WO2007008042A1

WO2007008042A1 - Cascode low-noise amplifier

Info

Publication number: WO2007008042A1
Application number: PCT/KR2006/002765
Authority: WO
Inventors: Sungmin Ock; Joonsuk Lee
Original assignee: Fci Inc
Priority date: 2005-07-14
Filing date: 2006-07-14
Publication date: 2007-01-18

Abstract

A cascode low-noise amplifier for enhancing linearity is provided. The cascode low-noise amplifier has a two-stage structure in which a common-emitter stage cascaded to a common-base stage, wherein a phase difference of 180 is produced between a third-order intermodulation signal resulting from a fundamental signal input to the common-base stage and a third-order intermodulation signal previously generated at the common-emitter stage. Accordingly, the fundamental signal and the third-order intermodulation signal can be adjusted to have a phase difference of 180 by disposing an impedance matching circuit between a common-emitter (or a common-source) stage of a buffer transistor and a common-base (or a common-gate) stage of an amplifier transistor, and the magnitude difference can be adjusted as well. In addition, the entire linearity of the cascode low-noise amplifier is enhanced by adjusting the impedance value of the impedance matching circuit to an appropriate value so as to cancel the impedance with the third- order intermodulation signal in the subsequent stage.

Description

CASCODE LOW-NOISE AMPLIFIER

Technical Field

[1] The present invention relates to a low-noise amplifier (LNA), and more particularly, to a cascode low-noise amplifier. Background Art

[2] A low-noise amplifier is generally used in a receiver stage for applications such as wireless communications and serves to detect and amplify signals from the outside and to output the amplified signals to a subsequent stage.

[3] When signals are detected by an antenna, the signals typically contain a variety of undesired signals as well as desired signals, which are very weak. Accordingly, when the received signals are amplified, the least increment of noise and the maximum assurance of linearity should be accomplished together. To this end, a variety of design schemes of semiconductor devices and circuits are applied to the low-noise amplifier. A cascode low-noise amplifier (LNA) is one example thereof.

[4] FlG. 1 is a circuit diagram illustrating an example of a conventional cascode low- noise amplifier.

[5] In the conventional cascode low-noise circuit, a drain at a common-source stage of a MOSFET Ql as a buffer transistor and a source at a common-gate stage of a MOSFET Q2 as an amplifier transistor are connected to each other. Gates of MOSFET Ql and MOSFET Q2 are connected to bypass capacitors Cl and C2, respectively.

[6] The cascode low-noise amplifier has an advantage that it exhibits a sufficient gain and a proper noise figure (NF) with low power consumption. On the other hand, the cascode low-noise amplifier has a disadvantage that it has a poor linearity.

[7] Several solutions have been suggested to overcome the above-mentioned problems.

Typically, the solutions were applied to a cascode low-noise amplifier with a MOSFET configuration, but not to a cascode low-noise amplifier with a BJT configuration. Disclosure of Invention Technical Problem

[8] In order to solve the aforementioned problems, an object of the present invention is to provide a cascode low-noise amplifier (LNA) having enhanced linearity and applicability to a bipolar junction transistor configuration.

Technical Solution

[9] According to an aspect of the present invention, there is provided a cascode amplifier having a two-stage structure in which a common-emitter stage is cascaded to a common-base stage, wherein a phase difference of 180 is produced between a third- order intermodulation signal resulting from a fundamental signal input to the common- base stage and a third-order intermodulation signal previously generated at the common-emitter stage.

[10] According to another aspect of the present invention, there is provided a cascode low-noise amplifier using a BJT (bipolar junction transistor) to enhance linearity, the amplifier comprising: an impedance matching circuit disposed between a common-em itter stage of a buffer transistor Q21 and a common-base stage of an amplifier transistor Q22 so that specific resistance value is realized in the impedance between the common-emitter stage and the common-base stage, thereby producing a phase difference of 180 between a fundamental signal and a third-order intermodulation signal.

[11] According to another aspect of the present invention, there is provided a cascode low-noise amplifier using a MOSFET (metal-oxide semiconductor field effect transistor) to enhance linearity, the amplifier comprising: an impedance matching circuit disposed between a common-source stage of a buffer transistor Q31 and a common-gate stage of an amplifier transistor Q32 so that specific resistance value is realized in the impedance between the common-source stage and the common-gate stage, thereby producing a phase difference of 180 between a fundamental signal and a third-order intermodulation signal. Brief Description of the Drawings

[12] Additional characteristics, advantages, structures, and operations according to the present invention will be described in detail with reference to the accompanying drawings in which:

[13] FIG. 1 is a circuit diagram illustrating an example of a conventional cascode low- noise amplifier;

[14] FIG. 2 is a schematic diagram illustrating a linearization method of a cascode low- noise amplifier according to an embodiment of the present invention;

[15] FIG. 3 is a simplified equivalent circuit for Volterra series analysis, which illustrates a typical common-emitter circuit;

[16] FIG. 4 is a graph illustrating a magnitude ratio and a phase difference between a fundamental signal and a third-order intermodulation signal according to a variation in collector resistance;

[17] FIG. 5 is a graph illustrating a phase difference according to variations in values of a capacitor and an inductor;

[18] FIG. 6 is a circuit diagram illustrating an example of a cascode low-noise amplifier with a bipolar junction transistor configuration according to an embodiment of the present invention; and [19] FlG. 7 is a circuit diagram illustrating an example of a cascode low-noise amplifier with a MOSFET configuration according to another embodiment of the present invention. Best Mode for Carrying Out the Invention

[20] Hereinafter, the present invention will be described with reference to the accompanying drawings.

[21] FlG. 2 is a schematic diagram illustrating a linearization method of a cascode low- noise amplifier according to an embodiment of the present invention.

[22] In a typical linearization method in a cascode amplifier, the entire linearity is enhanced by maximizing the linearity of a common-emitter (source) stage 210 and a common-base (gate) stage 220, in which the linearity of the common-emitter stage 210 is more important than that of the common-base stage 220.

[23] Since the cascode amplifier has a two-stage structure having the common-emitter stage 210 cascaded to the common-base stage 220, the entire linearity of the cascode amplifier can be also enhanced by canceling non-linear components generated at the stages 210 and 220 as well as by enhancing the linearity of the stages 210 and 220.

[24] Assuming that a phase difference of 180°is produced between a fundamental signal and a third-order intermodulation component generated at the common-emitter stage 210 and no phase difference is produced between a fundamental signal and an intermodulation signal output from the common-base stage 220, the phase difference of 180°is produced between a third-order intermodulation signal resulting from a fundamental signal input to the common-base stage 220 and the third-order intermodulation signal previously generated at the common-emitter stage 210. Here, the third-order intermodulation signal will be simply amplified in the common-base stage 220 like the fundamental signal.

[25] Therefore, it is possible to cancel the two third-order intermodulation signals having a phase difference of 180° and moreover, the amount of cancellation will become even greater by adjusting the magnitudes of the two third-order intermodulation signals more appropriately.

[26] As a matter of fact, the differences in the magnitude and phase between a fundamental signal and a third-order intermodulation signal in the common-emitter stage 210 of the cascode amplifier can be adjusted in various manners.

[27] The non-linearity of the common-emitter amplifier is affected collectively by characteristics of devices and impedances connected to a base, a collector and an emitter. However, in the conventional low-noise amplifier, circuits connected to the base and the emitter and impedances of the connected circuits have a critical influence on noise characteristics. [28] In order to adjust the phase difference without deteriorating the noise characteristics, it is necessary to change the circuits connected to the collector so as to realize appropriate impedance.

[29] Volterra series analysis based on a simplified equivalent circuit model is suitable for verifying the influence of the collector impedance on non-linear signals.

[30] FlG. 3 is a simplified equivalent circuit for Volterra series analysis, which illustrates a typical common-emitter circuit.

[31] In FlG. 3, Ql represents a bipolar junction transistor and Zc, Ze, and Zb represent a collector impedance component connected to a collector, an emitter impedance component connected to an emitter, and a base impedance component connected to a base, respectively. Moreover, vl, v2, and v3 represent major nodes for the Volterra series analysis, and the v3 node serves as an output node in the common-emitter circuit.

[32] The Volterra series analysis is widely used in analyzing non-linearity of a weakly non-linear circuit and is able to present the magnitudes and phases of the non-linear components.

[33] As the major non-linear components in the conventional bipolar junction transistor, there are collector current ic, base current ib, and base-emitter diffusion current ic . These components can be defined by Equation 1 as a function of a base-emitter voltage v be as follows.

[34] [Equation 1]

[35]

ⁱC=gm^vbe^~hgm2 ^v be⁺gm3 ^v be [36]

b β

[37]

¹ c ^{~ τ ~} drt ^l c

[38] In Equation 1, gm, gm2, and gm3 represent coefficients used for expressing the collector current ic as polynomials of the base-emitter voltage v DC , respectively, β represents a low-frequency current gain, and represents a forward transit time. [39] A first-order Volterra transfer function H (s) obtained for an input signal Vin=V

1 s cos( t)+V cos( t) using the Volterra series analysis can be expressed by Equation 2 as follows.

[40] [Equation 2]

[41]

[42]

[43]

[44]

[45]

Ad= Z_b Z_e+ Z_e Z_c+ Z_c Z_b

[46] In Equation 2, C represents base-emitter junction capacitance and Crepresents base-collector capacitance. [47] Similarly, a third-order Volterra transfer function H (s) obtained by using the

Volterra series analysis can be expressed by Equation 3 as follows. [48] [Equation 3]

[49]

G ₃ O) K(S) ²Ki- s)E(2sΛs)

[50] Here, [51]

[52] l + C_Λs Z_c L(s) [53]

[54]

[55]

S -J ® 1 ~J ® 2 ^■> As -J( Gi ₁ - W ₂)

[56] By using the Volterra transfer functions H (s) and H (s) defined in Equations 1 and 2, the differences of magnitude and phase between the fundamental signal and the third-order intermodulation signal output from the v3 node can be expressed as Equations 4 and 5 as follows.

[57] [Equation 4] [58]

[59] [Equation 5] [60]

phase(

[61] As a result, the magnitude ratio and the phase difference between the fundamental signal and the third-order intermodulation signal can be verified in Equations 4 and 5 by changing the values of specific variables.

[62] To examine the influence of collector impedance by using the Volterra transfer functions H (s) and H (s), the magnitude ratio and the phase difference between a fundamental signal and a third-order intermodulation signal were measured when the resistance value of the collector impedance Zc is varied.

[63] FIG. 4 is a graph illustrating a magnitude ratio and a phase difference between a fundamental signal and a third-order intermodulation signal according to a variation in collector resistance.

[64] It is recognized from FIG. 4 that there are noticeable variations in the magnitude ratio and the phase difference between the two signals according to a variation in collector resistance. Therefore, it is expected that a desired magnitude ratio and a desired phase difference can be realized by precisely adjusting the resistance value of the collector impedance Zc.

[65] The collector circuit for realizing a phase difference of 180°can be embodied in various manners. According to a feasible example for embodying the collector circuit, various resistance values of the collector impedance Zc are realized by connecting a resistor, a capacitor, and an inductor in parallel.

[66] FlG. 5 is a graph illustrating a phase difference according to variations in values of a capacitor and an inductor. The phase difference of 180°is realized by using a specific combination of capacitance and inductance.

[67] By using the above-mentioned concepts, it is possible to realize a cascode low- noise amplifier as illustrated in FlG. 6. In the cascode low-noise amplifier, it is possible to produce a phase difference of 180 between the fundamental signal and the third-order intermodulation signal in a collector node of Ql. Moreover, it is possible to realize a low-noise amplifier having an excellent linearity by taking advantage of the phase difference of 180°

[68] FlG. 6 is a circuit diagram illustrating an example of a cascode low-noise amplifier with a bipolar junction transistor configuration according to an embodiment of the present invention.

[69] The cascode low-noise amplifier shown in FlG. 6 includes an impedance matching circuit 30 which is disposed between a common-emitter stage 210 of a buffer transistor Q21 and a common-base stage 220 of an amplifier transistor Q22 so that a specific resistance value is realized in the impedance between the common-emitter stage 210 and the common-base stage 220, thereby producing a phase difference of 180°between the fundamental signal and the third-order intermodulation signal IM3.

[70] Since in a high frequency range, for example, in an RF range, the capacitance value of a capacitor between a base and a collector of a transistor is not negligible, the impedance matching circuit 30 includes a resistor R21, a capacitor C21, and an inductor L21 connected in parallel between the collector node of the buffer transistor Q21 and the emitter node of the amplifier transistor Q22. Therefore, it is possible to adjust the fundamental signal and the third-order intermodulation signal IM3 to have a phase difference of 180°thanks to the influence of the impedance matching circuit 30 disposed between the common-emitter stage 210 and the common-base stage 220, thereby adjusting the magnitude difference as well. As a result, the entire linearity of the cascode low-noise amplifier is enhanced by adjusting the impedance value of the impedance matching circuit 30 to an appropriate value so as to cancel the impedance with the third-order intermodulation signal (IM3) in the subsequent stage (common-base stage 220).

[71] FlG. 7 is a circuit diagram illustrating an example of a cascode low-noise amplifier with a MOSFET configuration according to another embodiment of the present invention.

[72] The cascode low-noise amplifier shown in FlG. 7 includes an impedance matching circuit 30 which is disposed between a common-source stage 210 of a buffer transistor Q31 and a common-gate stage 220 of an amplifier transistor Q32 so that a specific resistance value is realized in the impedance between the common-source stage 210 and the common-gate stage 220, thereby producing a phase difference of 180°between the fundamental signal and the third-order intermodulation signal IM3.

[73] Since in a high frequency range, for example, in an RF range, the capacitance value of a capacitor between a gate and a drain of a transistor is not negligible, the impedance matching circuit 30 includes a resistor R31, a capacitor C31, and an inductor L31 connected in parallel between the drain node of the buffer transistor Q31 and the source node of the amplifier transistor Q32. Therefore, it is possible to adjust the fundamental signal and the third-order intermodulation signal IM3 to have a phase difference of 180°thanks to the influence of the impedance matching circuit 30 disposed between the common-source stage and the common-gate stage, thereby adjusting the magnitude difference as well. As a result, the entire linearity of the cascode low-noise amplifier is enhanced by adjusting the impedance value of the impedance matching circuit 30 to an appropriate value so as to cancel the impedance with the third-order intermodulation signal (IM3) in the subsequent stage (common-gate stage 220).

[74] Since the subsequent common-gate stage 220 has a relatively good linearity, the phase difference produced between the fundamental signal and the third-order intermodulation signal (TM3) is negligible.

[75] While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. Industrial Applicability

[76] According to the present invention, it is possible to adjust a fundamental signal and a third-order intermodulation signal IM3 to have a phase difference of 180°by disposing an impedance matching circuit between a common-emitter (or a common- source) stage of a buffer transistor and a common-base (or a common-gate) stage of an amplifier transistor, and thus adjust the magnitude difference as well.

[77] As a result, it is possible to enhance the entire linearity of the cascode low-noise amplifier by adjusting the impedance value of the impedance matching circuit to an appropriate value so that the impedance is cancelled by a third-order intermodulation signal (IM3) in the subsequent stage (common-base stage or common-gate stage).

Claims

[2] A cascode low-noise amplifier using a BJT (bipolar junction transistor) to enhance linearity, the amplifier comprising: an impedance matching circuit disposed between a common-emitter stage of a buffer transistor and a common-base stage of an amplifier transistor so that a phase difference of 180 is produced between a fundamental signal and a third- order intermodulation signal.

[3] A cascode low-noise amplifier using a MOSFET (metal-oxide semiconductor field effect transistor) to enhance linearity, the amplifier comprising: an impedance matching circuit disposed between a common-source stage of a buffer transistor and a common-gate stage of an amplifier transistor so that a phase difference of 180 is produced between a fundamental signal and a third- order intermodulation signal.

[4] The cascode low-noise amplifier according to claim 2 or 3, wherein the impedance matching circuit has a structure in which a resistor, a capacitor, and an inductor are connected in parallel between a collector (or a drain) node of the buffer transistor and an emitter (or a source) node of the amplifier transistor.

[5] The cascode low-noise amplifier according to claim 4, wherein the impedance matching circuit adjusts the value of at least one of the resistor, the capacitor, and the inductor, so that a desired magnitude ratio and a desired phase difference are realized between the fundamental signal and the third-order intermodulation signal.