Dynamic current steering mixer

A dynamic current steering mixer. The dynamic current steering mixer comprises a Gilbert cell mixer core, a pair of load devices, a dynamic current steering cell, and a transconductor cell. The Gilbert cell mixer core has first and second nodes, receives a first differential input signal, and provides a differential output signal at the first nodes thereof. The load devices are respectively coupled between the first nodes of the Gilbert cell mixer core and a first fixed voltage. The dynamic current steering cell has third nodes coupled to the second nodes and fourth nodes. The transconductor cell is coupled between the fourth nodes and a second fixed voltage and receives a second differential input signal. The dynamic current steering cell alternately steers current of the transconductor cell to or away from the Gilbert cell mixer core.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a double-balanced mixer and, in particular, to a double-balance mixer with a dynamic current steering cell.

2. Description of the Related Art

Mixer circuits for high frequency applications constructed using metal oxide semiconductor (MOS) transistors are subject to a limited voltage supply (usually less than 2V) and high levels of flicker noise, having frequencies extending to several tens of MHz. Accordingly, the gain and output signal level required in such mixer circuits exceed those required in the equivalent bipolar circuits.

FIG. 1is a circuit diagram illustrating a conventional double balanced mixer circuit. The double balanced mixer circuit inFIG. 1includes differential pairs of MOSFETs (Q131-Q132and Q133-Q134). The drains of the pairs of MOSFETs are connected to an output terminal (Output-I+and Output-I−). The gates of the pairs of MOSFETs are connected to first input terminals (Input-II+and Input-II−). The double balanced mixer circuit inFIG. 1also includes active devices Q135, Q136, Q137and Q138. The sources of the MOSFET pair Q131-Q132are connected to the drains of the active devices Q135and Q136. The sources of the MOSFET pair Q133-Q134are connected to the drains of the active devices Q137and Q138. The gates of the active devices Q135, Q136, Q137and Q138are connected to the second input terminal (Input-I+and Input-I−). The sources of the active devices Q135, Q136, Q137and Q138are connected to the ground through an impedance unit (Degeneration Impedance).

FIG. 2is a circuit diagram of a conventional quadrature mixer disclosed by Raj a S Pullela et. al in ISSCC 2006. The conventional quadrature mixer200comprises an I-Mixer Quad210, a Q-Mixer Quad220, a 2x LO stage230, and a transconductor stage240. The I-Mixer Quad210inFIG. 2includes differential pairs of MOSFETs (M9-M10and M11-M12). The drains of the pairs of MOSFETs are connected to an output terminal (BBIp and BBIn). The gates of the pairs of MOSFETs are connected to first input terminals (LOIp and LOIn). The Q-Mixer Quad220inFIG. 2includes differential pairs of MOSFETs (M13-M14and M15-M16). The drains of the pairs of MOSFETs are connected to an output terminal (BBQp and BBQn). The gates of the pairs of MOSFETs are connected to first input terminals (LOQp and LOQn). The 2x LO stage230comprises MOSFETs M5, M6, M7and M8. Sources of the MOSFETs M9-M10and M11-M12are respectively connected to drains of the MOSFETs M5and M7and those of the MOSFETs M13-M14and M15-M16respectively connected to drains of the MOSFETs M6and M8. Gates of the MOSFETs M5and M7are connected to an input terminal2Lop and those of the MOSFETs M6and M8connected to an input terminal2Lon. MOSFETs M1and M3are connected between the sources of the MOSFETs M5-M6and a ground GND and MOSFETs M2and M4connected between the sources of the MOSFETs M7-M8and the ground. Gates of the MOSFETs M1and M3are connected to an input terminal RF+ and those of the MOSFETs M2and M4connected to an input terminal RF−.

BRIEF SUMMARY OF THE INVENTION

An embodiment of a dynamic current steering mixer comprises a Gilbert cell mixer core, a pair of load devices, a dynamic current steering cell, and a transconductor cell. The Gilbert cell mixer core has first and second nodes, receives a first differential input signal, and provides a differential output signal at the first nodes thereof. The load devices are respectively coupled between the first nodes of the Gilbert cell mixer core and a first fixed voltage. The dynamic current steering cell has third nodes coupled to the second nodes and fourth nodes. The transconductor cell is coupled between the fourth nodes and a second fixed voltage and receives a second differential input signal. The dynamic current steering cell alternately steers current of the transconductor cell to or away from the Gilbert cell mixer core.

An embodiment of a quadrature dynamic current steering mixer comprises first and second dynamic current steering mixers connected in parallel between first and second fixed voltages. Each of the first and second dynamic current steering mixers comprises a Gilbert cell mixer core, a pair of load devices, a dynamic current steering cell, and a transconductor cell. The Gilbert cell mixer core has first and second nodes, receives a first differential input signal, and provides a differential output signal at the first nodes thereof. The load devices are respectively coupled between the first nodes of the Gilbert cell mixer core and the first fixed voltage. The dynamic current steering cell has third nodes coupled to the second nodes and fourth nodes. The transconductor cell is coupled between the fourth nodes and the second fixed voltage and receives a second differential input signal. The dynamic current steering cell alternately steers current of the transconductor cell to or away from the Gilbert cell mixer core. There is a phase difference of 90° between the first differential input signals of the first and second dynamic current steering mixers.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 3is a circuit diagram of a dynamic current steering mixer according to an embodiment of the invention. The dynamic current steering mixer300comprises a Gilbert cell mixer core310, a pair of load devices RL, a dynamic current steering cell330, and a transconductor cell350. The Gilbert cell mixer core310has first nodes311and311′ and second nodes313and313′ and comprises differential pairs of NMOS transistors T7-T8and T9-T10coupled therebetween. The Gilbert cell mixer core310receives a local oscillator signal LO+/LO−, and provides an intermediate frequency (IF) signal IF+/IF− at the first nodes311and311′. The load devices RL are respectively coupled between the first nodes311and311′ of the Gilbert cell mixer core310and a supply voltage Vcc. The dynamic current steering cell330has third nodes331and331′ respectively coupled to the second nodes313and313′, and fourth nodes333and333′. The dynamic current steering cell330comprises a first switch335and a second switch335′. The first and second switches335and335′ alternately connect to the fourth nodes333and333′, the third node331and331′, and a fixed voltage Vdd according to a control signal2fo, respectively. Frequency of the control signal is twice of that of the local oscillator signal LO+/LO−. The transconductor cell350comprises NMOS transistors T1and T2coupled between the fourth nodes333and333′ and a ground GND. Gates of the NMOS transistors T1and T2receive a radio frequency (RF) signal RF+/RF−.

FIG. 4Ais a detailed circuit diagram of the dynamic current steering mixer inFIG. 3. InFIG. 4A, the dynamic current steering cell330comprises NMOS transistors T3, T4, T5and T6. The NMOS transistors T3and T6are coupled between the third nodes331and331′ and the fourth nodes333and333′. The NMOS transistors T4and T5are coupled between the fixed voltage Vdd and the fourth nodes333and333′. Gates of the NMOS transistors T3and T6receive the control signal2foand those of the NMOS transistors T4and T5are connected to a reference voltage Vref. Preferably, the fixed voltage Vdd is the same as the supply voltage Vcc.FIG. 4Bis a schematic diagram showing waveforms of the local oscillator signal LO+/LO− and the control signal2foinFIG. 4A. At zero-crossing points Toff of the local oscillator signal LO+/LO−, voltage level of the control signal2fois lower than the reference voltage Vref, allowing the NMOS transistors T3and T6to be turned off. Current (both DC and AC) of the transconductor cell350is thus steered to the NMOS transistors T4and T5. Since voltage level of the control signal2foexceeds the reference voltage Vref at non-zero-crossing points of the local oscillator signal LO+/LO−, the NMOS transistors T3and T6will be turned on. Current (both DC and AC) of the transconductor cell350is thus steered to the NMOS transistors T3and T6. It is well-known that flicker noise of the Gilbert cell mixer core310is proportional to a current injected therein at zero-crossing point. Therefore, steering current off the Gilbert cell mixer core310according to the dynamic current steering cell of the present invention can successfully suppress the flicker noise and would be insensitive to LO quality.

FIG. 4Cshows another embodiment of the dynamic current steering mixer of the present invention. The main difference between the dynamic current steering mixer inFIG. 4Aand inFIG. 4Cis that the Gilbert cell mixer core310and the load devices are folded down and coupled to the ground GND. The NMOS transistors T7, T8, T9and T10in the Gilbert cell mixer core310are replaced by PMOS transistors.

FIG. 4Dshows another embodiment of the dynamic current steering mixer of the present invention. The main difference between the dynamic current steering mixer inFIG. 4Aand inFIG. 4Dis that the NMOS transistors T7, T8, T9and T10in the Gilbert cell mixer core310are replaced by bipolar junction transistors (BJTs). It is noted that the Gilbert cell mixer core310and the load devices inFIG. 4Dcan also be folded down and coupled to the ground GND. In addition, the dynamic current steering cell comprising the NMOS transistors T3, T4, T5, and T6is an embodiment and the scope is not limited thereto. Bipolar junction transistors are also applicable to the dynamic current steering cell.

FIG. 5Ais a detailed circuit diagram of the dynamic current steering mixer inFIG. 3. InFIG. 5A, the dynamic current steering cell330comprises NMOS transistors T3, T4, T5and T6. The NMOS transistors T3and T6are coupled between the third nodes331and331′ and the fourth nodes333and333′. The NMOS transistors T4and T5are coupled between the fixed voltage Vdd and the fourth nodes333and333′. Gates of the NMOS transistors T3and T6are connected to a reference voltage Vref and those of the NMOS transistors T4and T5receive the control signal2fo. Preferably, the fixed voltage Vdd is the same as the supply voltage Vcc.FIG. 5Bis a schematic diagram showing waveforms of the local oscillator signal LO+/LO− and the control signal2foinFIG. 5A. At zero-crossing points Toff of the local oscillator signal LO+/LO−, voltage level of the control signal2foexceeds the reference voltage Vref, allowing the NMOS transistors T3and T6to be turned off. Current (both DC and AC) of the transconductor cell350is thus steered to the NMOS transistors T4and T5. Since voltage level of the control signal2fois lower than the reference voltage Vref at non-zero-crossing points of the local oscillator signal LO+/LO−, the NMOS transistors T3and T6are then turned on. Current (both DC and AC) of the transconductor cell350is thus steered to the NMOS transistors T3and T6. As a result, current of the transconductor cell350is steered off the Gilbert cell mixer core310at zero-crossing points Toff of the local oscillator signal LO+/LO− and flicker noise thereof is thus suppressed.

FIG. 5Cshows another embodiment of the dynamic current steering mixer of the present invention. The main difference between the dynamic current steering mixer inFIG. 5Aand inFIG. 5Cis that the Gilbert cell mixer core310and the load devices are folded down and coupled to the ground GND. The NMOS transistors T7, T8, T9and T10in the Gilbert cell mixer core310are replaced by PMOS transistors.

FIG. 5Dshows another embodiment of the dynamic current steering mixer of the present invention. The main difference between the dynamic current steering mixer inFIG. 5Aand inFIG. 5Dis that the NMOS transistors T7, T8, T9and T10in the Gilbert cell mixer core310are replaced by bipolar junction transistors (BJTs). It is noted that the Gilbert cell mixer core310and the load devices inFIG. 5Dcan also be folded down and coupled to the ground GND. In addition, the dynamic current steering cell comprising the NMOS transistors T3, T4, T5, and T6is an embodiment and the scope is not limited thereto. Bipolar junction transistors are also applicable to the dynamic current steering cell.

FIG. 6Ais a schematic diagram of current of a Gilbert cell mixer core in a conventional double balanced mixer and a dynamic current steering mixer according to an embodiment of the invention. InFIG. 6A, current of the Gilbert cell mixer core in a conventional double balanced mixer exceeds that of a dynamic current steering mixer according to an embodiment of the invention at zero-crossing points (represented by a dashed-line box).FIGS. 6B and 6Care respectively schematic diagrams of current of the NMOS transistors in a Gilbert cell mixer core of a conventional double balanced mixer and a dynamic current steering mixer according to an embodiment of the invention. InFIG. 6B, current of the NMOS transistors in a Gilbert cell mixer core of a conventional double balanced mixer is about 1 mA. InFIG. 6C, current of the NMOS transistors in a Gilbert cell mixer core of a dynamic current steering mixer according to an embodiment of the invention is only about 0.3 mA.FIG. 6Dshows noise figure of a conventional double balanced mixer and a dynamic current steering mixer according to an embodiment of the invention. InFIG. 6D, noise figure of a conventional double balanced mixer is 15.3 dB at 10.4 kHz and that of a dynamic current steering mixer according to an embodiment of the invention is only 12.3 dB at 10.4 kHz.

FIG. 7Ais a circuit diagram of a quadrature dynamic current steering mixer according to an embodiment of the invention. InFIG. 7A, the quadrature dynamic current steering mixer700comprises an I-Quad mixer710and a Q-Quad mixer760. The I-Quad mixer710and the Q-Quad mixer760are both dynamic current steering mixers as disclosed inFIG. 5Aand connected in parallel between a supply voltage Vcc and a ground GND. The Gilbert mixer core310in the I-Quad mixer710receives a local oscillator signal LOIP/LOIN and that in the Q-Quad mixer760a local oscillator signal LOQP/LOQN. The dynamic current steering cell330in the I-Quad mixer710is controlled by a control signal2LOP and that in the Q-Quad mixer760controlled by a control signal2LON. The I-Quad mixer710generates an IF signal IFIP/IFIN and the Q-Quad mixer760an IF signal IFQP/IFQN. Since the I-Quad mixer710and the Q-Quad mixer760in the quadrature dynamic current steering mixer700are both dynamic current steering mixers, noise figure of the quadrature dynamic current steering mixer700according to an embodiment of the invention is also improved. It is noted that the disclosed variants of the dynamic current steering mixer inFIG. 4AandFIG. 5Acan also be used in the quadrature dynamic current steering mixer.

FIG. 7Bis a schematic diagram showing waveforms of the local oscillator signals LOIP/LOIN and LOQP/LOQN and the control signals2LOP and2LON. InFIG. 7B, at a zero-crossing point t1of the local oscillator signal LOIP/LOIN, since voltage level of the control signal2LOP exceeds that of the reference voltage Vref, current Ia also exceeds current Ib. Meanwhile, voltage level of the reference voltage Vref also exceeds that of the control signal2LON, and current Id thus exceeds current Ic. In other words, the currents Ia and Id consume most current at zero-crossing points of the local oscillator signal LOIP/LOIN and the currents Ib and Ic consume most current at zero-crossing points of the local oscillator signal LOQP/LOQN. As a result, current reuse is accomplished by combining current Ia with Id, and combining Ic with Ib.

FIG. 8Ais a circuit diagram of a variant of the quadrature dynamic current steering mixer according to an embodiment of the invention. The quadrature dynamic current steering mixer inFIG. 8Adiffers from that inFIG. 7Aonly in that drains of the NMOS transistors TI3, TI4, TI5, and TI6in the I-Quad mixer710are respectively connected with drains of the NMOS transistors TQ4, TQ3, TQ6, and TQ5in the Q-Quad mixer760. Nodes A inFIG. 8Arepresent the drains of the NMOS transistors TI3and TQ4connected and nodes B inFIG. 8Arepresent the drains of the NMOS transistors TI4and TQ3are connected, and so forth. In such configuration, dynamic current steering and current combination are achieved and noise figure and current consumption are thus improved.

FIG. 8Bis a circuit diagram of a variant of the quadrature dynamic current steering mixer inFIG. 8A, differing only in that the I-Quad mixer810and the Q-Quad mixer860are both dynamic current steering mixer as disclosed inFIG. 4A.