Mixer Circuits and Methods with Improved Spectral Purity

BACKGROUND

[0001] The present invention relates to mixer circuits, and in particular, to mixer circuits and methods with matched bias currents.

[0002] Mixer circuits are used in a variety of applications including modulation and demodulation, for example. Fig. 1 is a prior art mixer circuit. Mixer circuit 100 is sometimes referred to as a double balanced mixer because it includes two mixer circuits with outputs coupled together to common resistors. Mixer circuit 100 includes two sets of differential transistors 101-102 and 104-105 that each receive a first differential input signal ("inl+" and "inl-"). The differential transistors 101-102 ("Ml" and "M2") receive a bias current from transistor 103 that has a drain connected to the sources of both transistors 101 and 102 and a source connected to a current source 109. Similarly, differential transistors 104-105 ("M4" and "M5") receive a bias current from transistor 106 that has a drain connected to the sources of both transistors 104 and 105 and a source connected to current source 109. Bias transistors 103 and 106 may also perform a voltage-to-current function. A second input voltage signal in2+ may be received at the gate of transistor 103, and a complementary input voltage signal in2- may be received at the gate of transistor 106. These signals (i.e., in2+ and in2-) will be converted to currents and coupled to the differential transistors.

[0003] One problem with existing mixer circuits, such as mixer circuit 100, is that device imperfections may result in spectral contamination. Ideally, it is desirable to match Ml, M2, M4 and M5. Likewise, it is desirable to match M3 and M6. However, manufacturing variations may cause components of the circuit to be mismatched. Such mismatches may cause the output of the circuit may include a variety of unwanted frequencies. For example, mismatch in devices M3 and M6 may cause odd harmonic distortion and mismatches in the differential devices Ml and M2 or M4 and M5 may cause even harmonic distortion.

[0004] Figs. 2-3 illustrate odd harmonics that are generated from mismatch in the circuit of Fig. 1. Waveform 201 is an example input signal, inl+. When inl+ is high and inl- is low, the current in transistor 103 ("M3"), I ₀₁ , is flowing through Ml and resistor 107 and the current in transistor 106 ("M6"), I _02> is flowing through M5 and resistor 108. When inl+ goes low and inl- goes high, the currents "commutate" (i.e., reverse) and the current in M3, I ₀₁ , flows through M2 and resistor 108 and the current in M6, 1 ₀₂ , is flows through M4 and

resistor 107. Ideally, the currents in M3 and M6 are precisely the same. IfI ₀₁ and I ₀₂ are identical, then the differential output voltage will be zero at all times as inl+ and inl- reverse polarity. However, if devices M3 and M6 are mismatched, the currents I ₀₁ and I ₀₂ will be different. Waveform 202 illustrates the change in the differential output voltage resulting from mismatched currents I ₀₁ and I ₀₂ . When inl+ is high and inl- is low, the differential output voltage is given by the following equation:

Vo_diff= outl - out2,

outl = Vdd- I _ol R,

out2 = Vdd - I _o2 R,

Vo_diff= (I _o2 - I _ol ) R,

When inl+ goes low and inl- goes high, currents I ₀₁ and I ₀₂ commutate and the differential output voltage is given as follows:

Vo_diff= outl - out2,

outl = Vdd- I _o2 R,

out2 = Vdd- I _ol R,

Vo_diff= - (I _o2 - I _ol ) R,

If I ₀₁ and I ₀₂ are the same, the differential output will be zero. But if I ₀₁ and I ₀₂ are mismatched, Vo_diff will transition with the input signals inl+ and inl- as shown in waveform 202. Waveforms 204-206 illustrate the undesired spectral components that will appear at the output of the circuit as result of mismatch in I ₀₁ and I ₀₂ . For example, waveform

204 illustrates a spectral component in Vo_diff having a frequency equal to the transition frequency of the input signals inl+ and inl-. Waveform 205 illustrates a second order harmonic of the fundamental frequency, which will be zero because a full cycle of waveform

205 will occur during a single half-cycle of the input. Waveform 206 illustrates a third order harmonic. Fig. 3 shows the odd harmonic components resulting from current mismatch in mixer 100. As shown in Fig. 3, mismatches in devices M3 and M6 will produce different current I ₀₁ and I ₀₂ that result in odd harmonic components at the output of the mixer. Such spectral impurities may adversely affect the operation of the system in which the mixer is used.

[0005] Fig. 4 illustrates even harmonics that are generated using the circuit of Fig. 1. Mismatches in differential transistors 101 and 102 and/or 104 and 105 may further contribute to the spectral impurity at the output. Waveform 401 shows inl+ and inl- as sinusoids. If transistors 101 and 102 are matched, the currents generated by these transistors in response to inputs inl+ and inl- will be the same. However, if these devices are mismatched, then the currents will be different. If one transistor (e.g., Ml) has a larger transconductance than the other transistor (e.g., M2) because of mismatch, then the difference can be modeled as an offset voltage ("Voff" ) at the gate of Ml. Waveform 402 shows inl+ offset by an offset voltage Voff. Waveform 402 is illustrative in understanding the effects of mismatch on the output of the circuit. Waveform 402 illustrates that the crossover points (i.e., switching points) of inl+ and inl- are no longer coincident. When the crossover point of one pair of transistors (e.g., Ml and M2) shifts, then there will be periods of time when two transistors are turned on at the same time (e.g., Ml and M4) and are conducting current to one of the outputs. Likewise, for a short amount of time two transistors will be turned off at the same time (e.g., M2 and M5) and provide no current to the other output. This phenomena is illustrated in waveform 403, which shows the current in Ml generated by inl+. Comparing waveforms 402 and 403 it can be seen that the amount of time Ml is conducting current to the output is increased by an amount of time δt as a result of mismatch. Consequently, Ml may still be conducting current through resistor 107 when M4 turns on and conducts current through resistor 107. Thus, rather than an ideal situation, where I ₀₁ and I ₀₂ are alternately coupled to resistor 107 as shown in diagram 404, there is a short period of time, δt, where both currents are coupled to the output at the same time as shown in diagram 405. Plot 406 illustrates the current through resistor 107 ("I _R107 ") and resistor 108 ("I _R I _OS ") as a function of time. As shown in 406, mismatches in the differential devices may cause both transistors Ml and M4 to conduct current into resistor 107 for a short period of time, δt, resulting in a series of current pulses that occur twice per period of inl+ and inl-. Similarly, such mismatches may cause both transistors M2 and M5 to be turned off (i.e., zero current) for a short period of time, δt, resulting in a series of negative current pulses that occur twice per period of inl + and inl-. The effect of these current pulses is shown in plot 407. If the currents I ₀₁ and I ₀₂ are equal, then the current pulses will generate differential output voltage ("Vo_diff ') pulses equal to 2I ₀ R having a period of T ₀ . Since the current pulses occur twice for every period of inl+ and inl- (i.e., T ₀ is one-half the period of the input signal), the fundamental frequency of the output pulses will be twice the frequency of the input, and the output will include additional harmonics at even multiples of the input signal frequency.

[0006] In some cases, harmonic impurities resulting from manufacturing variances may be very small and effectively negligible. However, variations in the manufacturing process may cause different devices to exhibit different levels of harmonics. When the variances are sufficiently large, harmonic impurity may impact system performance, and in some cases may even cause the system to be completely inoperable. Thus, some portion of the circuits produced by the manufacturing process may have to be discarded, thereby affecting the "yield" of the process. Reducing the harmonic impurity caused by manufacturing variations would improve the production yield.

[0007] It is generally desirable to reduce the amount of harmonic impurity at the output of a mixer. Moreover, it is generally desirable to improve the yield of a manufacturing process. Thus, there is a need for improved mixer circuits and methods for improving the spectral purity of mixer circuits. There is also a need to provide matched bias currents through each branch of a mixer circuit.

SUMMARY [0008] Embodiments of the present invention include circuits and methods for improving the spectral purity of mixer circuits. In one embodiment the present invention includes a mixer circuit comprising a first transistor having a gate, a source and a drain, a second transistor having a gate, a source and a drain, a first capacitance coupled between the source of the first transistor and the source of the second transistor and a bias circuit having an input, a first output coupled to the source of the first transistor and a second output coupled to the source of the second transistor.

[0009] In another embodiment, the present invention includes a mixer circuit comprising a first transistor having a gate, a source and a drain, a second transistor having a gate, a source and a drain, a third transistor having a gate, a source and a drain, a fourth transistor having a gate, a source and a drain, a first capacitance coupled between the source of the first transistor and the source of the second transistor, a second capacitance coupled between the source of the third transistor and the source of the fourth transistor and a bias circuit having first and second inputs, a first output coupled to the source of the first transistor, a second output coupled to the source of the second transistor, a third output coupled to the source of the third transistor and a fourth output coupled to the source of the fourth transistor.

[0010] A mixer circuit comprising a first transistor having a gate, a source and a drain, a second transistor having a gate, a source and a drain, a first capacitor coupled between the

source of the first transistor and the source of the second transistor, a first resistor having a first terminal coupled to the source of the first transistor and a second terminal coupled to a first reference voltage, a second resistor having a first terminal coupled to the source of the second transistor and a second terminal coupled to the first reference voltage, a second capacitor having a first terminal coupled to the source of the first transistor and a third capacitor having a first terminal coupled to the source of the second transistor.

[0011] In another embodiment the present invention includes a mixer circuit including a bias circuit, the bias circuit comprising a first transistor having a gate, a source, and a drain, wherein the drain is coupled to a first input of the mixer circuit, a first sensing circuit coupled to the source of the first transistor, and a first comparison circuit coupled to the first sensing circuit, wherein the first comparison circuit receives a feedback signal from the first sensing circuit and a reference voltage, and in accordance therewith, generates a bias control signal to control the bias at the gate of the first transistor.

[0012] In one embodiment, the bias circuit further comprises an amplifier having at least one input, an output coupled to the gate of the first transistor, and a bias control input coupled to an output of the first comparison circuit.

[0013] In one embodiment, the first sensing circuit comprises a resistor coupled to the source of the first transistor.

[0014] Li one embodiment, the bias circuit further comprises a first low pass filter.

[0015] In one embodiment, the first comparison circuit is a first amplifier coupled to receive the reference voltage and feedback signal.

[0016] In one embodiment, the first amplifier generates a current into a bias circuit of a second amplifier for changing the bias of an input signal at the gate of the first transistor.

[0017] In one embodiment, the first comparison circuit comprises first differential transistors having a first input coupled to the first sensing circuit and a second input coupled to the reference voltage.

[0018] In one embodiment, the bias circuit further comprises a second transistor having a gate, a source, and a drain, wherein the drain is coupled to a second input of the mixer circuit, a second sensing circuit coupled to the source of the second transistor, and a second comparison circuit coupled to the second sensing circuit, wherein the second comparison

circuit receives a feedback signal from the second sensing circuit and the reference voltage, and in accordance therewith, generates a second bias control signal to control the bias at the gate of the second transistor.

[0019] In one embodiment, the bias circuit further comprises an amplifier having at least one input, an output coupled to the gate of the second transistor, and a bias control input coupled to an output of the second comparison circuit.

[0020] In one embodiment, the bias circuit further comprises a resistor coupled between the source of the first transistor and the source of the second transistor.

[0021] In one embodiment, the mixer includes four bias inputs coupled to four of said bias circuits.

[0022] In another embodiment, the present invention includes a method of controlling bias current in a mixer circuit, the method comprising sensing a bias current through a first transistor to generate a feedback signal, comparing the feedback signal to a reference voltage, and in accordance therewith, generating a bias control signal, and adjusting the bias of an amplifier coupled to at least one input of the mixer circuit to reduce the offset of an input signal to the mixer circuit.

[0023] In one embodiment, the method further comprises low pass filtering the bias control signal.

[0024] In one embodiment, sensing comprises coupling a current at a source terminal of the first transistor through a resistor.

[0025] In one embodiment, the method further comprises amplifying the bias control signal.

[0026] In one embodiment, the method further comprises translating the bias control signal into a current and coupling the current to an amplifier through a current mirror.

[0027] In another embodiment, the present invention includes a mixer circuit comprising first and second transistors coupled to receive a first differential input signal, a third transistor coupled to provide bias current to the first transistor, a first sense circuit coupled to sense current through the third transistor, a first comparison circuit coupled to the first sense circuit and to a reference voltage, the first comparison circuit generating a bias control signal, and a first amplifier coupled to an input signal and providing the input signal to a gate of the third

transistor, wherein the first amplifier receives the bias control signal on a bias control input and changes the level of offset of the input signal at the gate of the third transistor.

[0028] In one embodiment, the first sense circuit comprises a first resistor coupled to a source of the third transistor.

[0029] In one embodiment, the circuit further comprises a low pass filter coupled between the first sense circuit and the bias control input of the first amplifier.

[0030] In one embodiment, the circuit further comprises a fourth transistor coupled to provide bias current to the second transistor, a capacitor coupled between a source of the first and second transistors, a second sense circuit coupled to sense current through the fourth transistor, a second comparison circuit coupled to the second sense circuit and to a reference voltage, the second comparison circuit generating a second bias control signal, and a second amplifier coupled to the input signal and providing the input signal to a gate of the fourth transistor, wherein the second amplifier receives the second bias control signal on a bias control input and changes the level of offset of the input signal at the gate of the fourth transistor.

[0031] The following detailed description and accompanying drawings provide a better understanding of the nature and advantages of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS [0032] Fig. 1 is a prior art mixer circuit.

[0033] Figs. 2-3 illustrate odd harmonics that are generated using the circuit of Fig. 1.

[0034] Fig. 4 illustrates even harmonics that are generated using the circuit of Fig. 1.

[0035] Fig. 5 illustrates a mixer circuit according to one embodiment of the present invention.

[0036] Fig. 6 is an example of a mixer circuit using transistor biasing according to one embodiment of the present invention.

[0037] Fig. 7 is an example of a mixer circuit using resistor biasing according to one embodiment of the present invention.

[0038] Fig. 8 is an example of a mixer circuit according to another embodiment of the present invention.

[0039] Fig. 9 is an example of a mixer circuit according to yet another embodiment of the present invention.

[0040] Fig. 10 illustrates one advantageous application of mixer circuits according to embodiments of the present invention.

[0041] Fig. 1 IA is an example of a bias circuit for a mixer circuit according to another embodiment of the present invention.

[0042] Fig. 1 IB is an example of a bias circuit for a mixer circuit according to yet another embodiment of the present invention.

[0043] Fig. 12 illustrates an example of a bias circuit according to another embodiment of the present invention.

DETAILED DESCRIPTION

[0044] Described herein are circuits and methods for reducing spectral impurity in mixer circuits. In the following description, for purposes of explanation, numerous examples and specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be evident to one skilled in the art that embodiments of the present invention may include other equivalent embodiments or basic modifications of the examples shown below. For example, while the embodiments and examples below are presented using NMOS transistors, other transistors such as PMOS or bipolar may be used. Thus, the invention, as defined by the claims, may include some or all of the features in these examples alone or in combination with other features described below along with equivalents.

[0045] Fig. 5 illustrates a mixer circuit 500 according to one embodiment of the present invention. Mixer circuit 500 includes a first transistor 501 ("Ml") having a gate, a source and a drain and a second transistor 502 ("M2") having a gate, a source and a drain. Ml and M2 may be matched devices, but may not be perfectly matched because of manufacturing variations, for example. Rather than a typical differential pair, where the sources are connected together, a capacitance, such as a capacitor 503 ("Cl"), is coupled between the source of Ml and the source of M2. Mixer circuit 500 also includes a bias circuit 510, which may be any one of a variety of architectures. Bias circuit 510 has a first output coupled to the source of Ml and a second output coupled to the source M2. The term bias circuit, as used herein, means a circuit that provides DC bias currents to the switching transistors Ml and M2

and couples an AC signal to the sources (or emitters, in a bipolar implementation) of these devices. Bias circuit 510 provides bias currents il and i2 to the sources of Ml and M2. Bias circuit 510 also includes an input for receiving an input signal, "in2." The input signal is coupled to the sources of Ml and M2. For example, if the input signal is an AC current signal, bias circuit 510 may couple the input to currents il and i2. Currents il and i2 may thereby have both AC and DC components. If the input signal is a voltage signal, bias circuit 510 may include voltage-to-current conversion functionality so that the input signal is translated into an AC component on the current signals il and i2. A variety of voltage-to- current converters may be used for this function. An output of a voltage-to-current converter may be coupled to the sources of Ml and M2 through internal capacitors, for example.

[0046] Advantageously, Cl reduces the harmonic impurity caused by mismatch between Ml and M2. Because the sources of Ml and M2 are not connected together, each device may operate at a different source voltage. Thus, if the devices are mismatched, a voltage will be stored on Cl that will compensate for the mismatch and the crossover points will shift closer to a 50% duty cycle. Accordingly, the even harmonics illustrated in Fig. 4 may be reduced or effectively eliminated and the output spectral purity is improved.

[0047] Fig. 6 is an example of a mixer circuit 600 using transistor biasing according to one embodiment of the present invention. Mixer circuit 600 includes a first transistor 601 ("Ml") having a source coupled through capacitor 603 ("Cl") to the source of a second transistor 602 ("M2"). The bias circuit in this example includes transistor 604 ("M3"), transistor 605 ("M4") and a bias current source 606 ("Ibias"). The sources of M3 and M4 receive a bias current from source 606. The drain of M3 is coupled to the source of Ml and the drain of M4 is coupled to the source of M2. Thus, the bias currents in M3 and M4 are provided to the sources of Ml and M2. It is to be understood that the bias current in M3 and M4 may be implemented using any of a variety of techniques including gate biasing, for example.

Additionally, a second input signal, "in2," may be coupled to the gate of both transistors M3 and M4. If "in2" is a voltage signal, M3 and M4 provide voltage-to-current conversion functionality and will convert the voltage input signal into a current signal (e.g., as an AC component of the bias current). M3 and M4 may be matched devices, but may not be perfectly matched because of manufacturing variations. Thus, if M3 and M4 were perfectly matched, Ml and M2 may receive substantially the same currents from M3 and M4. Such currents are coupled to the sources of Ml and M2 and combined with differential input signal "inl+" and "inl-." The resulting current signal is provided to outputs "outl" and "out2." In

the example of Fig. 6, even order harmonics are reduced by including capacitor Cl. Mismatch in M3 and M4 may also cause even order harmonic impurity because mismatched bias currents in M3 and M4 will change the crossover point in Ml and M2. However, such even order harmonic effects may be reduced by increasing the size of M3 and M4 to reduce current mismatches caused by manufacturing variations. This technique may be effective because in many applications the frequency of "in2" is lower than the frequency of "+/-inl." Therefore, the sizes of M3 and M4 can be increased without impacting system performance.

[0048] Fig. 7 is an example of a mixer circuit 700 using resistor biasing according to one embodiment of the present invention. Mixer circuit 700 includes a first transistor 701 ("Ml") having a source coupled through capacitor 703 ("C 1 ") to the source of a second transistor 702 ("M2"). The bias circuit in this example includes resistor 705 ("Rl") and resistor 706 ("R2"). Rl has a first terminal coupled to the source of Ml and a second terminal coupled to a reference voltage (e.g., ground). Similarly, R2 has a first terminal coupled to the source of M2 and a second terminal coupled to the reference voltage (e.g., ground). Thus, Rl and R2 are used to set the bias current in Ml and M2. Additionally, a second input signal, "in2," may be coupled to the sources of Ml and M2. For example, "in2" may be received on the first terminal of capacitor 707 ("C2") and capacitor 708 ("C3"). The second terminal of capacitor 707 may be coupled to the source of Ml, and the second terminal of capacitor 708 may be coupled to the source of M2. Thus, if "in2" is a current signal, it will be AC coupled to the sources of Ml and M2 where it will be combined with differential input signal "inl+" and "inl-." The resulting current signal is provided to outputs "outl" and "out2." Since the absolute value of resistors can be matched accurately on an integrated circuit process, resistor biasing, together with capacitor Cl, reduces both even and odd harmonic impurity.

[0049] Resistor biasing in mixer circuit 700 also reduces harmonics generated by the input signal +/-inl. Ideally, "+/-inl" should be a sinusoid. However, in many applications (e.g., the local oscillator in a wireless application) "inl" is typically a square wave. If the input to Ml and M2 is a square wave, the output spectrum may include the frequency spectrum of the other mixer input, in2, to appear at the output at odd intervals of the fundamental frequency of "inl." Generally, it is desirable to smooth out the input square wave to suppress these odd harmonics. This may be accomplished by reducing the amplitude of the "inl ." However, one advantage of resistor biasing is that when "inl" increases in amplitude, the odd harmonic content does not substantially increase. The reason for this is that as the amplitude of "inl" increases, the average DC voltage at the sources of Ml and M2 also increases. The increase

in DC voltage at the sources, in turn, increases the bias current through Ml and M2. Increasing the bias current in these devices also increases the point at which the devices will switch in response to differential input signal +/-inl. Because the switching point is increased, the time it takes for "+/-inl" to switch Ml and M2 is increased to approximately the same amount of time as if the signal had been at a lower amplitude. Thus, the present invention is less susceptible to spectral impurities stemming from larger differential signals.

[0050] Fig. 8 is an example of a mixer circuit 800 according to another embodiment of the present invention. Mixer circuit 800 is sometimes referred to as a "double balanced" mixer because it includes two mixer stages. Mixer circuit 800 includes a first mixer comprising transistors 801 ("Ml"), transistor 802 ("M2"), bias transistor 804 ("M3"), bias transistor 805 ("M4") and capacitor 803 ("Cl"). The gate of Ml receives a first input signal, "inl+," and the gate of M2 receives the complement of the first input signal, "inl-." A first terminal of capacitor Cl is coupled to the source of Ml and a second terminal of Cl is coupled to the source of M2. Ml and M2 are biased by M3 and M4 and bias current source 813. A second input signal, "in2," is received at the gates of M3 and M4, converted into a current and coupled to the sources of Ml and M2. Mixer circuit 800 further includes a second mixer comprising transistors 806 ("M6"), transistor 807 ("M7"), bias transistor 809 ("M8"), bias transistor 810 ("M9") and capacitor 808 ("C2"). The gate of M7 also receives the first input signal, "inl+," and the gate of M6 receives the complement of the first input signal, "inl-." A first terminal of capacitor C2 is coupled to the source of M6 and a second terminal of C 1 is coupled to the source of M7. M6 and M7 are biased by M8 and M9 and bias current source 813. A second input signal, "in2," is received at the gates of M8 and M9, converted into a current and coupled to the sources of M6 and M7.

[0051] The drains of Ml and M6 are coupled to a first load impedance 811 ("Zl") and the drains of M2 and M6 are coupled to a second load impedance 812 ("Z2"). Load impedances 811 and 812 may be active, complex or real impedances and may include capacitors, inductors or resistors for implementing narrowband, wideband or tuned filters, for example. In one embodiment, load impedance Zl is a resistor having a first terminal coupled to the drain of Ml and M6 and a second terminal coupled to a reference voltage such as a power supply, "Vdd." Additionally, load impedance Z2 may also be a resistor having a first terminal coupled to the drain of M2 and M7 and a second terminal coupled to a reference voltage such as a power supply, "Vdd." For a symmetric implementation, devices Ml, M2,

M6 and M7 are the same size, M3, M4, M8 and M9 are the same size and the load impedances are the same (e.g., resistors with the same resistance value).

[0052] In another embodiment, in2 may be a single-ended signal, rather than a fully differential signal, and coupled to only one-half of the double balanced mixer. For example, in2 may only be received at the inputs of either M3 and M4 or M8 and M9. hi this approach the double balanced mixer circuit would have one-half the gain.

[0053] Fig. 9 is an example of a mixer circuit 900 according to yet another embodiment of the present invention. Mixer circuit 900 is another example of a double balanced mixer. Example mixer circuit 900 is the same as example mixer circuit 800 except for the bias circuit. The bias circuit in mixer 900 includes, in the first mixer stage, resistor 905 ("Rl"), resistor 906 ("R2"), capacitor 907 ("C2"), capacitor 908 ("C3"), transistor 909 ("M5") and bias current "Ilbias" for biasing M5. In the second mixer stage, the bias circuit includes resistor 915 ("R3"), resistor 916 ("R4"), capacitor 917 ("C5"), capacitor 918 ("C6"), transistor 919 ("M6") and bias current "I2bias" for biasing M6.

[0054] In the first mixer stage, a first terminal of Rl is coupled to the source of Ml and the second terminal of Rl is coupled to a reference voltage (e.g., ground). Similarly, a first terminal of R2 is coupled to the source of M2 and the second terminal of R2 is coupled to the reference voltage (e.g., ground). Rl and R2 set the bias current for Ml and M2 in the first stage of the mixer. Likewise, R3 and R4 set the bias current for M3 and M4 in the second stage of the mixer. A first terminal of R3 is coupled to the source of M3 and the second terminal of R3 is coupled to a reference voltage (e.g., ground). Similarly, a first terminal of R4 is coupled to the source of M4 and the second terminal of R4 is coupled to the reference voltage (e.g., ground). Rl, R2, R3 and R4 may be matched elements on an integrated circuit process to that the currents provided to Ml, M2, M3 and M4 are substantially the same.

[0055] In mixer circuit 900, transistors M5 and M6 and current sources Ilbias and I2bias provide voltage-to-current functionality. However, these are only examples of how voltage- to-current conversion may be implemented. Many other techniques could also be used. The bias circuit for mixer 900 receives a differential voltage input signal "in2+" and "in2-." In2+ is received at the gate of M5 and converted into a current. The drain of M5 is coupled to a first terminal of capacitor C2 and capacitor C3. The other terminal of capacitor C2 is coupled to the source of Ml so that the signal in2+ is coupled to Ml. Likewise, the other terminal of capacitor C3 is coupled to the source of M2 so that in2+ is also coupled to M2. The

complement of in2+ (i.e., in2-) is received at the gate of M6 and coupled through capacitors C5 and C5 to the sources of M3 and M4. The combined signals +/-inl and +/- in2 are provided on differential outputs "outl" and "out2." The outputs are coupled to supply Vdd through load impedances 920 ("Zl") and 921 ("Z2"). As mentioned above with reference to Fig. 7, resistor biasing provides improved matching and tracking across manufacturing processes. Thus, even order harmonics are reduced by introducing capacitors Cl and C4, and odd order harmonics may be reduced via the improved tracking of R1-R4.

[0056] While the above circuit shows in2 as a differential voltage signal, it is to be understood that a variety of other signals may be received at the input of mixer 900. For example, more generally, transistors M5 and M6 and bias currents Ilbias and I2bias could be dependent current sources 998 and 999 (or alternatively, part of the same bias system) that receive either a voltage input signal or a current input signal and provide a current output signal. Thus, the circuit may include either a voltage-to-current converter or a current-to- current converter for providing a current signal to the mixer. Additionally, in2 may be a differential voltage signal (as shown) or a differential current signal. In2 may also be a single ended signal, and the dependent current source (i.e., the voltage-to-current or current-to- current converter) may produce a differential output to both halves of the double balanced mixer. In yet another implementation, in2 may be a single-ended signal, rather than a fully differential signal, and may be coupled to only one-half of the double balanced mixer. For example, ui2 may only be received at the inputs of C2 and C3, for example, and C5, C6, M5, and I2bias are not included. As in the single ended case mentioned above with reference to Fig. 8, such an implementation would have one-half the gain.

[0057] Fig. 10 illustrates one advantageous application of mixer circuits according to embodiments of the present invention. In one embodiment, mixer circuits according to the present invention may be used to up-convert signals in a wireless transmitter. Fig. 10 shows an example of a wireless transmitter 1000. Wireless transmitter 1000 includes a baseband processor 1001 for providing digital signals to be transmitted, hi this example, the transmitter is a direct conversion architecture including a quadrature path and an in-phase path. However, the present invention is also advantageous in other transmitter architectures. The in-phase path may include digital-to-analog converter 1002 ("D/A"), filter 1003 and mixer 1004. The quadrature path may include D/A 1005, filter 1006 and mixer 1007. The digital signals are received in each D/A over N-bit signals lines, for example, converted into analog signals, filtered and up-converted to an RF frequency by modulating the analog signal

with in-phase and quadrature local oscillator signals ("LOi" or "LO _Q "). The modulated signals are then summed and coupled through driver 1010, which may be a power amplifier, to antenna 1020. Features and advantages of the present invention include eliminating filter requirements in the transmission channel. As shown in Fig. 10, a bandpass filter 1010 ("BP Filter") is typically required to filter out unwanted spectral impurities such as the even and odd order harmonics illustrated in Fig. 2-4. However, the present invention eliminates the need for a bandpass filter after the mixer because the spectral impurities in the mixer output signal have been reduced or effectively eliminated. Thus, BP Filter 1010 is shown as a dashed box to illustrate that such component is no longer necessary.

[0058] The present invention is particularly advantageous for wideband or multi-band transmission applications. For example, in one embodiment, a mixer circuit receives a local oscillator signal having a frequency in the range of 3-10GHz on a first input and an analog signal having a bandwidth of up to 250MHz on a second input. A transmitter using embodiments of the present invention may transmit a signal across the 3-10GHz range because the bandpass filter, which previously would have prevented such wideband transmission, has been eliminated. Previously, harmonic impurity would have affected other systems operating in other bandgroups, and a bandpass or narrowband filter at the output of the system was required to reduce the effects of such harmonics. Using embodiments of the present invention, a static narrow band filter at the output may be eliminated, which allows the system to transmit across a wide range of frequencies such as, for example, from 3- 10GHz. It is to be understood that these techniques may also be used in narrowband applications where harmonic spurs are to be suppressed, hi particular, when the harmonics are reduced or eliminated the requirements of a narrowband filter may be advantageously relaxed.

[0059] Fig. 1 IA is an example of a bias circuit for a mixer circuit according to another embodiment of the present invention. Circuit 1 IOOA provides matched bias currents to a mixer circuit by controlling the bias of transistors M3 and M4. For example, bias circuit 1100 may include a bias transistor ("M3") 1101, sensing circuit 1102A, and comparison circuit 1104. The bias circuit in Fig. 1 IA may be half of bias circuit that provides bias current in a complete mixer. Bias transistor 1101 in Fig. 11 provides a bias current to one branch of the mixer circuit. For example, the drain of transistor 1101 can be coupled to the source of transistor 1130. Another bias transistor 1120 may be coupled to the source of transistor 1131. In this case, transistors Ml and M2 and capacitor Cl form the switching pair

of a mixer cell. It is to be understood that multiple feedback circuits including a sensing circuit and comparison circuit could be used to control bias in the same mixer cell (as here, where two feedback circuits are used because a capacitor is between the sources of Ml and M2), or in different mixer cells (e.g., where one feedback circuit is used for each mixer cell because the sources of Ml and M2 are coupled together without a capacitor).

[0060] Circuit 1 IOOA receives an input signal In2, which is coupled to the gate of both M3 and M4. Li a differential system, In2 may be one half of a differential signal (e.g., 1Ώ2+/1Ώ2- as shown below). For the reasons discussed above, it is desirable to reduce the mismatch in currents il and i2 at the drains of M3 and M4, respectively. One source of mismatch results from mismatch in devices M3 and M4. In this embodiment, the present invention reduces mismatch by sensing a bias current through the bias transistor to generate a feedback signal. The feedback signal may then be compared to a reference voltage, and accordingly, a bias control signal can be generated. The bias control signal may be used to control the bias point of the bias transistor (e.g., M3 or M4). In one embodiment, the input signal is provided to amplifier 1106 having a bias control input. The bias control signal may be coupled to the bias control input of amplifier 1106 to control the output level of the amplifier output signal, and thereby control the bias point of the bias transistor. Therefore, the bias control signal is used to adjust the bias of the amplifier, and consequently, the gate of M3/M4. From Fig. 1 IA it can be seen that a feedback loop 1121 is created from the sensing circuit to the gate of the bias transistor. The feedback loop operates to make the DC component of the bias current at the drain of the bias transistors proportional to a constant reference, which in this example is a voltage, but could be a current, for example. The feedback loop may also include a low pass filter 1103. In this example, the low pass filter is coupled to the output of the sensing circuit 1102A, but the filter could be positioned in other locations in the feedback path (e.g., between the comparison circuit and the bias control input of amplifier 1106 as illustrated below).

[0061] The input signal In2 is received at amplifier 1106. In some embodiments, amplifier 1106 may be buffer, which is intended to be included in the use of the term "amplifier." The output of amplifier 1106 is provided to the gate of bias transistor 1101 ("Ml"). The control signal received at the gate of transistor 1101 may include an offset voltage. This offset voltage, together with the mismatch in devices M3/M4, may cause mismatch in the bias currents provided to the two legs of the mixer circuit (i.e., transistors 1130 and 1131). The feedback loops in circuit 1 IOOA causes the currents in bias transistor M3 and M4 to be

proportional to a reference voltage, and accordingly, the currents through transistors 1101 and 1102 are more closely matched.

[0062] The feedback loop in the example circuit 1 IOOA includes sensing circuit 1102A, low pass filter 1103, comparison circuit 1104, and amplifier 1106 (which could be a buffer as described above). Sensing circuit 1102A is coupled to the source of transistor 1101. Low pass filter 1103 is coupled to sensing circuit 1102A. One input of comparison circuit 1104 is coupled to the output of low pass filter 1103. The other input of comparison circuit 1104 is coupled to a reference voltage VR _E F- The output of comparison circuit 1104 generates a bias control signal, which is coupled to a bias control input of amplifier 1106. Amplifier 1106 receives input signal In2 and may amplifier or buffer In2 to the gate of M3. The feedback loop changes the bias of amplifier 1106 so that the current in M3 is approximately proportional to the reference voltage.

[0063] Sensing circuit 1102A is used to sense the current through transistor 1101. Sensing circuit 1102A can also be a circuit that linearizes the current response of transistor 1101. A resistor is one example of sensing circuit 1102 A. The low pass filter 1103 filters the offset voltage signal in the feedback signal output of the sensing circuit 1102A. The comparison circuit 1104 compares difference between the feedback signal from the sensing circuit and a reference voltage V _REF and generates a bias control signal. One example of a bias control signal is an analog voltage or current that is proportional to the different between the reference voltage and the feedback signal. In one embodiment, the feedback loop may include an amplifier (as shown below) to produce a high loop gain. High loop gain reduces the significance of any mismatch occurring in the measurement of the feedback signal. The output of comparison circuit 1104 is applied to the bias control input of amplifier 1106. Amplifier 1106 receives input signal hτ2 and generates an output signal with the AC characteristics of the inputs signal but with a DC bias that will result in a bias current m M3 proportional to the reference voltage.

[0064] Low pass filter 1103 functions as a high pass filter in the feed forward path, canceling out of the effect of the offset voltage and mismatch. The transfer function of the feedback loop is a high pass filter in the feed forward path because low pass filter 1103 subtracts the offset voltage from the input signal In2. Because the feedback loop removes the offset voltage, the bias currents through transistors 1101 and 1120 in the mixer circuit are

closely matched. The corner frequency of the high pass filter can be reduced to reduce filtering of input signal In2 by increasing the resistance of LPF 1103.

[0065] Fig. 1 IB is an example of a bias circuit for a mixer circuit according to yet another embodiment of the present invention. In this example, a differential signal Inl+/Inl- is provided to a first set of mixer inputs at the gates of Ml, M2, M5, and M6. The differential signal InI may be a local oscillator signal, for example. Bias transistors MS ₁ and M-I ₁ are used to provide DC bias and AC signal to the sources of Ml and M2, respectively. Similarly, bias transistors M3 ₂ and M4 ₂ are used to provide DC and AC signals to the sources of M5 and M6. A second differential signal to be mixed is provided to the gates of bias transistors M3i and M4 ₁ and M3 ₂ and M4 ₂ through bias circuits including sense circuits (here resistors 1102A-D), amplifiers 1106A-D, comparison circuits 1103 A-D, and low pass filters ("LPF") 1104A-D. Each bias transistor 1110-1113 may include a separate bias circuit that compares a voltage corresponding to the transistors bias current to a reference voltage and generates a control signal for maintaining all the bias currents proportional to a reference.

[0066] Fig. 12 illustrates an example of a bias circuit according to another embodiment of the present invention. Bias circuit 1200 includes two n-channel bias transistors 1201 ("MS ₁ ") and 1202 ("M3 ₂ ") and two feedback loops. N-channel transistors 1201 and 1202 set the bias current for two inputs of a mixer circuit 1290, and a similar bias circuit 1292 may be used for two other inputs of the mixer circuit. For example, the drain of transistor 1201 can be coupled to the source of transistor 1130 in Fig. 1 IB, and the drain of transistor 1202 can be coupled to the source of transistor 1131.

[0067] In this example, a differential inputs signal hτ2+ and In2- are coupled to the same amplifier 1240, and the differential outputs of amplifier 1240 are use to provide the input signal to two different bias circuit inputs to the mixer. For example, hτ2+ is coupled to the gate of transistor 1224 ("Ml 3") and hi2- is coupled to the gate of transistor 1225 ("M14"). M13 and M14 have sources coupled together through resistor R6. Thus, amplifier 1240 is a differential amplifier. The amplifier may further include current sources 1228 and 1229 and load impedances 1226 and 1227. A first output of amplifier 1240 (e.g., at the drain of M14) may be coupled to the gate of M3 _l5 and a second output of amplifier 1240 (e.g., at the drain of M 13) may be coupled to the gate of M3 ₂ . A feedback loop includes bias transistor 1201 ("Ml") having a source coupled to sense resistor 1203 ("Rl") to translate the bias current into a voltage feedback signal. The feedback signal is coupled through resistor 1206 ("R2")

to a comparison circuit. R2 may be used to isolate the gate capacitance of M16 from MS ₁ . In this example, comparison circuit is a differential amplifier comprising differential transistors 1210 ("M15") and 1211 ("Ml 6"), bias transistor 1212 ("M5"), and load resistors 1208 ("R4") and 1209 ("R5") for comparing the feedback signal to a reference voltage, V _R E _F2> and generating a bias control signal. A bias control signal in this example is a differential voltage signal appearing at the drains of Ml 5 and Ml 6. The bias control signal is coupled to the gate inputs of a second differential amplifier including differential transistors 1214 ("M6") and 1215 ("M7"), and bias transistor 1216 ("M8"). The output of this differential amplifier is a current, which may be mirrored through transistor 1219 ("MlO") and transistor 1218 ("M9") control the bias of amplifier 1240. hi this example the mirror ratio is 5:1. hi one embodiment, a low pass filter comprising resistor 1207 ("R9") and capacitor 1205 ("Cl") is coupled between the gate of MlO and the gate of M9. The gate of M9 may be the bias control input of amplifier 1240, for example. The second feedback loop similarly includes transistor 1202 ("M3 ₂ "), sense resistor 1204 ("R3"), resistor 1236 ("R20"), amplifier 1235, transistor 1220 ("Ml 1"), low pass filter 1230 ("LPF"), and transistor 1221 ("M12").

[0068] hi this example, resistor 1291 ("R91") is coupled between the sources of transistors 1201 and 1202. Thus these devices will act differential and show improved gain performance. While this technique may result in some odd order distortion, the adverse effects of such distortion may be outweighed by the benefits of the additional gain.

[0069] Referring to the embodiment of Fig. 9, mixer circuit 900 receives a differential input signal hi2+/In2-. Input signal Lα2+ is passed through a high pass filter (HPF) in the feed forward path formed by capacitor 907 and resistor 905 as well as capacitor 908 and resistor 906. Input signal In2- is passed through a high pass filter in the feed forward path formed by capacitor 917 and resistor 915 as well as capacitor 918 and resistor 916. The corner frequency of these high pass filters should be low enough so as not to filter out input signal In2+/In2-. The capacitances of the capacitors in the HPFs can be increased to lower the corner frequency of the high pass filters. However, in order to lower the corner frequency to reduce attenuation of input signal In2 to an acceptable level, the capacitors have to be increased to an unacceptably large size. The resistors cannot be increased without affecting the strength of the bias current through the mixer, which is also undesirable.

[0070] One advantage of the embodiment in Figs. 1 IA-B and 12 is that it allows the corner frequency of the high pass filter in the feed forward path to be lowered by increasing the

resistance of resistor 1207 in the LPF. The low pass filter in the feedback path acts as a high pass filter in the feed forward path to effectively remove offset voltage and mismatch. Resistor 1207 can be increased to lower the cutoff frequency without consuming a large amount of die area, and capacitor 1205 does not need to be increased to lower the corner frequency. A lower cutoff frequency in the feedback path reduces a lower cutoff frequency in the feed forward path, allowing wideband signals to be processed without mismatch in the mixer currents.

[0071] The above description illustrates various embodiments of the present invention along with examples of how aspects of the present invention may be implemented. The above examples and embodiments should not be deemed to be the only embodiments, and are presented to illustrate the flexibility and advantages of the present invention as defined by the following claims, hi particular, while the embodiments and examples above were presented using NMOS transistors, other transistors such as PMOS or bipolar may also be used. Based on the above disclosure and the following claims, other arrangements, embodiments, implementations and equivalents will be evident to those skilled in the art and may be employed without departing from the spirit and scope of the invention as defined by the claims.