Transconductance-enhancing passive frequency mixer

A transconductance-enhancing passive frequency mixer comprises a transconductance amplification stage, a frequency mixing stage, and an output transresistance amplifier. The transconductance amplification stage has a pre-amplification transconductance-enhancing structure, so that the transconductance is greatly enhanced, thereby obtaining the same transconductance value at a lower bias current. A radio-frequency current is modulated by the frequency mixing stage to generate an output mid-frequency current signal. The mid-frequency current signal passes through the transresistance amplifier, to form voltage output, and finally obtain a mid-frequency voltage signal. The transresistance amplifier has a transconductance-enhancing structure, thereby further reducing input impedance, and improving current utilization efficiency and port isolation. The frequency mixer has the characteristics of low power consumption, high conversion gain, good port isolation, and the like.

FIELD OF THE INVENTION

The present invention relates to a transconductance enhanced passive frequency mixer, which comprises a transconductance stage with enhanced transconductance, a passive frequency mixing switch pair, and an output transresistance amplifier. The transconductance amplification stage employs a pre-amplified transconductance enhancement structure, so that the transconductance is greatly enhanced, and thereby the same transconductance value can be achieved at lower bias current; intermediate frequency (IF) current signal output is generated from radio frequency (RF) current under the modulation action of the frequency mixer stage. Voltage output is generated via a transresistance amplifier, and finally IF voltage signal is obtained. The transresistance amplifier employs a transconductance enhancement structure, so that the input impedance is further decreased, and current utilization and port isolation are improved. The frequency mixer structure has advantages such as low power consumption, high conversion gain, and high port isolation, etc.

BACKGROUND OF THE INVENTION

Conventional Gilbert frequency mixers have balanced indexes and high port isolation, and operate reliably. However, as the requirement for single-chip of RF receivers increases and the RF technology advances, sometimes the performance of conventional Gilbert frequency mixers can not meet the present demand in many actual applications. For example, in the case that the frequency mixer stage employs an active frequency mixing structure, the flicker noise will have some adverse effect in zero IF receiver structure; if a passive frequency mixing structure can be employed, the flicker noise will be greatly decreased since a passive frequency mixer does not have quiescent current; in addition, the linearity of passive frequency mixer is usually higher than the linearity of active frequency mixer.

In the classic frequency mixing structure, as depicted inFIG. 1A, when RF voltage is converted into RF current in the transconductance stage, the transconductance value is limited and the conversion gain is low at low bias current, because the signal received by the receiver is generally low and the transconductance is only the transconductance of the input transistor in the conventional structure. It will be of great significance for performance improvement of the entire frequency mixer if higher transconductance value can be achieved at the same bias current in the transconductance stage by one approach. Based on that ideal, a novel transconductance circuit structure is designed successfully in the present invention; as a result, the transconductance value in the circuit of transconductance stage is greatly enhanced.

For the output stage of a passive frequency mixer, the input resistance of the transresistance amplifier in the output stage must be decreased as far as possible, owing to considerations of a series of problems, such as port isolation, linearity, and conversion gain, etc. In the present invention, the object is attained by means of a transresistance amplifier that employs a transconductance enhancement structure.

SUMMARY OF THE INVENTION

Technical Problem

The object of the present invention is to provide a passive frequency mixer with a transconductance enhanced transconductance stage, in which the same transconductance value can be achieved at lower bias current and IF current signal is output and generated from RF current under the modulation action of the frequency mixer stage. Voltage output is generated via a transresistance amplifier, and finally IF voltage signal is obtained. The transresistance amplifier employs a transconductance enhancement structure, so that the input impedance is further decreased, and current utilization and port isolation are improved. The frequency mixer structure has advantages such as low power consumption, high conversion gain, and high port isolation, etc.

Technical Solution

The object of the present invention is attained with the following solution: a frequency mixer, which usually comprises a transconductance stage, a frequency mixer stage, and a load output stage. In a conventional frequency mixer, the circuit structure of the transconductance stage is simple, the transconductance gain is low, and the transconductance is usually the transconductance value gmp, as shown inFIG. 1A. RF signal is converted into RF current through the transconductance stage, the RF current generates down-converted signal at intermediate frequency at the output terminal under the modulation action of the frequency mixer stage, and then the down-converted signal is converted into voltage signal through the load output stage. In that process, it will be greatly helpful for improvement of conversion gain and noise performance in the frequency mixer if higher current signal can be obtained at the output terminal of the transconductance stage. A transconductance stage circuit with transconductance enhancement function is designed in the passive frequency mixer disclosed in the present invention.FIG. 1Bshows the voltage-current conversion relationship in the circuit structure after transconductance is enhanced. The working principle will be detailed below.

In the transconductance enhanced passive frequency mixer of the present invention, as shown inFIG. 2, the RF current signal outputted from points A and B in the transconductance stage is coupled to the frequency mixer stage, converted through a frequency conversion process by a frequency mixing switch, and then enters into the transresistance amplifier. Viewed from the source terminal of PM6, the resistance of the circuit in the load stage is very low in the output frequency band, because the circuit in the load stage employs a transconductance enhanced common-gate input structure. When the signal frequency is relatively low, with the extremely low equivalent input resistance of PM6and PM7, the output points A and B in the transconductance stage is equivalent to AC ground for low frequency signal output, and therefore the oscillation amplitude of IF signal is as low as possible at the port; thus the isolation between IF port and RF port is improved. Likewise, for input RF signal, with a capacitor C7connected to the frequency mixing output terminal, the points A and B in the transconductance stage is equivalent to AC ground; thus, the oscillation amplitude of RF voltage at the node is decreased, and current utilization and linearity are improved. A resonant network composed of L0and C4is connected between the gate electrodes of NM0and NM1in the transconductance stage, and the resonant of the parallel resonant network is near the input RF frequency; thus, the resonant network can inhibit IF and low frequency signals from gain and thereby ensures stability of the positive feedback transconductance enhancement circuit. In the input frequency band, with the AC short circuit effect of capacitor C7and low on-state resistance of the switch pair, the transconductance enhancement circuit creates a low-impedance node in the positive feedback loop, and ensures that the gain of the positive feedback loop at the frequency is lower than1, and thereby ensures stability. The transconductance enhancement function is implemented as follows: for RF signal near 2.4 GHz, suppose the positive and negative inputs of RF signal are +vRFand −vRFrespectively. The absolute value of gain of amplifier tubes PM0and PM1at points C and D is A0, then the current flowing through resistor R2is gmpvRF(suppose the transconductance value of all PM tubes is gmp), and the current direction is drawn out of point A; the negative terminal of RF signal passes through PM1, amplified to A0vRFat point D, coupled to the gate electrode of NM0via C3, and converted to current A0gmnvRFat the drain electrode of NM0, and the current direction is also drawn out of point A; then, the sum of RF current drawn out of point A is gmpVRF+A0gmnvRF. In the meanwhile, point B has RF current of the same size, and the direction is injected into point B; wherein, the resistor R4and R5are used to balance the phase delay of the current generated from the drain electrodes of PM0and PM1after the current flowing through R2and R3. Here, a differential RF current output is generated between point A and B, and is coupled to the switch frequency mixer stage via C5and C6; the output stage employs transconductance enhancement technology and can generate low input impedance in the output frequency band; the IF current obtained through frequency mixing is completely drawn into the load stage, and finally IF output voltage is generated in the output load by means of a current mirror.

In the transconductance enhanced passive frequency mixer of the present invention, capacitors C5and C6are connected in series between the transconductance stage and the frequency mixer stage to block the effect of DC signal. The output from the frequency mixer stage is connected to a capacitor C7, so as to filter off the high-frequency signal after frequency mixing. The output IF signal is amplified through the load stage.FIG. 3shows the waveform (low-amplitude low-frequency sinusoidal wave) at the output terminal without transconductance gain structure and the waveform (high-amplitude low-frequency sinusoidal wave) at the output terminal with transconductance gain structure, wherein, the local oscillation frequency is 2.45 GHz; it is seen clearly fromFIG. 3: the conversion gain is greatly increased owing to the improvement of transconductance structure.

Beneficial Effects

the transconductance stage of the transconductance enhanced passive frequency mixer can increase the RF current converted from RF input voltage. The main working principle is: the current is multiplied with the local oscillator signal via a passive dual balance switch pair, so as to accomplish frequency mixing function. The switch stage employs a passive frequency mixing method, and does not have static power consumption, and eliminates flicker noise from switch stage. The output stage employs transconductance enhancement technology, and can generate low input impedance in the output frequency band; the IF current obtained through frequency mixing is completely drawn into the load stage, and finally IF output voltage is generated in the output load by means of a current mirror. The output terminal of the frequency mixer stage is connected to a capacitor, so that the output terminal is equivalent to AC ground for the RF signal from the transconductance stage; thus, the RF current generated in the transconductance stage can be drawn into the frequency mixing switch as much as possible. Since the transresistance stage has low input impedance, the IF voltage fluctuation at the input terminal of the transresistance stage is very low; thus, voltage feeding of IF signal to the output terminal of transconductance stage is reduced, the output voltage of transconductance stage is stabilized, and current utilization and linearity are improved. The transconductance enhanced passive frequency mixer described above has advantages such as high transconductance in the transconductance stage, low power consumption, and high conversion gain.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The transconductance stage of the transconductance enhanced passive frequency mixer can increase the RF current converted from RF input voltage. The main working principle is: the current is multiplied with the local oscillator signal via a passive dual balance switch pair, so as to accomplish frequency mixing function. The switch stage employs a passive frequency mixing method, and does not have static power consumption, and eliminates flicker noise from switch stage. The output stage employs transconductance enhancement technology, and can generate low input impedance in the output frequency band; the IF current obtained through frequency mixing is completely drawn into the load stage, and finally IF output voltage is generated in the output load by means of a current mirror. The output terminal of the frequency mixer stage is connected to a capacitor, so that the output terminal is equivalent to AC ground for the RF signal from the transconductance stage; thus, the RF current generated in the transconductance stage can be drawn into the frequency mixing switch as much as possible. The transimpedance stage has low input impedance, and the IF voltage fluctuation at the input terminal of the transimpedance stage is very low; thus, voltage feeding of IF signal to the output terminal of transconductance stage is reduced, the output voltage of transconductance stage is stabilized, and current utilization and linearity are improved. The transconductance enhanced passive frequency mixer described above has advantages such as high transconductance in the transconductance stage, low power consumption, and high conversion gain.

The main structure of the transconductance enhanced passive frequency mixer disclosed in the present invention comprises a transconductance enhanced transconductance/amplifier stage, a passive mixer stage, a load output stage, and a bias circuit, etc. The transconductance/amplifier stage comprises P-channel metal oxide field-effect transistors (hereinafter referred to as PMOS transistors) PM0and PM1, and N-channel metal oxide field-effect transistors (hereinafter referred to as NMOS transistors) NM0and NM1, and cross-coupled capacitor and LC resonant circuits. The bias voltages of PM0and PM1are obtained from bias voltage1via R0and R1respectively. L0, C2, C3, and C4are used to enhance transconductance; the output RF currents of the transconductance stage are led out between R2and R4and between R3and R5respectively. The currents are coupled to the frequency mixing switch stage via C5and C6respectively; in the frequency mixer stage, PM2-PM5is used as the core circuit, and the results after frequency mixing are outputted from the drain electrodes of PM2and PM5, wherein, the drain electrodes of PM2and PM4are short connected, and the drain electrodes of PM3and PM5are short connected. The load output stage mainly comprises PM6-PM15and NM2-NM3. PM6, PM7, PM10, PM11, NM2, and NM3constitute the first stage differential amplification circuit of the load output stage, and the bias of NM2and NM3are provided by bias voltage2. The stages are outputted from the drain terminals of PM6and PM7, and connected to a second stage of differential source follower constituted by PM8, PM9, PM12, and PM13; then, the signals are outputted from the drain electrodes of PM12and PM13, and connected to a third stage of differential common-source amplification circuit constituted by PM14, PM15, R6, and R7; finally, IF signals are outputted from the drain terminals of PM14and PM15.

The upper plates of capacitors C0and C1are connected to the positive input signal terminal and negative input signal terminal respectively. The lower plate of C0is connected to the gate electrode of PM0; the lower plate of C1is connected to the gate electrode of PM1; the upper plate of capacitor C2is connected to the drain terminal of PM0, the upper plate of capacitor C3is connected to the drain terminal of PM1, the lower plate of C2is connected to the lower plate of C4, the lower plate of C3is connected to the upper plate of C4, the upper plate of C4is connected to the positive terminal of L0, the lower plate of C4is connected to the negative terminal of L0, the positive terminal of L0and the upper plate of C4are connected to the gate electrode of NM1, and the negative terminal of L0and the lower plate of C4are connected to the gate electrode of NM0; the source electrodes of PM0and PM1are connected to the supply voltage, the positive terminal of resistor R0is connected to the gate electrode of PM0, the negative terminal of R0is connected to the positive terminal of resistor R1, and the negative terminal of R1is connected to the gate electrode of PM1. The positive terminal of resistor R2is connected to the drain terminal of PM0, the negative terminal of R2is connected to the positive terminal of R4, and the negative terminal of R4is connected to the drain electrode of NM0; the positive terminal of resistor R3is connected to the drain terminal of PM1, the negative terminal of R3is connected to the positive terminal of R5, and the negative terminal of R5is connected to the drain electrode of NM1. The source electrodes of NM0and NM1are grounded. The upper plate of RF coupling capacitor C5is connected to the negative terminal of R2and positive terminal of R4, the upper plate of RF coupling capacitor C6is connected to the negative terminal of R3and positive terminal of R5, the lower plate of C5is connected to the source electrodes of PM2and PM3, and the lower plate of C6is connected to the source electrodes of PM4and PM5. The positive terminal of local oscillator signal is connected to the gate electrodes of PM3and PM4, and the negative terminal of local oscillator signal is connected to the gate electrodes of PM2and PM5. The drain electrodes of PM2and PM4are connected to the upper plate of C7, and the drain electrodes of PM3and PM5are connected to the lower plate of C7. The positive output of the switch stage (i.e., the upper plate of C7) is connected to the source electrode of PM6and drain electrode of PM10in the load stage, the negative output of the switch stage (i.e., the lower plate of C7) is connected to the source electrode of PM7and drain electrode of PM11in the load stage. The gate electrodes of PM6and PM7are connected to bias voltage4for bias; the drain electrode of PM6is connected to the drain electrode of NM2and gate electrode of PM8; the drain electrode of PM7is connected to the drain electrode of NM3and gate electrode of PM9; the drain electrodes of PM8and PM9are grounded, forming a source follower; the source electrodes of NM2and NM3are grounded, and the gate electrodes of NM2and NM3are connected to bias voltage2for bias. The source electrodes of PM10-PM15are connected to the supply voltage; the source electrode of PM8is connected to the drain electrode of PM12and gate electrode of PM14; the source electrode of PM9is connected to the drain electrode of PM13and gate electrode of PM15; the drain electrode of PM14serves as the positive terminal of output voltage and is connected to the positive terminal of resistor R7, and the negative terminal of resistor R7is grounded; the drain electrode of PM15serves as the negative terminal of output voltage and is connected to the positive terminal of resistor R8, and the negative terminal of R8is grounded.

While the present invention has been illustrated and described with reference to some preferred embodiments, the present invention is not limited to these. Those having ordinary skills in the art should recognize that various variations and modifications can be made without departing from the spirit and scope of the present invention as defined by the accompanying claims.