This patent application is based on Taiwan, R.O.C. patent application No. 98116506 filed on May 19, 2009.
The present invention relates to a mixer, and more particularly to a mixer with high linearity and a low operating voltage.
In a wireless transmitter or a radio frequency (RF) transmitter, a mixer is a widely-used frequency conversion unit.
The transconductor 21 comprises two n-type transistors Ml and M2. The drain of M1 is coupled to the first current path of the switch quad 22; The drain of M2 is coupled to the second current path of the switch quad 22. The gate of M1 receives a voltage signal Vin+; The gate of M2 receives a voltage signal Vin−. Further, the sources of M1 and M2 are mutually coupled. An n-type transistor Ms is coupled between the source of M1 and ground. The gate of the transistor Ms is inputted in a stable voltage such that the n-type transistor Ms provides a current source.
For the wireless transmitter, a signal swing of an input/output signal needs to be large to enhance a signal-to-noise ratio (SNR) so as to allow the input/output signal to be immune from a noise and to reduce a local oscillation leakage (LO leakage) effect. However, since electronic apparatuses have a trend of a decreasing size, an integrated circuit (IC) needs to be smaller and smaller while an operating voltage has to become lower and lower. Therefore, under such low operating voltage condition, it is an issue to be discussed as how the transmission signal swing can remain large when the mixer is designed in the wireless transmitter.
On the other hand, as shown in
As a result, one object of the present invention is to provide a mixer with high linearity to avoid a non-linear problem in a transconductor of a conventional mixer.
Another object of the present invention is to provide a mixer with a low operating voltage for lowering the operating voltage and still maintaining a large input/output signal swing.
The present invention discloses a mixer comprising a transconductor and a switch circuit. The transconductor receives a pair of differential voltage signals and outputs a pair of differential current signals. The transconductor comprises a first resistor and a second resistor, and a differential amplifier. The differential amplifier comprises a first input end, a second input end, a first output end, and a second output end, wherein the pair of differential voltage signals are transmitted to the first input end and the second input end via the first resistor and the second resistor respectively, and the pair of differential current signals are outputted from the first input end and the second input end respectively. The transconductor further comprises a first current source and a second current source, coupled to the first input end and the second input end respectively. The switch circuit comprises a first switch, a second switch, a third switch, and a fourth switch, where the first switch and the second switch are coupled to the first input end, the third switch and the fourth switch are coupled to the second input end, the first switch and the third switch are mutually coupled to provide an output for the mixer, and the second switch and the fourth switch are mutually coupled to provide another output for the mixer. The first switch, the second switch, the third switch, and the fourth switch control whether to allow the pair of differential current signals to pass therethrough according to a pair of differential control signals; wherein, the first output end is coupled to the first switch and the second switch, such that the first output end and the first input end of the differential amplifier form a negative feedback loop, and the second output end is coupled to the third switch and the fourth switch, such that the second output end and the second input end of the differential amplifier form another negative feedback loop.
The present invention will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed description and accompanying drawings, in which:
The switch quad 42 comprises four switches 421, 422, 423 and 424. The switch 421 comprises a transistor M1 and an isolation circuit 4211, the switch 422 comprises a transistor M2 and an isolation circuit 4221, the switch 423 comprises a transistor M3 and an isolation circuit 4231, and the switch 424 comprises a transistor M4 and an isolation circuit 4241. Sources of the transistors M1 and M2 are both coupled to the positive input end of the differential amplifier 411; sources of the transistors M3 and M4 are both coupled to the negative input end of the differential amplifier 411. Drains of the transistors M1 and M3 are mutually coupled to provide an output end 43 of the mixer 40; drains of the transistors M2 and M4 are mutually coupled to provide another output end 44 of the mixer 40.
The switches 421, 422, 423 and 424 control whether to allow the differential current signals I+ and I− to pass through according to a pair of differential control signals. The pair of differential control signals, comprising a first control signal and a second control signal, are transmitted to each gate of the transistors M1 and M2 via the isolation circuits 4211 and 4221 respectively, to control the switches 421 and 422 whether to allow the current signal I+ to pass through. The first control signal and the second control signal are also transmitted to each gate of the transistors M4 and M3 via the isolation circuits 4241 and 4231 respectively, to control the switches 424 and 423 whether to allow the current signal I− to pass through (functions of the isolation circuits 4211, 4221, 4231 and 4241 are described later). The pair of differential control signals can be generated by a local oscillator. By controlling a frequency of the pair of differential control signals adequately to switch the switches 421, 422, 423 and 424, the frequency of the pair of differential current signals I+ and I− can be converted into a desired frequency and then be outputted to the output ends 43 and 44.
As shown in
When the transconductor 41 is a fully differential circuit and R1=R2, Eq.(1) can be simplified to
Therefore, the relation between the output current Iout and the input voltage Vin of the transconductor 41 become linear; that is, the transconductor 41 has a linear transconductance. Consequently, the mixer 40 has high linearity by using the transconductor 41.
In
While the first control signal and the second control signal are high frequency signals, the signals, outputted from the positive output end and the negative output end, are low frequency signals; the isolation circuits 4211, 4221, 4231 and 4241 can be realized as shown in
The mixer 40 additionally can operate with a low operating voltage. The following takes computing a required gate operating voltage of the transistor M1 for example, whereas each required gate operating voltage of the transistors M2, M3, and M4 is similar. The meaning of the required gate operating voltage is that, throughout the mixer 40 operation, the gate has to maintain above such voltage value, or the mixer 40 cannot function well. With reference to
V
G1
=V
a
+V
GS1 Eq.(2)
Wherein, Va is the voltage of point a, and VGS1 is a gate-to-source voltage of the transistor M1. Since point a is the positive input end of the differential amplifier 411, Va is a common mode input voltage of the differential amplifier 411, denoted as Vicm in the following. VGS1 includes a direct current and an alternating current. The direct current is generated from biasing the transistor M1. Note that the transistor M1 needs to operate in a saturation region so that the mixer 40 can function well. Therefore, such direct current needs to be at least VDsat1+VTH1, wherein VDsat1 and VTH1 represent a drain saturation voltage and a threshold voltage of the transistor M1 respectively. The alternating current is generated from a voltage variation (denoted as ΔVGS1) of the input voltage Vin, whose computation is as follows:
Provided that a transconductance of the transistor M1 is gm1, then:
ΔVGS1=ΔID1/gm1 Eq.(3)
Wherein, ΔID1 is the drain current of the transistor M1. Since I+ is equal to the sum of both drain currents of the transistors M1 and M2, ΔID1=I+/2. Consequently, Eq.(3) can be represented as:
Provided that a maximal amplitude of the input voltage Vin, i.e., a maximal difference between the differential voltage signals Vin+ and Vin−, is Vs, according to Eq.(2) and Eq.(4), a required gate operating voltage of the transistor M1, VG1min, is derived as:
Wherein, Vicm is a drain saturation voltage of the transistor M5, VDsat5.
With reference to a conventional mixer 20 as shown in
Wherein, VDsats, VDsat1 and VDsat3 represent each drain saturation voltage of transistors Ms, M1 and M3 respectively in the conventional mixer 20, VTH3 represents a threshold voltage of the transistor M3 in the conventional mixer 20, and Vs represents a maximal difference between differential voltage signals Vin+ and Vin−.
Suppose the conventional mixer 20, according to
With reference to
While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not to be limited to the above embodiments. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.
Number | Date | Country | Kind |
---|---|---|---|
098116506 | May 2009 | TW | national |