Transconductance amplifier having common mode feedback circuit and method of operating the transconductance amplifier

A complementary transconductance amplifier having a common mode feedback circuit includes a first-type transconductor, a second-type transconductor and a common mode feedback circuit. The first-type transconductor generates a first differential output signal pair in response to a differential input signal pair under the control of a first control signal. The second-type transconductor generates a second differential output signal pair in response to the differential input signal pair under the control of a second control signal. The common mode feedback circuit generates the second control signal in response to the first and second differential output signal pairs under the control of a common mode control signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Korean Patent Application No. 2004-91678 filed on Nov. 11, 2004, the contents of which are herein incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a transconductance amplifier, and more particularly to a complementary transconductance amplifier having a common mode feedback circuit and a method of amplifying the transconductance of the amplifier.

2. Description of Related Art

A transconductance amplifier is a circuit block that amplifies a voltage signal with a predetermined gain. Transconductance amplifiers may be used in applications such as Gm-C filters. The transconductance amplifier needs to have a high operating frequency, a low operating current, a high linearity and a broad tuning range. A typical transconductance amplifier has a structure of a differential amplifier and has a function of controlling a variable gain.

FIG. 1is a circuit diagram illustrating an example of a conventional transconductance amplifier, and is disclosed in U.S. Pat. No. 6,271,688. Referring toFIG. 1, input voltages VIP and VIM are applied to the gates of NMOS transistors MN1and MN2. An NMOS transistor MN3, coupled between the sources of the NMOS transistors MN1and MN2, operates in a triode region and functions as a variable resistor. Transconductance (Gm), which is represented as IO/VI, is controlled by controlling a control voltage VC that is applied to a gate of the NMOS transistor MN3. The transconductance amplifier ofFIG. 1includes a common mode feedback circuit (CMFB)12to stabilize an output common mode voltage.

FIG. 2is a circuit diagram illustrating another example of a conventional transconductance amplifier, and is disclosed in U.S. Pat. No. 5,332,937. Referring toFIG. 2, NMOS transistors MN4and MN5, to which input voltages VIP and VIM are applied, operate in a triode region. Drain voltages of the NMOS transistors MN4and MN5are controlled in response to the control voltage VC, which is applied to bases of bipolar transistors Q1and Q2. Therefore, transconductance (Gm) is controlled by way of controlling the control voltage VC.

FIG. 3is a circuit diagram illustrating still another example of a conventional transconductance amplifier, and is described in Martinez et al., “A 60-mW 200-MHz continuous time seventh-order linear phase filter with on-chip automatic tuning system,” IEEE J. Solid-State Circuits, February 2003, Vol. 38, Issue 2, pp. 216-225. The transconductance amplifier ofFIG. 3has a complementary structure. A common mode feedback circuit32enables the output common mode voltage to have a constant value even though the transconductance (Gm) ofFIG. 3changes. Transconductance amplifiers having a complementary structure such as the transconductance amplifier ofFIG. 3have reduced current consumption and noise.

In the transconductance amplifier ofFIG. 3, the transconductance of an N-type transconductor comprising NMOS transistors MN6and MN7is controlled by a control voltage VCN, and the transconductance of a P-type transconductor comprising PMOS transistors MP1and MP2is controlled by a control voltage VCP. In the transconductance amplifier ofFIG. 3, one of the control voltages VCP and VCN needs to have a fixed value because the control voltages VCP and VCN are independent from each other. Accordingly, the transconductance (Gm) control range of the transconductance amplifier ofFIG. 3may be reduced by about 50% as compared with the transconductance amplifier ofFIG. 1, and a degree of symmetry between the N-type transconductor and the P-type transconductor may be reduced.

The noise of the transconductance amplifier ofFIG. 3may be increased because of current sources IP1and IP2, placed between the supply voltage VDD and the P-type transconductor, and current sources IN1and IN2, placed between ground GND and the N-type transconductor.

The transconductance amplifier ofFIG. 3may have a lower linearity than the transconductance amplifier ofFIG. 1having NMOS transistors MN4and MN5, which operate in a triode region, because the PMOS transistors MP1and MP2and the NMOS transistors MN1and MN2operate in a saturation region.

Accordingly, there is a need for a transconductance amplifier that has an increased linearity, a reduced operating current, an improved noise characteristic and a broad tuning range of transconductance (Gm).

SUMMARY OF THE INVENTION

According to an exemplary embodiment of the present invention, a transconductance amplifier includes a first-type transconductor, a second-type transconductor and a common mode feedback circuit.

The first-type transconductor generates a first differential output signal pair in response to a differential input signal pair under the control of a first control signal. The second-type transconductor generates a second differential output signal pair in response to the differential input signal pair under the control of a second control signal. The common mode feedback circuit generates the second control signal in response to the first and second differential output signal pairs under the control of a common mode control signal.

According to an exemplary embodiment of the present invention, the first control signal is a signal received from an exterior of the transconductance amplifier.

According to an exemplary embodiment of the present invention, the first-type transconductor is coupled to a first supply voltage and the second-type transconductor is coupled to a second supply voltage that is complementary to the first supply voltage.

According to an exemplary embodiment of the present invention, a method of amplifying transconductance includes generating a first differential output signal pair in response to a differential input signal pair under the control of a first control signal, generating a second differential output signal pair in response to the differential input signal pair under the control of a second control signal, and generating the second control signal in response to the first and second differential output signal pairs under the control of a common mode control signal.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Exemplary embodiments of the present invention are disclosed herein.

FIG. 4is a circuit diagram illustrating a transconductance amplifier according to an exemplary embodiment of the present invention. Referring toFIG. 4, the transconductance amplifier includes an N-type transconductor430, a P-type transconductor410, and a common mode feedback circuit420.

The N-type transconductor430generates a first differential output signal pair IOP1and IOM1in response to a differential input signal pair VIP and VIM under the control of a control signal VCN. The P-type transconductor410generates a second differential output signal pair IOP2and IOM2in response to the differential input signal pair VIP and VIM under the control of a control signal VCP. The common mode feedback circuit420generates the control signal VCP in response to the first differential output signal pair IOP1and IOM1, and the second differential output signal pair IOP2and IOM2under the control of a common mode control signal VCM.

The N-type transconductor430includes NMOS transistors MN10and MN11and NPN transistors Q3and Q4. The NMOS transistor MN10has a gate to which the input signal VIP is applied and a source coupled to the ground. The NPN transistor Q3has a base to which the control signal VCN is applied, an emitter coupled to a drain of the NMOS transistor MN10and a collector coupled to a first output terminal TO1. The NPN transistor Q4has a base to which the control signal VCN is applied, an emitter coupled to a drain of the NMOS transistor MN11and a collector coupled to a second output terminal TO2.

The P-type transconductor410includes PMOS transistors MP3to MP6. The PMOS transistor MP3has a gate to which the input signal VIP is applied and a source coupled to a supply voltage VDD. The PMOS transistor MP4has a gate to which the input signal VIM is applied and a source coupled to the supply voltage VDD. The PMOS transistor MP5has a gate to which the control signal VCP is applied, a source coupled to the drain of the PMOS transistor MP3and a drain coupled to the first output terminal TO1. The PMOS transistor MP6has a gate to which the control signal VCP is applied, a source coupled to the drain of the PMOS transistor MP4and a drain coupled to the second output terminal TO2.

Hereinafter, referring toFIG. 4, the operation of the transconductance amplifier according to an exemplary embodiment of the present invention will be described.

The transconductance amplifier ofFIG. 4is a complementary transconductance amplifier including the N-type transconductor430and the P-type transconductor410. The N-type transconductor430includes the NMOS transistors MN10and MN11and the NPN transistors Q3and Q4. The P-type transconductor410includes the PMOS transistors MP3to MP6. The differential output signal IOP1and the differential output signal IOP2are summed to generate a first output current IOP. The differential output signal IOM1and the differential output signal IOM2are summed to generate a second output current IOM.

The common mode feedback circuit420generates the control signal VCP based on the first output current IOP and second output current IOM. When a common mode component of the two signals IOP and IOM is increased, the control signal VCP is increased and the differential output signals IOP2and IOM2, which are output from the P-type transconductor410, are decreased. When the common mode component of the two signals IOP and IOM is decreased, the control signal VCP is decreased and the differential output signals IOP2and IOM2are increased. The common mode feedback circuit420negatively feeds back the common mode component. As such, when a common mode component is negatively fed back, voltages of the output terminals TO1and TO2may be stabilized.

FIG. 5is a circuit diagram illustrating an example of the common mode feedback circuit420depicted inFIG. 4. Referring toFIG. 5, the common mode feedback circuit420includes a common mode component comparator422and a voltage level limiting circuit424. The common mode component comparator422compares the first output signal IOP and the second output signal IOM of the transconductance amplifier with the common mode control signal VCM and outputs the compared result. The voltage level limiting circuit424limits a variation range of an output signal of the common mode component comparator422to generate the control signal VCP.

Hereinafter, the operation of the common mode feedback circuit ofFIG. 5will be described.

The reference voltage VREF1that is applied to a gate of the NPN transistors Q5and Q6may be set to have an appropriate voltage level. The control voltage VCN, which is applied to the N-type transconductor430, may be used as the reference voltage VREF1. Common mode components of the differential output signals IOP and IOM are compared with the common mode control signal VCM. The differential mode components of the differential output signals IOP and IOM will not affect the voltage level of a node N51because NMOS transistors MN13and MN14are connected in parallel.

When the common mode component of the differential output signals IOP and IOM is increased, the current flowing through the NPN transistor Q6is increased. Accordingly, the current flowing through the PMOS transistor MP10and the current flowing through the PMOS transistor MP9, which is a current-mirror connected to the PMOS transistor MP10, increase. Because the common mode control signal VCM maintains a constant voltage level, the current flowing through the NPN transistor Q5may not vary. Therefore, the voltage of the node N51, which is the control signal VCP of the P-type transconductor410, increases. When the control signal VCP is increased, the differential output signal pair IOP2and IOM2of the P-type transconductor410is decreased.

When the common mode component of the differential output signal pair IOP2and IOM2is decreased, the current flowing through the NPN transistor Q6is decreased. Accordingly, the current flowing through the PMOS transistor MP10and the current flowing through the PMOS transistor MP9, which is a current-mirror connected to the PMOS transistor MP10, decrease. Because the common mode control signal VCM maintains a constant voltage level, the current flowing through the NPN transistor Q5may not vary. Therefore, the voltage of the node N51, which is the control signal VCP of the P-type transconductor410, decreases. When the control signal VCP is decreased, the differential output signal pair IOP2and IOM2of the P-type transconductor410is increased.

The transconductance amplifier ofFIG. 4according to an exemplary embodiment of the present invention may stabilize the voltage on the output terminals TO1and TO2using a common mode feedback circuit420. Further, the transconductance amplifier ofFIG. 4receives only one control signal VCN from an external source to control the N-type transconductor430, and the output signal of the common mode feedback circuit420is used as the control voltage VCP of the P-type transconductor430. Accordingly, in the transconductance amplifier according to an exemplary embodiment of the present invention shown inFIG. 4, the degree of symmetry may be maintained even though the transconductance (Gm) changes.

FIG. 6is a circuit diagram illustrating a transconductance amplifier according to an exemplary embodiment of the present invention. The transconductance amplifier ofFIG. 6is a complementary transconductance amplifier including an N-type transconductor440and a P-type transconductor410.

Referring toFIG. 6, the N-type transconductor440includes the NMOS transistors MN10, MN11, and NMOS transistors MN15and MN16. The input signal VIP is applied to the gate of the NMOS transistor MN10and the source of the NMOS transistor MN10is coupled to the ground GND. The input signal VIM is applied to the gate of the NMOS transistor MN11and the source of the NMOS transistor MN11is coupled to the ground GND. The NMOS transistor MN15has a gate to which the control signal VCN is applied, a source coupled to the drain of the NMOS transistor MN10and a drain coupled to the first output terminal TO1. The NMOS transistor MN16has a gate to which the control signal VCN is applied, a source coupled to the drain of the NMOS transistor MN11and a drain coupled to the second output terminal TO2.

The transconductance amplifier ofFIG. 6may be implemented in a semiconductor integrated circuit using a CMOS fabrication process because the N-type transconductor440, the P-type transconductor410and the common mode feedback circuit420are comprised of MOS transistors. The operation of the transconductance amplifier ofFIG. 6is substantially similar to the operation of the transconductance amplifier ofFIG. 4. Therefore, a detailed description referring to the particular operation of the transconductance amplifier ofFIG. 6is omitted.

FIG. 7is a circuit diagram illustrating a transconductance amplifier according to an exemplary embodiment of the present invention. The transconductance amplifier ofFIG. 7is a complementary transconductance amplifier including an N-type transconductor430and a P-type transconductor450.

Referring toFIG. 7, the P-type transconductor450includes the PMOS transistors MP3and MP4and PNP transistors Q7and Q8. The input signal VIP is applied to the gate of the PMOS transistor MP3and the source of the PMOS transistor MP3is coupled to the supply voltage VDD. The input signal VIM is applied to the gate of the PMOS transistor MP4and the source of the PMOS transistor MP4is coupled to the supply voltage VDD. The PNP transistor Q7has a base to which the control signal VCP is applied, an emitter coupled to the drain of the PMOS transistor MP3and a collector coupled to the first output terminal TO1. The PNP transistor Q8has a gate to which the control signal VCP is applied, an emitter coupled to the drain of the PMOS transistor MP4and a collector coupled to the second output terminal TO2.

In the transconductance amplifier ofFIG. 7, each of the drain voltages of the NMOS transistors MN10and MN11, to which the differential input signals VIP and VIM are applied, has a voltage level corresponding to the voltage level of the control signal VCN minus a base-emitter voltage (Vbe) of the NPN transistors Q3and Q4. Each of the drain voltages of the PMOS transistors MP3and MP4to which the differential input signals VIP and VIM are applied has a voltage level corresponding to the voltage level of the control signal VCP plus a base-emitter voltage (Vbe) of the PNP transistors Q7and Q8. Therefore, the drain voltages of the MOS transistors MN10, MN11, MP3and MP4, to which the differential input signals VIP and VIM are applied, may be stabilized.

FIG. 8Ais a graph illustrating simulation results of a Gm-C filter constructed using the conventional transconductance amplifier ofFIG. 2when a bias current is varied.FIG. 8Bis a graph illustrating simulation results of a Gm-C filter constructed using the transconductance amplifier ofFIG. 4when a bias current is varied. Referring toFIG. 8AandFIG. 8B, the linearity of the transconductance amplifier ofFIG. 4may be similar to the linearity of the conventional transconductance amplifier ofFIG. 2. However, the bias current I_bias of the transconductance amplifier ofFIG. 4is lower than the bias current I_bias of the conventional transconductance amplifier ofFIG. 2, in achieving substantially the same transconductance (Gm). Referring toFIG. 8AandFIG. 8B, the bias current I_bias needed to achieve a Gm of 30 uS (micro-Siemens) is about 67 uA for the conventional transconductance amplifier ofFIG. 2, and about 34 uA for the transconductance amplifier ofFIG. 4. The bias current I_bias of the transconductance amplifier according to an exemplary embodiment of the present invention may be reduced to about one half of the bias current I_bias of the conventional transconductance amplifier.

FIG. 9is a graph that illustrates an output current Iout versus an input voltage Vi for the transconductance amplifier ofFIG. 4. Referring toFIG. 9, the output current of the N-type transconductor is consistent with the output current of the P-type transconductor, and the complementary characteristic is maintained.

FIG. 10is a graph that illustrates an input noise voltage versus a frequency for the transconductance amplifier ofFIG. 4. Referring toFIG. 10, the noise voltage of the transconductance amplifier ofFIG. 4is smaller than the noise of the conventional transconductance amplifier ofFIG. 2. The simulation result represents that the noise voltage of the transconductance amplifier ofFIG. 4is approximately 65% of the noise of the conventional transconductance amplifier ofFIG. 2.

FIG. 11is a circuit diagram illustrating a transconductance amplifier according to an exemplary embodiment of the present invention. In the transconductance amplifierFIG. 11, the N-type transconductor430and the P-type transconductor460are not fully symmetric to each other. Referring toFIG. 11, the transconductance amplifier includes the N-type transconductor430, a P-type transconductor460, and the common mode feedback circuit420. The P-type transconductor460includes PMOS transistors MP7to MP10and resistors R1and R2. The PMOS transistor MP7has a gate to which the control signal VCP is applied and a source coupled to the supply voltage VDD. The PMOS transistor MP8has a gate to which the control signal VCP is applied and a source coupled to the supply voltage VDD. The resistor R1has a first terminal coupled to a drain of the PMOS transistor MP7. The resistor R2has a first terminal coupled to a drain of the PMOS transistor MP8. The PMOS transistor MP9has a gate to which the input signal VIP is applied, a source coupled to a second terminal of the resistor R and a drain coupled to the first output terminal TO1. The PMOS transistor MP10has a gate to which the input signal VIM is applied, a source coupled to a second terminal of the resistor R2and a drain coupled to the second output terminal TO2.

Hereinafter, the operation of the transconductance amplifier ofFIG. 11will be described.

In the circuit ofFIG. 11, the input transistor pair MN10and MN11in the N-type transconductor430is comprised of NMOS transistors operating in the triode region, and the input transistor pair MP9and MP10in the P-type transconductor460is comprised of source-degenerate PMOS transistors operating in a saturation region. The resistors R1and R2are used to stabilize operation of the PMOS transistors MP7and MP8in the triode region and improve the linearity of the P-type transconductor460. Further, the resistors R1and R2are used to control an amount of variation of transconductance Gm of the P-type transconductor460in response to the control signal VCN. The complementary transconductance amplifier ofFIG. 11may cancel a non-linearity generated between both transconductors430and460by using the P-type transconductor460and the N-type transconductor430that are asymmetric to each other.

FIG. 12is a graph that illustrates transconductance versus an input voltage for the transconductance amplifier ofFIG. 11. Referring toFIG. 11, the range of the input voltage Vi that is needed to keep the transconductance Gm substantially constant is narrower than the case ofFIG. 8Bthat represents the transconductance Gm of the transconductance amplifier ofFIG. 4. The range of the input voltage Vi is narrower because the input transistor pair MP9and MP10of the P-type transconductor460operate in the saturation region.

In the transconductance amplifier ofFIG. 11, the input transistor pair MN10and MN11of the N-type transconductor430operate in the triode region, and the input transistor pair MP9and MP10of the P-type transconductor460operates in the saturation region. As such, when the two transconductors that are complementary to each other operate in the different regions, a third order non-linearity generated in each of the transconductors may be cancelled. This principle is described in Morozov et al., “A realization of low-distortion CMOS transconductor amplifier,” IEEE Trans. Circuit and Systems, September 2001, Vol. 48, Issue 9, pp. 1138-1141. In Morozov et al., an example is disclosed in which non-linearity components offset each other in the two N-type transconductors connected in parallel, the two transconductors each operating in the different regions of operation.

FIG. 13is a graph that illustrates a third order differentiation coefficient versus an input voltage for the N-type transconductor430and the P-type transconductor460of the transconductance amplifier ofFIG. 11. InFIG. 13, the output current Iout when the input voltage Vi is 0V represents the third order non-linearity coefficient of the output current Iout. Referring toFIG. 13, the third order non-linearity coefficient is about 2×10−6for the N-type transconductor430and about −2×10−6for the P-type transconductor460. That is, the third order non-linearity coefficient of the N-type transconductor has substantially the same absolute value as the third order non-linearity coefficient of the P-type transconductor, but the signs are opposite.

Accordingly, the third order non-linearity coefficient of the transconductance amplifier including the N-type transconductor430and the P-type transconductor460is about “0” (zero). Further, the transconductance amplifier ofFIG. 11may have a low third order inter-modulation product (IM3) when a 2-tone input voltage of an acceptable level is applied.

FIG. 14is a graph that illustrates 2-tone third order differentiation characteristics for the conventional transconductance amplifier ofFIG. 2and the transconductance amplifier ofFIG. 11. Referring toFIG. 14, the IM3of the complementary transconductance amplifier ofFIG. 11according to an exemplary embodiment of the present invention has a lower value than the IM3of the conventional transconductance amplifier ofFIG. 2.

Referring toFIG. 15, the transconductance amplifier includes an N-type transconductor530, a P-type transconductor510and a common mode feedback circuit520. The N-type transconductor530has an input terminal pair Vi+ and Vi−, an output terminal pair IO+ and IO− and a control terminal CONT. The input signals VIP and VIM are received through the input terminal pair Vi+ and Vi−, and the output signals IO+ and IO− are outputted through the output terminal pair IO+ and IO−. The control signal VCN is received through the control terminal CONT. The P-type transconductor510has an input terminal pair Vi+ and Vi−, an output terminal pair IO+ and IO− and a control terminal CONT. The input signals VIP and VIM are received through the input terminal pair Vi+ and Vi−, and the output signals IO+ and IO− are outputted through the output terminal pair IO+ and IO−. The control signal VCP is received through the control terminal CONT. The common mode feedback circuit520generates the control signal VCP in response to the differential output signal pair IOP and IOM under the control of the common mode control signal VCM. The transconductance amplifier according to an exemplary embodiment of the present invention, which is shown inFIG. 15, is a complementary transconductance amplifier having the N-type transconductor530and the P-type transconductor510between the supply voltage VDD and the ground voltage GND.

FIG. 16is a block diagram illustrating a transconductance amplifier when an external control signal is applied to a P-type transconductor according an exemplary embodiment of the present invention.

In the transconductance amplifier ofFIG. 16, the control signal VCP is applied to the P-type transconductor510, and the output signal of the common mode feedback circuit520is applied to the control terminal CONT of the N-type transconductor530as a control signal VCN. The remaining portions ofFIG. 16are substantially similar toFIG. 15, and the operation of the circuit ofFIG. 16is substantially similar toFIG. 15. Therefore, the particular operation of the circuit ofFIG. 16is omitted.

As described above, the transconductance amplifier according to an exemplary embodiment of the present invention has a complementary structure, improved linearity and lower power consumption. Further, an adjustment range of transconductance of the transconductance amplifier according to an exemplary embodiment of the present invention may be broadened compared with the conventional transconductance amplifier.

While exemplary embodiments of the present invention have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the scope of the invention.