# Patent

## Title
Amplifier and the method thereof

## 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.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a Continuation of U.S. patent application Ser. No. 11/964,103, filed on Dec. 26, 2007, now U.S. Pat. No. 7,633,345, the entirety of which is incorporated by reference herein.


BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates in general to high speed communication, and in particular, to an amplifier and the method thereof in high speed communication.
2. Description of the Related Art
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.
In low noise amplifier (LNA) applications, the most critical parameters are noise figure NF and linearity. Noise figure NF is defined as 10 Log(SNRin/SNRout), with SNRin and SNRout 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.
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.
Thus a need exists for an amplifier amplifying an input signal without introducing noise to the amplified signal.
BRIEF SUMMARY OF THE INVENTION
A detailed description is given in the following embodiments with reference to the accompanying drawings.
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.
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
The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein:
FIG. 1 is a block diagram of an exemplary receiver according to the invention.
FIG. 2 is a block diagram of an exemplary transconductor amplifier according to the invention.
FIG. 3 is a circuit schematic of an exemplary transconductor amplifier according to the invention.
FIG. 4 is another circuit schematic of an exemplary transconductor amplifier according to the invention.
FIG. 5 is yet another circuit schematic of an exemplary transconductor amplifier according to the invention.
FIG. 6 is still another circuit schematic of an exemplary transconductor amplifier according to the invention.
FIG. 7 is a block diagram of a conventional transconductor amplifier.



DETAILED DESCRIPTION OF THE INVENTION
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.
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.
Antenna 100 receives input signal RFin, 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 VI and VQ, 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 DI and DQ for performing subsequent baseband operations. Input RF signal RFin 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 RFin, and typically have an oscillation frequency substantially equivalent to the center frequency of input RF signal RFin. 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.
LNA 104 may be a variable gain amplifier amplifying weak RF signal RFin 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.
FIG. 7 is a block diagram of a conventional resistive shunt feedback transconductor, comprising feedback resistor Ri, load resistor RL, and transistor Q70. Feedback resistor Ri is coupled between the drain and gate terminals of transistor Q70. Load resistor RL is coupled to transistor Q70 providing amplified output voltage Vout. Transconductor circuit 7 has a negative gain −A and input impedance Ri/(1+A). The channel thermal noise of transistor Q70 directly contributes to a noise component in amplified output voltage Vout.
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.
Impedance matching circuit 20 comprises first resistor RS, second resistor RF, current source I200, and first transistor Q200. First resistor RS is coupled to second resistor RF and transistor Q200. Second resistor RF is coupled in shunt to the drain terminal of transistor Q200.
Impedance matching circuit 20 is a shunt feedback circuit with gain (−A), receiving input signal Sin to perform impedance matching thereon and producing first output voltage VS. First resistor RS is typically selected as 50 ohm to provide input impedance matching for input signal Sin and generate matched input signal Vi. Input signal Vi comprises first signal voltage Vg,s and first noise voltage Vg,n, and output voltage VS comprises second signal voltage Vd,s and second noise voltage Vd,n. Second resistor RF provides a feedback path between the drain terminal and gate terminal of first transistor Q200. First transistor Q200 is an NPN type MOSFET transistor receiving first signal voltage Vg,s at the gate thereof to establish signal current Id,s and first signal voltage Vd,s at the drain. First transistor Q200 has intrinsic channel thermal noise Id,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 Id,n establishes first noise voltage Vd,n at the drain terminal and second noise voltage Vg,n at the gate terminal of transistor Q200.
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 gm1 and second transconductor 220 has negative transconductance −gm2. First transconductor amplifier circuit 222 receives input voltage Vi and second transconductor 220 receives output voltage VS, together establish output current IOUT by (Vi*gm1−Vs*gm2). Output current IOUT also comprises signal component Iout,s and noise component Iout,n. Since impedance matching circuit 20 has negative gain (−A), first input signal voltage Vg,s and second output signal voltage Vd,s have opposite numerical signs, or, output signal voltage Vd,s equals to −(Vg,s*A), resulting in strengthened signal component Iout,s equaling to (Vg,s*gm1+Vg,s*A*gm2). On the contrary, first input noise voltage Vg,n and second output noise voltage Vd,n share an identical numerical sign, or, representing Vg,n by (Id,n*RS) and Vd,n by (Id,n*(RS+RF)), producing reduced noise component Iout,n equaling to (Id,n*RS*gm1−Id,n*(RS+RF)*gm2). First noise current (Id,n*RS*gm1) from first transconductor circuit 222 and second current (Id,n*(RS+RF)*gm2) from first transconductor circuit 220 can cancel each other out by choosing transconductance gm1 and gm2 according to Equation (1):

gm1/gm2=1+RF/RS  (1)

thereby producing zero noise component Iout,n in IOUT, or, a noiseless output current.

While first conductance gm1 is positive and second conductance gm2 is negative in the embodiment, it would be apparent for the people in the art that negative first conductance gm1 and positive second conductance gm2 also serves the purpose of reducing the noise component in output current IOUT, proper circuitry modification to the LNA in FIG. 2 can be made without deviating from the principle of the invention.
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.
Impedance matching circuit 20 in FIG. 3 is identical to FIG. 2, transconductor circuit 32 comprises transistors Q320, Q322, Q324, current source I320, and capacitor C320.
Transistor Q324 provides positive transconductance gm1 in the first transconductor amplifier, and transistors Q320 and Q322 provide negative transconductance gm2 in the second transconductor amplifier in FIG. 2. The signal component in output current IOUT is (Vg,s*gm1+Vg,s*A*gm2) and the noise component is removed by selecting the first and second transconductance gm1 and gm2 according to Equation (1).
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.
Impedance matching circuit 20 in FIG. 4 is identical to FIG. 2, transconductor circuit 42 comprises transistors Q420, Q422, current source I420, and capacitor C420.
Transistor Q422 provides positive transconductance gm1 in the first transconductor amplifier, and transistor Q420 provides negative transconductance gm2 in the second transconductor amplifier in FIG. 2. Transconductor circuit 42 provides better device matching than transconductor circuit 32 in FIG. 3, since transistor Q422 providing positive transconductance gm1 and transistor Q422 providing negative transconductance gm2 share a common output loading I420. The noise component is removed by selecting the first and second transconductance gm1 and gm2 according to Equation (1).
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 IOUT.
Impedance matching circuit 20 in FIG. 5 is identical to FIG. 2, transconductor circuit 52 comprises transistors Q520 through Q526, current source I520, and capacitor C520.
Transistor Q526 provides positive transconductance gm1 in the first transconductor amplifier, and transistor Q524 provides negative transconductance gm2 in the second transconductor amplifier in FIG. 2. Control signal Sc controls transistors Q520 and Q522 to provide variable gain control to output current IOUT. When Control signal Sc turns transistor Q520 on, the output current IOUT is reduced, providing a low gain mode. The noise component is removed by selecting the first and second transconductance gm1 and gm2 according to Equation (1).
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 Sin to provide amplified output current IOUT.
Impedance matching circuit 60. Transconductor circuit 62 comprises transistors Q620a,b and Q622a,b, current source I620, and capacitors C620a,b.
Transistors Q622a,b provide positive transconductance gm1 in the first transconductor amplifier, and transistors Q622a,b provide negative transconductance gm2 in the second transconductor amplifier. The noise component is removed by selecting the first and second transconductance gm1 and gm2 according to Equation (1).
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.

## Claims

1. An amplifier amplifying an input signal to generate an output current, comprising:
a first transconductor circuit, coupled to a first transistor, receiving a first noise voltage to generate a first noise current; and
a second transconductor circuit, coupled in parallel to the first transconductor circuit, receiving a second noise voltage to generate a second noise current, such that the first and second noise currents mutually cancel each other so as to reduce a noise component in the output current, and the first and second transconductor circuits are operated in a current mode.

2. The amplifier of claim 1, wherein the first transistor is arranged to generate an amplified signal according to the input signal with a negative gain, the first transconductor circuit is arranged to generate a first signal current according to the input signal, and the second transconductor circuit is arranged to generate a second signal current according to the amplified signal such that the first and second signal currents accumulates to increase a signal component in the output current when summing up together.

3. The amplifier of claim 1, wherein a transconductance of the first transconductor circuit is positive and a transconductance of the second transconductor circuit is negative.

4. The amplifier of claim 1, wherein:
the first transistor having a first gate, a first drain, and a first source;
the first transconductor circuit comprises a second transistor having a second gate coupled to the first gate, a second drain, and a second source coupled to the ground,
the second transconductor circuit comprises:
a third transistor having a third gate coupled to the first drain, a third drain coupled to a voltage source, and a third source coupled to a current source; and
a fourth transistor having a fourth gate coupled to the ground through a capacitor, a fourth drain coupled to the second drain, and a fourth source coupled to the third source and the current source.

5. The amplifier of claim 1, further comprising a third transconductor circuit, wherein:
the first transistor having a first gate, a first drain, and a first source;
the second transconductor circuit comprises a second transistor having a second gate coupled to the first gate, a second drain, and a second source coupled to a current source,
the third transconductor circuit comprises a third transistor having a third gate coupled to the first drain, a third drain coupled to a voltage source, and a third source coupled to the second source and the current source.

6. The amplifier of claim 5, further comprising:
a fourth transistor having a fourth gate, a fourth drain coupled to the voltage source, and a fourth source coupled to the second drain; and
a fifth transistor having a fifth gate, a fifth drain, and a fifth source coupled to the second drain.

7. The amplifier of claim 1, wherein the input signal is a differential signal pair, circuit components in the amplifier are constructed by transistor pairs.

8. A method of amplifying an input signal to generate an output current in an amplifier, 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;
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 mutually cancel each other out to reduce a noise component in the output current; and
wherein the first and second transconductance have the opposite signs, and the first and second transconductor circuits are operated in a current mode.

9. The method of claim 8, wherein the first transconductance is positive and the second transconductance is negative.

10. The method of claim 8, wherein the input signal is a differential signal pair, circuit components in the first and second transconductor circuits are constructed by transistor pairs.

11. The method of claim 8, wherein the first transistor is arranged to generate an amplified signal according to the input signal with a negative gain, the first transconductor circuit is arranged to generate a first signal current according to the input signal, and the second transconductor circuit is arranged to generate a second signal current according to the amplified signal such that the first and second signal currents accumulates to increase a signal component in the output current when summing up together.