Abstract:
An amplifier amplifying an input signal and the method thereof. The amplifier comprises first and second transconductor circuits. The first transconductor circuit, coupled to the first transistor, receives the first noise voltage to generate a first noise current. The second transconductor circuit, coupled in parallel to the first transconductor circuit, receives the second noise voltage to generate a second noise current such that the first and second noise currents cancel each other out to reduce a noise component in the output current when summing up together, and the first and second transconductor circuits are operated in a current mode.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a Continuation of U.S. patent application Ser. No. 11/964,103, filed Dec. 26, 2007, the entirety of which is incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates in general to high speed communication, and in particular, to an amplifier and the method thereof in high speed communication. 
         [0004]    2. Description of the Related Art 
         [0005]    The demand for high speed communication systems remains high in the recent years. The amplifier in a high speed communication system needs to meet stringent requiems such as broadband input matching, high gain, wide bandwidth, and low noise figure (NF). In addition, it is highly desirable to implement the amplifier in CMOS technology in order to perform a high level of integration, poising design challenges arising from the inferior RF characteristics of CMOS, including large parasitic capacitance, low transconductance, and low supply voltage. 
         [0006]    In low noise amplifier (LNA) applications, the most critical parameters are noise figure NF and linearity. Noise figure NF is defined as 10 Log(SNR in /SNR out ), with SNR in  and SNR out  being the input and output signal to noise ratio. Low noise amplifier typically includes a matching network immediately after an antenna to improve noise figure NF. The linearity may be defined by Third Order Input Intercept Point, representing the point at which the power in the third-order product and the fundamental tone intersect. 
         [0007]    Two common impedance matching techniques, namely, common gate and resistive shunt feedback circuit configuration are typically used to construct the input impedance matching in LNA, providing wideband impedance matching. Both suffer from channel thermal noise of the transistor, accounted for the dominant noise source in an LNA. 
         [0008]    Thus a need exists for an amplifier amplifying an input signal without introducing noise to the amplified signal. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    A detailed description is given in the following embodiments with reference to the accompanying drawings. 
         [0010]    An amplifier amplifying an input signal to generate an output current is disclosed, comprising first and second transconductor circuits. The first transconductor circuit, coupled to the first transistor, receives the first noise voltage to generate a first noise current. The second transconductor circuit, coupled in parallel to the first transconductor circuit, receives the second noise voltage to generate a second noise current such that the first and second noise currents cancel each other out to reduce a noise component in the output current when summing up together, and the first and second transconductor circuits are operated in a current mode. 
         [0011]    According to another embodiment of the invention, a method of amplifying an input signal to generate an output current in an amplifier is provided, comprising providing a first transistor having a channel thermal noise to establish a first noise voltage, providing a resistance device receiving the channel thermal noise to establish a second noise voltage, providing a first transconductor circuit with first transconductance to receive the first noise voltage to generate a first noise current, and providing a second transconductor circuit with second transconductance to receive the second noise voltage to generate a second noise current, such that the first and second noise currents cancel each other out to reduce a noise component in the output current. The first and second transconductance have the opposite signs, and the first and second transconductor circuits are operated in a current mode. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
           [0013]      FIG. 1  is a block diagram of an exemplary receiver according to the invention. 
           [0014]      FIG. 2  is a block diagram of an exemplary transconductor amplifier according to the invention. 
           [0015]      FIG. 3  is a circuit schematic of an exemplary transconductor amplifier according to the invention. 
           [0016]      FIG. 4  is another circuit schematic of an exemplary transconductor amplifier according to the invention. 
           [0017]      FIG. 5  is yet another circuit schematic of an exemplary transconductor amplifier according to the invention. 
           [0018]      FIG. 6  is still another circuit schematic of an exemplary transconductor amplifier according to the invention. 
           [0019]      FIG. 7  is a block diagram of a conventional transconductor amplifier. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
         [0021]      FIG. 1  is a block diagram of an exemplary direct conversion receiver according to the invention, comprising antenna  100 , RF filter  102 , low noise amplifier (LNA)  104 , mixer  106 , filter  108 , amplifier  110  and analog-to-digital converter (ADC)  112 . Antenna  100  is coupled to RF filter  102 , LNA  104 , mixer  106 , filter  108 , amplifier  110 , and subsequently to ADC  112 . 
         [0022]    Antenna  100  receives input signal RF in , filtered by RF filter  102  to remove out-of-band signals, amplified by LNA  104 , modulated in mixer  106  with local oscillation signals LO_I and LO_Q to produce inphase and quadrature output voltages V I  and V Q , which in turn are filtered by filter  108 , amplified by amplifier  110 , converted to digital in analog-to-digital converter  112  to generate digital data to D I  and D Q  for performing subsequent baseband operations. Input RF signal RF in  comprises inphase and quadrature components, and may be a single ended signal or a differential signal pair. Local oscillation signals LO_I and LO_Q are supplied by a local oscillator (not shown) including a phase lock loop (not shown) and are 90 degree out of phase to each other. Local oscillation signals LO_I and LO_Q may also be single ended signals or differential signal pairs corresponding to input RF signal RF in , and typically have an oscillation frequency substantially equivalent to the center frequency of input RF signal RF in . Filter  108  may be a channel-select filter performing channel selection at intermediate frequency (IF). Amplifier  110  may be a programmable gain amplifier (PGA) amplifying filtered to change the amplifier gain thereof. 
         [0023]    LNA  104  may be a variable gain amplifier amplifying weak RF signal RF in  to generate an amplified RF signal without introducing noise. Mixer  106  comprises a Gilbert Cell mixer that includes a transconductor stage transforming the amplified RF signal to a current signal and a quad switching stage mixing the current signal with local oscillation signals LO_I and LO_Q. 
         [0024]      FIG. 7  is a block diagram of a conventional resistive shunt feedback transconductor, comprising feedback resistor R i , load resistor R L , and transistor Q 70 . Feedback resistor R i  is coupled between the drain and gate terminals of transistor Q 70 . Load resistor R L  is coupled to transistor Q 70  providing amplified output voltage V out . Transconductor circuit  7  has a negative gain −A and input impedance R i /(1+A). The channel thermal noise of transistor Q 70  directly contributes to a noise component in amplified output voltage V out . 
         [0025]      FIG. 2  is a block diagram of an exemplary low noise amplifier according to the invention, comprising impedance matching circuit  20  and transconductor amplifier  22  coupled thereto. 
         [0026]    Impedance matching circuit  20  comprises first resistor R S , second resistor R F , current source I 200 , and first transistor Q 200 . First resistor R S  is coupled to second resistor R F  and transistor Q 200 . Second resistor R F  is coupled in shunt to the drain terminal of transistor Q 200 . 
         [0027]    Impedance matching circuit  20  is a shunt feedback circuit with gain (−A), receiving input signal S in  to perform impedance matching thereon and producing first output voltage V S . First resistor R S  is typically selected as 50 ohm to provide input impedance matching for input signal S in  and generate matched input signal V i . Input signal V i  comprises first signal voltage V g,s  and first noise voltage V g,n , and output voltage V S  comprises second signal voltage V d,s  and second noise voltage V d,n . Second resistor R F  provides a feedback path between the drain terminal and gate terminal of first transistor Q 200 . First transistor Q 200  is an NPN type MOSFET transistor receiving first signal voltage V g,s  at the gate thereof to establish signal current I d,s  and first signal voltage V d,s  at the drain. First transistor Q 200  has intrinsic channel thermal noise I d,n  arising from the random thermal motion of the carrier and the drift in the field, which is the dominant noise source in impedance matching circuit  20 . Channel thermal noise I d,n  establishes first noise voltage V d,n  at the drain terminal and second noise voltage V g,n  at the gate terminal of transistor Q 200 . 
         [0028]    Transconductor amplifier  22  comprises first transconductor amplifier circuit  222  and first transconductor amplifier circuit  220  coupled in parallel to each other. First transconductor amplifier circuit  222  has positive transconductance g m1  and second transconductor  220  has negative transconductance −gm2. First transconductor amplifier circuit  222  receives input voltage V i  and second transconductor  220  receives output voltage V S , together establish output current I OUT  by (V i *g m1 −V s *g m2 ). Output current I OUT  also comprises signal component I out,s  and noise component I out,n . Since impedance matching circuit  20  has negative gain (−A), first input signal voltage V g,s  and second output signal voltage V d,s  have opposite numerical signs, or, output signal voltage V d,s  equals to −(V g,s *A), resulting in strengthened signal component I out,s  equaling to (V g,s *g m1 +V g,s *A*g m2 ). On the contrary, first input noise voltage V g,n  and second output noise voltage V d,n  share an identical numerical sign, or, representing V g,n  by (I d,n *R S ) and V d,n  by (I d,n *(R S +R F )), producing reduced noise component I out,n  equaling to (I d,n *R S *g m1 −I d,n *(R S +R F )*g m2 ). First noise current (I d,n *R S *g m1 ) from first transconductor circuit  222  and second current (I d,n *(R S +R F )*g m2 ) from first transconductor circuit  220  can cancel each other out by choosing transconductance g m1  and g m2  according to Equation (1): 
         [0000]        g   m1   /g   m2 =1 +R   F   /R   S   (1) 
         [0000]    thereby producing zero noise component I out,n  in I OUT , or, a noiseless output current. 
         [0029]    While first conductance g m1  is positive and second conductance g m2  is negative in the embodiment, it would be apparent for the people in the art that negative first conductance g m1  and positive second conductance g m2  also serves the purpose of reducing the noise component in output current I OUT , proper circuitry modification to the LNA in  FIG. 2  can be made without deviating from the principle of the invention. 
         [0030]      FIG. 3  is a circuit schematic of an exemplary transconductor amplifier in  FIG. 2 , comprising impedance matching circuit  20  and transconductor circuit  32  coupled thereto. 
         [0031]    Impedance matching circuit  20  in  FIG. 3  is identical to  FIG. 2 , transconductor circuit  32  comprises transistors Q 320 , Q 322 , Q 324 , current source I 320 , and capacitor C 320 . 
         [0032]    Transistor Q 324  provides positive transconductance g m1  in the first transconductor amplifier, and transistors Q 320  and Q 322  provide negative transconductance g m2  in the second transconductor amplifier in  FIG. 2 . The signal component in output current I OUT  is (V g,s *g m1 +V g,s *A*g m2 ) and the noise component is removed by selecting the first and second transconductance g m1  and g m2  according to Equation (1). 
         [0033]      FIG. 4  is another circuit schematic of an exemplary transconductor amplifier in  FIG. 2 , comprising impedance matching circuit  20  and transconductor circuit  42  coupled thereto. 
         [0034]    Impedance matching circuit  20  in  FIG. 4  is identical to  FIG. 2 , transconductor circuit  42  comprises transistors Q 420 , Q 422 , current source I 420 , and capacitor C 420 . 
         [0035]    Transistor Q 422  provides positive transconductance g m1  in the first transconductor amplifier, and transistor Q 420  provides negative transconductance g m2  in the second transconductor amplifier in  FIG. 2 . Transconductor circuit  42  provides better device matching than transconductor circuit  32  in  FIG. 3 , since transistor Q 422  providing positive transconductance g m1  and transistor Q 422  providing negative transconductance g m2  share a common output loading I 420 . The noise component is removed by selecting the first and second transconductance g m1  and g m2  according to Equation (1). 
         [0036]      FIG. 5  is yet another circuit schematic of an exemplary transconductor amplifier in  FIG. 2 , comprising impedance matching circuit  20  and transconductor circuit  52  coupled thereto. Transconductor amplifier  5  provides a variable gain control to amplified output current I OUT . 
         [0037]    Impedance matching circuit  20  in  FIG. 5  is identical to  FIG. 2 , transconductor circuit  52  comprises transistors Q 520  through Q 526 , current source I 520 , and capacitor C 520 . 
         [0038]    Transistor Q 526  provides positive transconductance g m1  in the first transconductor amplifier, and transistor Q 524  provides negative transconductance g m2  in the second transconductor amplifier in  FIG. 2 . Control signal S c  controls transistors Q 520  and Q 522  to provide variable gain control to output current I OUT . When Control signal S c  turns transistor Q 520  on, the output current I OUT  is reduced, providing a low gain mode. The noise component is removed by selecting the first and second transconductance g m1  and g m2  according to Equation (1). 
         [0039]      FIG. 6  is still another circuit schematic of an exemplary transconductor amplifier in  FIG. 2 , comprising impedance matching circuit  60  and transconductor circuit  62  coupled thereto. Amplifier  6  is implemented in differential circuit configuration, taking a differential input pair S in  to provide amplified output current I OUT . 
         [0040]    Impedance matching circuit  60 . Transconductor circuit  62  comprises transistors Q 620a,b  and Q 622a,b , current source I 620 , and capacitors C 620a,b . 
         [0041]    Transistors Q 622a,b  provide positive transconductance g m1  in the first transconductor amplifier, and transistors Q 622a,b  provide negative transconductance g m2  in the second transconductor amplifier. The noise component is removed by selecting the first and second transconductance g m1  and g m2  according to Equation (1). 
         [0042]    While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.