Tunable power amplifier with wide frequency range

Abstract

A circuit includes an amplifier to amplify an input signal and generate an output signal. The circuit also includes a tuning network to tune frequency response of the amplifier. The tuning network includes at least one tunable capacitor, which includes at least one micro-electro mechanical system (MEMS) capacitor. The amplifier could include a first die, the at least one MEMS capacitor could include a second die, and the first die and the second die could be integrated in a single package. The at least one MEMS capacitor could include a MEMS superstructure over a control structure, which is to control the MEMS superstructure and tune the capacitance of the at least one MEMS capacitor.

Description

BACKGROUND

This relates generally to radio frequency (RF) power amplifiers, and more particularly to a tunable power amplifier with a wide frequency range.

A radio frequency (RF) power amplifier and its matching network are typically designed for operation at a fixed carrier frequency. At other carrier frequencies, the power amplifier often loses performance, including efficiency, output power, and linearity. For example, if a power amplifier loses linearity, it cannot satisfy its spectral mask requirements, and pre-distortion is often needed, which in turn is more complicated, consumes more power, and occupies more die area.

For modern wireless communication systems where the frequency range of interest is very wide, multiple power amplifiers may be needed to cover different frequency bands within the frequency range of interest. Unfortunately, using multiple power amplifiers can incur significant increases in cost, complexity, power consumption and die area. Having a single tunable power amplifier that can cover multiple frequency bands is a more attractive solution for these applications.

SUMMARY

In one example, a circuit includes an amplifier to amplify an input signal and generate an output signal. The circuit also includes a tuning network to tune the frequency response of the amplifier. The tuning network includes at least one tunable capacitor, which includes at least one micro-electro mechanical system (MEMS) capacitor.

In another example, a method includes amplifying an input signal and generating an output signal using an amplifier. The method also includes tuning frequency response of the amplifier using a tuning network coupled to the amplifier. The tuning network includes at least one tunable capacitor, and the at least one tunable capacitor includes a micro-electro mechanical system (MEMS) capacitor.

In a further example, a system includes an amplifier having multiple power amplifiers. The system also includes a tuning network having: (a) multiple inductors each coupled to one of the power amplifiers; and (b) multiple tunable capacitors coupled to the inductors. Each tunable capacitor includes at least one micro-electro mechanical system (MEMS) capacitor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example digital light processing (DLP) micro-electro mechanical system (MEMS) tunable capacitor.

FIGS. 2A and 2B illustrate an example tunable power amplifier in accordance with this description.

FIGS. 3 through 10 illustrate example operational and other details regarding high-power and low-power implementations of the tunable power amplifier in accordance with this description.

FIG. 11 illustrates an example of a tunable power amplifier in a package in accordance with this description.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

Example embodiments include a tunable power amplifier that covers a wide frequency range.

FIGS. 1 through 11 described below, and various examples that describe principles of example embodiments, are not limiting.

As described above, a radio frequency (RF) power amplifier and its matching network are typically designed for operation at a fixed carrier frequency. Operations at other carrier frequencies or over a frequency range of interest can be problematic for conventional RF power amplifiers. In accordance with this description, a power amplifier is provided on or over a die (such as a complementary metal-oxide-semiconductor or “CMOS” die) and integrated with tunable capacitors (such as micro-electro mechanical system or “MEMS” capacitors) and inductors in the same package. The MEMS capacitors are used to achieve higher amplifier performance and may be based on the Digital Light Processing (DLP) technology from Texas Instruments Incorporated of Dallas, Tex.

FIG. 1 illustrates an example DLP MEMS tunable capacitor 10. As shown in FIG. 1, the tunable capacitor 10 includes a superstructure having at least one deflectable metallic membrane 12. Hinges 13 hinge the membrane 12 over an underlying control section, which includes at least one electrode 14 formed over a substrate 16. A space D between the membrane 12 and the electrode 14 forms a dielectric. A control voltage V+ provided to the electrode 14 by a controller 18 creates an electrostatic force on the membrane 12 that controllably varies the distance D between the membrane 12 and the electrode 14 to establish a variable capacitance. Each tunable capacitor 10 has two states, a high capacitance state (minimum distance D between membrane 12 and electrode 14) and a low capacitance state (maximum distance D between membrane 12 and electrode 14). Thus, the capacitors 10 are bi-stable. Multiple capacitors 10 combined together, such as in an array, provide a wide tunable capacitance range that is obtained by controlling the states of multiple capacitors 10. A voltage may also or alternatively be provided to the membrane 12 to create the electrostatic force. The membrane 12 can be substantially planar and substantially parallel to the electrode 14 and move vertically to vary the distance D and thus the capacitance.

As described in more detail below, one or multiple MEMS capacitors 10 are used in a power amplifier's tuning network. This is beneficial since MEMS capacitors provide high RF power handling capability, wide tuning ranges, and good RF performances such as high quality factor, small insertion loss and high linearity compared to varactor diodes or ordinary switched capacitor banks. MEMS capacitors can also be used in smaller packages. As particular examples, a tunable MEMS capacitor (array of MEMS capacitors 10) may have a tolerance of 0.1 pF, a Q>100 at 1 GHz, and a Q>50 at 2 GHz, and the linearity may be IIP3>75 dBm. These values can help to achieve superior performance of a power amplifier at these frequencies. For example, if the variable capacitor has a high insertion loss, then the power amplifier loses output power and efficiency degrades.

In some implementations, the controller 18 provides digital levels of the control voltage V+ to individual MEMS capacitor 10 of a tunable capacitor array so that the capacitance can be set in steps. For example, in some implementations, the capacitance of a tunable MEMS capacitor array can be programmed and digitally tuned in 0.17 pF steps and have a range of 1 pF to 6.5 pF. However, other values and ranges can be provided as desired.

FIG. 2A illustrates an example tunable power amplifier 20 in accordance with this description. In this particular example, the tunable power amplifier 20 represents a tunable outphasing or LINC (Linear Amplification with Nonlinear Components) power amplifier, which could be formed at least partially on a CMOS die 22 or other semiconductor die. The tunable power amplifier 20 could have any suitable operating frequency range. For instance, the tunable power amplifier 20 could represent a tunable outphasing power amplifier with an operating frequency range from 1.5 GHz to 2.7 GHz.

As shown in FIG. 2A, the tunable power amplifier 20 includes amplifier sections comprising four power amplifiers P₁-P₄, four inductors L₁-L₄, three tunable capacitors C₁-C₂ and C_out, and a load resistor R. The tunable capacitors C₁-C₂ and C_out, along with inductors L₁-L₄, form a tuning network 24 for the power amplifier. A first differential outphasing input signal S₁/S₁′ is coupled to the power amplifiers P₁ and P₃, and a second differential outphasing input signal S₂/S₂′ is coupled to power amplifiers P₂ and P₄.

Each of the power amplifiers P₁-P₄ represents any suitable type(s) of switching power amplifier(s), such as a class-D or class-E amplifier. Each of the inductors L₁-L₄ represents any suitable inductive structure having any suitable inductance. In some implementations, the inductors L₁-L₄ have substantially equal inductances. Each of the tunable capacitors C₁-C₂ and C_out represents any suitable tunable capacitor(s), such as multiple MEMS capacitors 10 configured in parallel or other arrangement. In some implementations, the capacitors C₁-C₂ are tuned to have substantially equal capacitances. Each MEMS capacitor array forming the tunable capacitors C₁-C₂ and C_out could have its own controller 18, or multiple MEMS capacitor arrays forming the tunable capacitors C₁-C₂ and C_out could share a common controller 18. The load resistor R represents any suitable resistive structure having any suitable resistance.

FIG. 2B shows an example schematic of a frequency-tunable wideband class-E outphasing power amplifier 20. Simulation results obtained from this design are shown in FIGS. 3-10. Each class-E power amplifier P₁-P₄ consists of a core switching device (NMOS transistor N₂) that is driven by the outphasing waveform through a driver chain 26, a high voltage cascode NMOS transistor (N₁) biased properly, which can sustain higher voltage stress and is used here to obtain higher peak output power from the outphasing power amplifier 20, a capacitor C_P (in this design no external capacitor is used, the output capacitance of the power amplifier P₁-P₄ is used as C_P), and an inductor L_C which connects the power amplifiers P₁-P₄ to the power supply V_DD. Inductors L₁-L₄ and capacitors C₁ and C₂ are tuned for class-E operation. Capacitor C_out is the output capacitor connected to load resistance R through a balun T. Capacitors C₁-C₂ and C_out are built using tunable MEMS capacitors. The capacitors C₁-C₂ and C_out and inductors L₁-L₄ form the outphasing combiner as well as the matching network.

In addition, LC circuits tuned to specific harmonic frequencies can be connected between the drains of the cascode transistors N₁ of differential sides for waveform shaping, and those capacitors are also built using MEMS and can be tuned over wide frequency range of interest. Inductors or capacitors can be connected between the drains of the cascode transistors N₁ of S₁ and S₂ sides for improving back-off efficiency.

FIGS. 3 through 10 illustrate example operational and other details regarding high-power and low-power implementations of the tunable power amplifier 20 in accordance with this description. More specifically, FIGS. 3 through 6 illustrate example operational and other details regarding the high-power implementation of the tunable power amplifier 20, and FIGS. 7 through 10 illustrate example operational and other details regarding the low-power implementation of the tunable power amplifier 20. The tunable capacitors C₁-C₂ and C_out have a higher capacitance range (tuning range) in the high-power implementation of the power amplifier 20 than in the low-power implementation of the power amplifier 20. Both implementations of the power amplifier 20 cover the frequency range from about 1.5 GHz to about 2.7 GHz in the examples shown, although other frequency ranges could be used.

Parameters and results of the high-power implementation of the power amplifier 20 are shown in FIGS. 3 through 6. FIG. 3 shows the values of the tunable MEMS capacitors C₁-C₂ as a function of the tuned frequency of the power amplifier 20. The value of each MEMS capacitor C₁-C₂ is programmable between 35 pF and 70 pF. FIG. 4 shows the values of the MEMS capacitor C_out as a function of the tuned frequency of the power amplifier 20, where the value of the MEMS capacitor C_out is programmable between 3.7 pF and 9 pF.

FIG. 5 shows the efficiency of the power amplifier 20 as a function of frequency, where the efficiency is relatively high and consistent over the frequency range. FIG. 6 shows the output power of the power amplifier 20 as a function of frequency, which again is relatively high and consistent over the frequency range.

Parameters and results of the low-power implementation of the power amplifier 20 are shown in FIGS. 7 through 10. FIG. 7 shows the values of the tunable MEMS capacitors C₁-C₂ as a function of the tuned frequency of the power amplifier 20. The value of each MEMS capacitor C₁-C₂ is programmable between 3 pF and 9 pF. FIG. 8 shows the values of the MEMS capacitor C_out as a function of the tuned frequency of the power amplifier 20, where the value of the MEMS capacitor C_out is programmable between 3.2 pF to 8.5 pF.

FIG. 9 shows the efficiency of the power amplifier 20 as a function of frequency, where the efficiency is relatively consistent over the frequency range. FIG. 10 shows the output power of the power amplifier 20 as a function of frequency, which is relatively consistent over the frequency range.

FIG. 11 illustrates an example of a tunable power amplifier in a package 30 in accordance with this description. The package 30 includes a substrate 32 with the CMOS die 22 positioned on an upper surface 34 of the substrate 32. The CMOS die 22 includes the power amplifiers P₁-P₄ shown in FIG. 2. The package 30 also includes the inductors L₁-L₄ positioned proximate the CMOS die 22. The inductors L₁-L₄ are designed as spiral inductors using the metal layers of the package 30. One inductor L₁-L₄ is provided for each power amplifier P₁-P₄, respectively, as shown in FIG. 2.

Multiple MEMS modules 36 form each capacitor C₁-C₂, and one MEMS module 36 forms the capacitor C_out. Each MEMS module 36 could include three tunable MEMS capacitor banks coupled in parallel. The MEMS modules 36 forming each capacitor C₁-C₂ are coupled in parallel so that all nine MEMS capacitor banks are coupled in parallel. However, the number of MEMS modules 36 and the number of MEMS capacitor banks in each MEMS module 36 are for illustration only. Each MEMS module 36 can include a semiconductor die that is separate from the die 22.

In the illustrated example, the MEMS modules 36 are positioned on the upper surface 34 of the substrate 32 and are radially positioned around the CMOS die 22. A controller 18 may be provided for each MEMS module 36 to control the capacitance of that MEMS module 36, or a controller 18 could be provided in the package 30 to control the capacitance of multiple MEMS modules 36. In any case, each of the MEMS modules 36 can be digitally programmed by remote control signals provided to the package 30.

The capacitor C_out is positioned at a central portion of the package 30 between the two capacitors C₁-C₂, which are positioned at opposing ends of the package 30. The interconnects between the CMOS die 22, the inductors L₁-L₄, and the MEMS modules 36 can be formed by strip lines as the package 30 may operate at frequencies over 1 GHz. The CMOS die 22 is positioned in a central portion of the substrate 32 to help reduce or minimize distance, and thus associated parasitics, introduced by the interconnects between the CMOS die 22 and the tuning network formed by the inductors L₁-L₄ and the tunable capacitors C₁-C₂ and C_out. Input terminals of the power amplifier 20 can be formed proximate the CMOS die 22 to reduce or minimize parasitics. The external control signals that program the capacitors C₁-C₂ and C_out may be provided at any location on the package 30.

In this example, three MEMS modules 36 form each MEMS capacitor C₁-C₂, and one MEMS module 36 forms the MEMS capacitor C_out. Each MEMS capacitor C₁-C₂ is programmable, such as between 9 pF and 58.5 pF, and the MEMS capacitor C_out is programmable, such as between 3 pF and 19.5 pF. However, other values could also be supported. Also, in other embodiments, more or fewer MEMS modules 36 may be used to form the programmable MEMS capacitors C₁-C₂ and C_out of the power amplifier 20, where more MEMS modules 36 are used to create MEMS capacitors having a greater capacitance tuning range.

Although FIGS. 1 through 11 illustrate examples of tunable power amplifiers that are operable over a wide range of frequencies, various changes may be made to FIGS. 1 through 11. For example, the tunable power amplifier 20 could include any other suitable arrangement of power amplifiers, inductors, capacitors, and resistors. The power amplifiers could comprise of other types, such as pulse width modulated (PWM) power amplifier, linear power amplifiers etc. and suitable tunable matching networks can be designed accordingly. Also, each tunable capacitor C₁-C₂ and C_out could be implemented using any number of MEMS capacitors, including a single MEMS capacitor or multiple MEMS capacitors placed in any suitable arrangement. In addition, the values shown in the graphs in FIGS. 3-10 are examples only. Other embodiments of the tunable power amplifier 20 could have any other suitable operational characteristics.

In this description: (a) the terms “include” and “comprise,” and derivatives thereof, mean inclusion without limitation; (b) the term “or” is inclusive, meaning and/or; (c) the phrase “associated with,” and derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, have a relationship to or with, or the like; and (d) the phrase “at least one of,” when used with a list of items, means that different combinations of one or more of the listed items may be used, and only one item in the list may be needed. For example, “at least one of: A, B and C” includes any of the following: only A; only B; only C; a combination of A and B; a combination of A and C; a combination of B and C; and a combination of A and B and C.

Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims.

Claims

1. A circuit comprising: an amplifier having an amplifier input and an amplifier output, the amplifier having a frequency response; anda tuning network coupled to the amplifier, the tuning network configured to tune the frequency response, the tuning network including a tunable microelectromechanical system (MEMS) capacitor having a capacitance, the tunable MEMS capacitor including a MEMS superstructure over a control structure, the control structure configured to control the MEMS superstructure and tune the capacitance, the control structure including an electrode, and the MEMS superstructure including a membrane of which at least a portion is parallel to the electrode.

2. The circuit of claim 1, wherein: the amplifier includes a first die;the tunable MEMS capacitor includes a second die; andthe first die and the second die are integrated in a single package.

3. The circuit of claim 1, wherein the control structure is configured to electrostatically control a position of the MEMS superstructure.

4. The circuit of claim 1, wherein the tuning network includes multiple tunable MEMS capacitors.

5. A method comprising: amplifying an input signal and generating an output signal using an amplifier, the amplifier having a frequency response; andtuning the frequency response using a tuning network coupled to the amplifier, the tuning network including a tunable microelectromechanical system (MEMS) capacitor having a capacitance, the tunable MEMS capacitor including a MEMS superstructure over a control structure that controls the MEMS superstructure and tunes the capacitance, the control structure including an electrode, and the MEMS superstructure including a membrane of which at least a portion is parallel to the electrode.

6. The method of claim 5, wherein: the amplifier includes a first die;the tunable MEMS capacitor includes a second die; andthe first die and the second die are integrated in a single package.

7. The method of claim 5, wherein the control structure electrostatically controls a position of the MEMS superstructure.

8. The method of claim 5, wherein the tuning network includes multiple tunable MEMS capacitors.