC components are supplied to the main differential amplification unit  210 , the main differential amplification unit  210  amplifies a difference between the first input voltage Vin+ and the second input voltage Vin− and outputs the amplified voltage difference. 
   Particularly, the main differential amplification unit  210  with the current source I sb1  is a FDA, and thus has a high CMRR and IIP2. 
   The first transistor MN 21a  of the main differential amplification unit  210  is coupled with the third transistor MN 21b  of the auxiliary differential amplification unit  220 . The third transistor MN 21b  is biased due to a first bias voltage V bias1 . 
   The auxiliary differential amplification unit  220  uses a method of offsetting the non-linearity of the main differential amplification unit  210  with use of a pseudo differential amplifier (PDA). 
   More specifically, to improve the linearity, the transconductance of the third transistor MN 21b , which is typically expressed as “gm″,” is used to change a negative value of the transconductance gm″ of the first transistor MN 21a  into a positive value thereof, so that the transconductance gm″ can be ignored. 
   On the basis of the same circuit configuration, the second transistor MN 22a  of the main differential amplification unit  210  is coupled with the fourth transistor MN 22b  of the auxiliary differential amplification unit  220 , and the fourth transistor MN 22b  is biased due to a second bias voltage V bias2 . 
   To improve the linearity, the transconductance gm″ of the fourth transistor MN 22b  is used to change a negative value of the transconductance gm″ of the second transistor MN 22a  into a positive value thereof, so that the transconductance gm″ can be ignored. 
   That is, optimum values of the first and second bias voltages V bias1  and V bias2  that can reduce the non-linearity of the first and second transistors MN 21a  and MN 22a  of the illustrated differential amplification circuit are set such that an added value of a second derivative value of the transconductance (i.e., gm″) of the first and second transistors MN 21a  and MN 22a  with respect to a gate-source voltage and a second derivative value of the transconductance (i.e., gm″) of the third and fourth transistors MN 21b  and MN 22b  with respect to a gate-source voltage is minimum in the operation region of the entire circuit. 
   Due to the illustrated circuit configuration, the linearity of the differential amplification circuit can be improved. The biasing is applied such that the first and second transistors M 21a  and MN 22a  operate in a saturation region, while the third and fourth transistors MN 21b  and MN 22b  operate in a subthreshold region. 
   The auxiliary differential amplification unit  220  has almost no gain since current barely flows to the third and fourth transistors MN 21b  and MN 22b  of the auxiliary differential amplification unit  220 . Therefore, the CMMR is high because the CMMR performs operations that depend on the main differential amplification unit  210 . This high CMMR leads to high IIP2. As a result, the linearity can be improved along with enhancing the advantages of the differential amplification circuit. 
     FIG. 3  illustrates a simplified diagram of a linearity-improved differential amplification circuit according to another embodiment of the present invention. 
   The differential amplification circuit comprises a main differential amplification unit  310 , an auxiliary differential amplification unit  320 , a main bias unit  330 , an auxiliary bias unit  340 , and a current source I sb1 . 
   The main differential amplification unit  310  comprises a first transistor MN 31a , a second transistor MN 32a , a first capacitor C 31a , a second capacitor C 32a , a first load terminal R 31  and a second load terminal R 32 . 
   The auxiliary differential amplification unit  320  comprises a third transistor MN 31b , a fourth transistor MN 32b , and a third capacitor C 31b , and a fourth capacitor C 32b . 
   The main bias unit  330  comprises a fifth transistor MN b1 , a sixth transistor MN b2 , a first bias resistor R b1 , and a second bias resistor R b2 . 
   The auxiliary bias unit  340  comprises a seventh transistor MN b3 , a third bias resistor R b3 , a fourth bias resistor R b4 , and another current source I sb2 . 
   A gate terminal of the first transistor MN 31a  is coupled to a node {circle around ( 2 )}. A drain terminal of the first transistor MN 31a  is coupled to a node {circle around ( 4 )}, and a source terminal of the first transistor MN 31a  is coupled to a ground terminal. 
   A gate terminal of the second transistor MN 32a  is coupled to a node {circle around ( 3 )}. A drain terminal of the second transistor MN 32a  is coupled to a node {circle around ( 5 )}, and a source terminal of the second transistor M 32a  is coupled to the ground terminal. 
   A gate terminal of the third transistor MN 21b  is coupled to one common terminal between the third bias terminal R b3  and the third capacitor C 31b . A drain terminal of the third transistor MN 31b  is coupled to the node {circle around ( 4 )}, and a source terminal of the third transistor MN 31b  is coupled to the ground terminal. 
   A gate terminal of the fourth transistor MN 32b  is coupled to one common terminal between the fourth bias resistor R b4  and the fourth capacitor C 32b . A drain terminal of the fourth transistor MN 32b  is coupled to the node {circle around ( 5 )}, and a source terminal of the fourth transistor MN 32b  is coupled to the ground terminal. 
   A drain terminal of the fifth transistor MN b1  is coupled to a node {circle around ( 1 )}, and a gate terminal of the fifth transistor MN b1  is coupled to the node {circle around ( 2 )}. A source terminal of the fifth transistor MN b1  is coupled to the ground terminal. 
   A drain terminal of the sixth transistor MN b2  is coupled to the node {circle around ( 1 )}, and a gate terminal of the sixth transistor MN b2  is coupled to the node {circle around ( 3 )}. A source terminal of the sixth transistor MN b2  is coupled to the ground terminal. 
   One terminal of the first bias resistor R b1  is coupled to the node {circle around ( 1 )}, and the other terminal of the first bias resistor R b1  is coupled to the node {circle around ( 2 )}. 
   One terminal of the second bias resistor R b2  is coupled to the node {circle around ( 1 )}, and the other terminal of the second bias resistor R b2  is coupled to the node {circle around ( 3 )}, 
   An output terminal of the current source I sb1  is coupled to the node {circle around ( 1 )}, and an input terminal of the current source I sb1  is supplied with a power supply voltage V DD . 
   One terminal of the first capacitor C 31a  is coupled to the node {circle around ( 2 )}, and the other terminal of the first capacitor C 31s  is supplied with a first input voltage Vin+. 
   One terminal of the second capacitor C 32a  is coupled to the node {circle around ( 3 )}, and the other terminal of the second capacitor C 32a  is supplied with a second input voltage Vin−. 
   The first input voltage Vin+ is supplied to the other terminal of the first capacitor C 31a  and the other terminal of the third capacitor C 31b . The second input voltage Vin− is supplied to the other terminal of the second capacitor C 32a  and the other terminal of the fourth capacitor C 32b . 
   One end of the first load terminal R 31  is coupled to the node {circle around ( 4 )}, and the other end of the first load terminal R 31  is supplied with the power supply voltage V DD . 
   One end of the second load terminal R 32  is coupled to the node {circle around ( 5 )}, and the other end of the second load terminal R 32  is supplied with the power supply voltage V DD . 
   A first output terminal Vout+ is coupled to the node {circle around ( 4 )}, and a second output terminal Vout− is coupled to the node {circle around ( 5 )}. 
   The other terminal of the third bias resistor R b3  and the other terminal of the fourth bias resistor R b4  are coupled to a gate terminal of the seventh transistor MN b3 . The gate terminal and a drain terminal of the seventh transistor MN b3  are coupled to each other, 
   The drain terminal of the seventh transistor MN b3  is coupled to an output terminal of the other current source I sb2 , and a source terminal of the seventh transistor MN b3  is coupled to the ground terminal. 
   When the first input voltage Vin+ and the second input voltage Vin− are supplied, the first to fourth capacitors C 31a , C 32a , C 31b , and C 32b  block DC components of the first and second input voltages Vin+ and Vin−. 
   Herein, the first to fourth capacitors C 31a , C 32a , C 31b , and C 32b  serve as a DC-blocking circuit. 
   The first transistor MN 31a  is biased due to the current source I sb1  supplied to the first bias resistor R b1  from a power supply voltage V DD  terminal coupled to the fifth transistor M b1 . 
   The second transistor MN 32a  is biased due to the current source I sb1  supplied to the second bias resistor R b1  from the power supply voltage V DD  terminal coupled to the sixth transistor MN b2 . 
   Due to the above circuit configuration, when the first and second input voltages Vin+ and Vin− that do not have the DC components are supplied to the main differential amplification unit  310 , the main differential amplification unit  310  amplifies a difference between the first input voltage Vin+ and the second input voltage Vin− and outputs the amplified voltage difference. 
   Particularly, the main differential amplification unit  310  is a FDA comprising the current sources, and thus has a high CMRR and IIP2. 
   The first transistor MN 31a  of the main differential amplification unit  310  is coupled with the third transistor MN 31b  of the auxiliary differential amplification unit  320 . The third transistor MN 31b  is biased due to the auxiliary bias unit  340 . 
   The auxiliary differential amplification unit  320  uses a method of offsetting the non-linearity of the main differential amplification unit  310  with use of a PDA, 
   More specifically, to improve the linearity, the transconductance gm″ of the third transistor MN 31b  is used to change a negative value of the transconductance gm″ of the first transistor MN 31a , into a positive value thereof, so that the transconductance gm″ can be ignored. 
   On the basis of the same circuit configuration, the second transistor MN 32a  of the main differential amplification unit  310  is coupled with the fourth transistor MN 32b  of the auxiliary differential amplification unit  320 , and the fourth transistor MN 32b  is biased due to the auxiliary bias unit  340 . 
   To improve the linearity, the transconductance gm″ of the fourth transistor MN 32b  is used to change a negative value of the transconductance gm″ of the second transistor MN 32a  into a positive value thereof, so that the transconductance gm″ can be ignored. 
   That is, optimum values of bias voltages that can reduce the non-linearity of the first and second transistors MN 31a  and MN 32a  of the illustrated differential amplification circuit are set such that an added value of a second derivative value of the transconductance (i.e., gm″) of the first and second transistors MN 31a  and MN 32a  with respect to a gate-source voltage of the seventh transistor MN b3  of the auxiliary bias unit  340  and a second derivative value of the transconductance (i.e., gm″) of the third and fourth transistors MN 31b  and MN 32b  with respect to a gate-source voltage thereof is integrated to a minimum value in the operation region of the entire circuit. 
   Due to the illustrated circuit configuration, the linearity of the differential amplification circuit can be improved. The biasing is applied such that the first and second transistors MN 31a  and MN 32a  operate in a saturation region, while the third and fourth transistors MN 31b  and MN 32b  operate in a subthreshold region. 
   The auxiliary differential amplification unit  320  has almost no gain since current barely flows to the third and fourth transistors MN 31b  and MN 32b  of the auxiliary differential amplification unit  320 . Therefore, the CMMR is high because the CMMR performs operations that depend on the main differential amplification unit  310 . This high CMMR leads to high IIP2. As a result, the linearity can be improved along with enhancing the advantages of the differential amplification circuit, 
     FIG. 4  illustrates a graph of a linearity characteristic exhibited by a linearity-improved differential amplification circuit according to an embodiment of the present invention. 
   When an Ios level is approximately 31.000 as marked with a reference denotation m 2 , a corresponding value of IIP3 is approximately 17.170 dBm. As a reference denotation m 3  indicates, when an los level is approximately 91.000, a corresponding value of IIP3 is approximately 15.158 dBm. On the other hand, when an Ios level is approximately 0.000 as marked with a reference denotation m 1 , a corresponding value of IIP3 is approximately 6.810 dBm. 
   The los level of 0.000 (refer to m 1 ) indicates that the typical differential amplification circuit that does not comprise an auxiliary differential amplification circuit is used. 
   When the simulation on IIP3 is performed while changing a bias condition of the auxiliary differential amplification circuit, the linearity of the differential amplification circuit is improved in a wide bias region. 
   According to various embodiments of the present invention, the differential amplification circuit can overcome the voltage headroom limitation, increase the CMRR or IIP2, which are the advantages when using the differential amplification circuit, and improve the IIP3. 
   Also, the circuit configuration according to the embodiments of the present invention allows the differential amplification circuit to have the improved linearity. 
   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 as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.