U.S. patent application number 11/895427 was filed with the patent office on 2009-02-26 for low-noise amplifier.
Invention is credited to Jason H. Branch, Lawrence E. Connell, Patrick L. Rakers, Poojan A Wagh.
Application Number | 20090051441 11/895427 |
Document ID | / |
Family ID | 40364623 |
Filed Date | 2009-02-26 |
United States Patent
Application |
20090051441 |
Kind Code |
A1 |
Branch; Jason H. ; et
al. |
February 26, 2009 |
LOW-NOISE AMPLIFIER
Abstract
Methods and corresponding systems in a low noise amplifier
include selecting a selected sub-band for amplifying, wherein the
selected sub-band is one of a plurality of sub-bands, wherein each
sub-band is a portion of a frequency band, and wherein each
sub-band has a corresponding sub-band center frequency. Next, a
gate-source capacitor is adjusted so that a real part of an LNA
input impedance corresponds to a real part of a source impedance at
the selected sub-band center frequency. A match capacitor is also
adjusted so that the LNA input impedance corresponds to the complex
conjugate of the source impedance at the selected sub-band center
frequency. The gate-source capacitor and the match capacitor can
each be adjusted by recalling capacitor values from memory that
correspond to the selected sub-band, and connecting selected
capacitor components in response to the recalled capacitor
values.
Inventors: |
Branch; Jason H.; (Austin,
TX) ; Connell; Lawrence E.; (Naperville, IL) ;
Rakers; Patrick L.; (Kildeer, IL) ; Wagh; Poojan
A; (Sleepy Hollow, IL) |
Correspondence
Address: |
LAW OFFICE OF CHARLES W. BETHARDS, LLP
P.O. BOX 1622
COLLEYVILLE
TX
76034
US
|
Family ID: |
40364623 |
Appl. No.: |
11/895427 |
Filed: |
August 24, 2007 |
Current U.S.
Class: |
330/305 |
Current CPC
Class: |
H03F 2203/45544
20130101; H03F 1/56 20130101; H03F 3/45192 20130101; H03F 2200/213
20130101; H03F 2203/45386 20130101; H03F 2200/294 20130101; H03F
3/191 20130101 |
Class at
Publication: |
330/305 |
International
Class: |
H03F 3/191 20060101
H03F003/191 |
Claims
1. A method in a low-noise amplifier (LNA) comprising: selecting a
selected sub-band for amplifying, wherein the selected sub-band is
one of a plurality of sub-bands, wherein each sub-band is a portion
of a frequency band, and wherein each sub-band has a corresponding
sub-band center frequency; adjusting a gate-source capacitor so
that a real part of an LNA input impedance corresponds to a real
part of a source impedance at a selected sub-band center frequency
corresponding to the selected sub-band; and adjusting a match
capacitor so that the LNA input impedance corresponds to the
complex conjugate of the source impedance at the sub-band center
frequency.
2. The method in an LNA according to claim 1 wherein the adjusting
the match capacitor comprises: selecting one or more match
capacitor components from a plurality of match capacitor components
to produce a selected combination of match capacitor components;
and connecting the selected combination of match capacitor
components.
3. The method in an LNA according to claim 1 wherein the match
capacitor comprises a metal-insulator-metal (MIM) capacitor, and
wherein the adjusting the match capacitor comprises: selecting one
or more MIM match capacitor components from a plurality of MIM
match capacitor components to produce a selected combination of MIM
match capacitor components; and connecting the selected combination
of MIM match capacitor components.
4. The method in an LNA according to claim 1 wherein the adjusting
the gate-source capacitor comprises: selecting one or more
gate-source capacitor components from a plurality of gate-source
capacitor components to produce a selected combination of
gate-source capacitor components; and connecting the selected
combination of gate-source capacitor components.
5. The method in an LNA according to claim 1 wherein the LNA has an
LNA Q factor, and wherein the LNA Q factor has a value greater than
a Q factor needed to receive the frequency band with less than a
predetermined input reflection coefficient.
6. The method in an LNA according to claim 1 wherein an input
reflection coefficient curve of the LNA is less than a minimum
input reflection coefficient over a frequency range at least equal
to a center frequency step size, wherein the center frequency step
size is a distance between adjacent sub-band center
frequencies.
7. The method in an LNA according to claim 1 further comprising
amplifying an input signal within the selected sub-band and
rejecting an input signal outside the selected sub-band.
8. The method in an LNA according to claim 1 wherein the adjusting
a gate-source capacitor and the adjusting a match capacitor
comprises: recalling from memory a gate-source capacitor value
associated with the selected sub-band; configuring an adjustable
gate-source capacitor in response to the gate-source capacitor
value associated with the selected sub-band; recalling from memory
a match capacitor value associated with the selected sub-band; and
configuring an adjustable match capacitor in response to the match
capacitor value associated with the selected sub-band.
9. A low-noise amplifier (LNA) comprising: first and second LNA
input terminals for coupling the LNA to a source having a source
impedance, wherein the LNA has an LNA input impedance across the
first and second LNA input terminals; a first transistor, of a
first conductivity type, having a first gate electrode, a first
source electrode, and a first drain electrode, wherein the first
gate electrode is coupled to the first LNA input terminal; an
adjustable gate-source capacitor coupled between the first gate
electrode and the first source electrode; a source inductor coupled
between the first source electrode and the second LNA input
terminal; an adjustable match capacitor coupled between the first
and second LNA input terminals; a match inductor coupled between
the first and second LNA input terminals; and a controller coupled
to the adjustable gate-source capacitor and the adjustable match
capacitor, wherein the controller is responsive to a selected
sub-band for amplifying, wherein the selected sub-band is one of a
plurality of sub-bands, wherein each sub-band is a portion of a
frequency band, and wherein each sub-band has a corresponding
sub-band center frequency, and wherein the controller sets the
adjustable gate-source capacitor so that a real part of the LNA
input impedance corresponds to a real part of the source impedance
at the selected sub-band center frequency, and wherein the
controller sets the adjustable match capacitor so that the LNA
input impedance is the complex conjugate of the source impedance at
a selected sub-band center frequency corresponding to the selected
sub-band.
10. The LNA of claim 9 wherein the adjustable gate-source capacitor
comprises: a plurality of gate-source capacitor components, wherein
each gate-source capacitor component has a first terminal coupled
to a first gate-source capacitor terminal; and a plurality of
gate-source capacitor switches, wherein each gate-source capacitor
switch is coupled between a second terminal of the gate-source
capacitor component and a second gate-source capacitor terminal,
and wherein each of the plurality of gate-source capacitor switches
is coupled to the controller for controlling a selection of one or
more gate-source capacitor components coupled between the first and
second gate-source capacitor terminals.
11. The LNA of claim 9 wherein the adjustable match capacitor
comprises one or more voltage variable capacitors.
12. The LNA of claim 9 wherein the adjustable match capacitor
comprises: a plurality of first match capacitor components, wherein
each first match capacitor component has a first terminal coupled
to a first match capacitor terminal; a plurality of second match
capacitor components, wherein each second match capacitor component
has a second terminal coupled to a second match capacitor terminal;
and a plurality of match capacitor switches, wherein each match
capacitor switch is connected between a second terminal of a first
match capacitor component and a first terminal of a second match
capacitor component, and wherein each of the plurality of match
capacitor switches is coupled to the controller for controlling a
selection of one or more match capacitor components coupled between
the first and second match capacitor terminals.
13. The LNA of claim 12 wherein the adjustable match capacitor
comprises: a plurality of first bias resistors, wherein each first
bias resistor has a first terminal coupled to a second terminal of
a first match capacitor component; a plurality of second bias
resistors, wherein each second bias resistor has a second terminal
coupled to a first terminal of a second match capacitor component;
and a bias voltage source coupled to a second terminal of the
plurality of first bias resistors and a first terminal of the
plurality of second bias resistors.
14. The LNA of claim 9 further comprising a load coupled between a
voltage source and the first drain electrode of the first
transistor.
15. The LNA of claim 9 wherein the controller comprises a memory
for storing configuration data associated with each of the
plurality of sub-bands for configuring the adjustable gate-source
capacitor, and for configuring the adjustable match capacitor.
16. A low-noise amplifier (LNA) comprising: first and second LNA
input terminals for coupling the LNA to a source having a source
impedance, wherein the LNA has an LNA input impedance across the
first and second LNA input terminals; a first transistor, of a
first conductivity type, having a first gate electrode, a first
source electrode, and a first drain electrode, wherein the first
gate electrode is coupled to the first LNA input terminal; a second
transistor, of a first conductivity type, having a second gate
electrode, a second source electrode, and a second drain electrode,
wherein the second gate electrode is coupled to the second LNA
input terminal; a first adjustable gate-source capacitor coupled
between the first gate electrode and the first source electrode; a
second adjustable gate-source capacitor coupled between the second
gate electrode and the second source electrode; a source inductor
coupled between the first source electrode and the second source
electrode; an adjustable match capacitor coupled between the first
and second LNA input terminals; a match inductor coupled between
the first and second LNA input terminals; and a controller coupled
to the first and second adjustable gate-source capacitors and to
the adjustable match capacitor, wherein the controller is
responsive to a selected sub-band for amplifying, wherein the
selected sub-band is one of a plurality of sub-bands, wherein each
sub-band is a portion of a frequency band, and wherein each
sub-band has a corresponding sub-band center frequency, and wherein
the controller sets the first and second adjustable gate-source
capacitors so that a real part of the LNA input impedance
corresponds to a real part of the source impedance at the selected
sub-band center frequency, and wherein the controller sets the
adjustable match capacitor so that the LNA input impedance is the
complex conjugate of the source impedance at a selected sub-band
center frequency corresponding to the selected sub-band.
17. The LNA of claim 16 wherein the first and second adjustable
gate-source capacitors each comprise: a plurality of gate-source
capacitor components, wherein each gate-source capacitor component
has a first terminal coupled to a first gate-source capacitor
terminal; and a plurality of gate-source capacitor switches,
wherein each gate-source capacitor switch is coupled between a
second terminal of the gate-source capacitor component and a second
gate-source capacitor terminal, and wherein each of the plurality
of gate-source capacitor switches is coupled to the controller for
controlling a selection of one or more gate-source capacitor
components coupled between the first and second gate-source
capacitor terminals.
18. The LNA of claim 16 wherein the adjustable match capacitor
comprises one or more voltage variable capacitors.
19. The LNA of claim 16 wherein the adjustable match capacitor
comprises: a plurality of first match capacitor components, wherein
each first match capacitor component has a first terminal coupled
to a first match capacitor terminal; a plurality of second match
capacitor components, wherein each second match capacitor component
has a second terminal coupled to a second match capacitor terminal;
and a plurality of match capacitor switches, wherein each match
capacitor switch is connected between a second terminal of a first
match capacitor component and a first terminal of a second match
capacitor component, and wherein each of the plurality of match
capacitor switches is coupled to the controller for controlling a
selection of one or more match capacitor components coupled between
the first and second match capacitor terminals.
20. The LNA of claim 19 wherein the adjustable match capacitor
comprises: a plurality of first bias resistors, wherein each first
bias resistor has a first terminal coupled to a second terminal of
a first match capacitor component; a plurality of second bias
resistors, wherein each second bias resistor has a second terminal
coupled to a first terminal of a second match capacitor component;
and a bias voltage source coupled to a second terminal of the
plurality of first bias resistors and a first terminal of the
plurality of second bias resistors.
21. The LNA of claim 16 further comprising a load coupled between a
voltage source and the first drain electrode of the first
transistor, and coupled between the voltage source and the second
drain electrode of the second transistor.
22. The LNA of claim 16 wherein the controller comprises a memory
for storing configuration data associated with each of the
plurality of sub-bands for configuring the first and second
adjustable gate-source capacitors, and for configuring the
adjustable match capacitor.
Description
BACKGROUND
[0001] 1. Field
[0002] This disclosure relates generally to amplifiers, and more
specifically to adjusting an input impedance of an
inductively-degenerated low-noise amplifier.
[0003] 2. Related Art
[0004] In the field of communications, a major component in a
receiver is the low noise amplifier (LNA), which can be a first
amplifier in a series of amplifiers that are used to demodulate and
process a low-level signal received by an antenna. A good LNA
amplifies extremely low-power signals without adding excessive
noise, thus preserving the required signal-to-noise ratio (SNR) of
the system at extremely low power levels. Additionally, the LNA
amplifies the received signal without introducing significant
distortion.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] The present invention is illustrated by way of example and
is not limited by the accompanying figures, in which like
references indicate similar elements. Elements in the figures are
illustrated for simplicity and clarity and have not necessarily
been drawn to scale.
[0006] FIG. 1 is a high-level block diagram of a low noise
amplifier (LNA) system having an LNA input coupled to a source in
accordance with one or more embodiments;
[0007] FIG. 2 is a high-level schematic diagram of a model of an
input impedance of the LNA of FIG. 1 in accordance with one or more
embodiments;
[0008] FIG. 3 depicts an adjustable gate-source capacitor circuit
in accordance with one or more embodiments;
[0009] FIG. 4 depicts an adjustable match capacitor circuit in
accordance with one or more embodiments;
[0010] FIG. 5 is a high-level flowchart depicting a process that
can be executed in an LNA in accordance with one or more
embodiments;
[0011] FIG. 6 depicts a high-level block diagram of an LNA system
coupled to a source in accordance with one or more embodiments;
[0012] FIG. 7 depicts a high-level block diagram of an LNA system
coupled to a source in accordance with one or more embodiments;
and
[0013] FIG. 8 is a graph of frequency versus input reflection
coefficient curves corresponding to a plurality of sub-bands within
a frequency band in accordance with one or more embodiments.
DETAILED DESCRIPTION
[0014] An LNA is an electronic amplifier, which can be used e.g.,
in communication systems to amplify low power signals, such as
wireless signals captured by an antenna. The LNA has an impact on
the quality and capabilities of a receiver because the effective
noise of all the subsequent stages is reduced by the gain of the
LNA, and the noise of the LNA is injected directly into the
received signal.
[0015] FIG. 1 is a high-level block diagram of a low noise
amplifier (LNA) system in accordance with one or more embodiments.
As illustrated, LNA system 100 includes a source 102 coupled to an
LNA 104 at first and second terminals 106 and 108, respectively.
Source 102 provides a low-power signal to the LNA for
amplification. Source 102 includes signal source 110 and source
impedance 112. Source impedance 112 can be a resistance, as
illustrated with dashed lines in FIG. 1. In other embodiments,
source impedance 112 can be complex, including both real (e.g.,
resistive) and imaginary (e.g., reactive) parts.
[0016] In one embodiment, source 102 can include signal source 110
and calibration source 114, which can be selectively coupled
through switch 116 and source impedance 112 to first terminal 106.
Signal source 110 can be implemented with an antenna coupled to a
transmission line, wherein a low-power signal can be received from
a wireless source (e.g., a cell phone tower, a wireless network
access point, a satellite antenna, or the like). In other
embodiments, signal source 110 can be a wireline source, wherein a
low-power signal can be received from a fiber optic cable,
telephone cable, coaxial cable, or the like.
[0017] Whether source 102 is wireless or wireline, a matching
network (not shown) can be included in source 102 (e.g., between
the antenna and the transmission line) to produce a predetermined
source impedance 112. Thus, impedance 112 can represent the
impedance of source 102.
[0018] Calibration source 114 can, in some embodiments, be
implemented with a low-power signal generator, which, for example,
can output a selected waveform at a selected frequency (e.g., a
radio frequency signal) in order to simulate signals that may be
output by signal source 110. The output from calibration source 114
can be used to calibrate LNA 104, and also as a means to more
comprehensively calibrate a receiver by allowing a controlled RF
signal to bypass the antenna. The RF signal can be used to
determine settings used by controller 134 that tune an input
matching network to a particular frequency band and/or frequency
sub-band. The calibration source 114 can also be used to measure
the performance of LNA 104, and also the performance of the
receiver in which LNA 104 is used.
[0019] In some embodiments, the low-level signal provided by
calibration source 114 can be demodulated and measured at the
output of a receiver that incorporates LNA 104 (other than LNA 104,
the receiver is not shown). This low-level signal can be used to
determine the LNA 104 performance for a particular frequency band
or sub-band based on the settings of adjustable match capacitor
120, and adjustable gate-source capacitors 128 and 132. The
settings for adjustable match capacitor 120, and adjustable
gate-source capacitors 128 and 132 can then be programmed, or
adjusted, in lookup table 138 in controller 134, as described in
more detail below. Feedback 142 can be produced from performance
metrics and/or measurements taken either at the output of LNA 104,
or elsewhere in a receiver that uses LNA 104. Feedback 142 can then
be used to calibrate or otherwise adjust the settings for
adjustable match capacitor 120, and adjustable gate-source
capacitors 128 and 132.
[0020] Switch 116 can be used to select an output from signal
source 110 or calibration source 114. Switch 116 can be any one of
several known switch types for switching a low-power analog signal
source such as, for example, an analog single-throw switch that can
toggle between the antenna and calibration source 114. Switch 116
can be controlled by a signal from controller 134, discussed in
more detail below.
[0021] In one embodiment, LNA 104 can include match inductor 118
and adjustable match capacitor 120 connected in parallel across
first and second LNA input terminals 106 and 108. First transistor
122, which, in the embodiment shown, is a N-channel transistor
(e.g., a transistor having a first or one conductivity type of a
transistor), can have a first or one current electrode (e.g., a
source electrode), and a second or another current electrode (e.g.,
a drain electrode), and a control electrode (e.g., a gate
electrode). The gate electrode can be coupled to the first LNA
input terminal 106, and the drain electrode can be coupled to load
124. The source electrode of first transistor 122 can be connected
to a first terminal of source inductor 126. A center tap of source
inductor 126 can be connected to ground or a virtual ground.
An-adjustable gate-source capacitor 128 can be coupled between the
gate electrode and the source electrode of first transistor
122.
[0022] A second N-channel transistor 130 can have a drain electrode
coupled to load 124, a source electrode coupled to a second
terminal of source inductor 126, and a gate electrode coupled to
second LNA input terminal 108. An adjustable gate-source capacitor
132 can be coupled between the gate electrode and the source
electrode of second transistor 130. In the embodiment of FIG. 1,
transistors 122 and 130 are configured as a differential,
common-source amplifier, wherein transistors 122 and 130 have
similar characteristics (e.g., they are matched transistors).
[0023] In various embodiments, load 124 can be implemented with a
high impedance load, such as a current source, or an inductor,
depending upon what is needed to match a subsequent circuit, such
as a mixer or amplifier stage, connected to the output (e.g., the
drain terminals of first and second transistors 122 and 130) of LNA
104. If the output of LNA 104 is coupled to a low input impedance
mixer or amplifier (not shown), the output of LNA 104 can be a
current. Alternatively, if such a coupled mixer or amplifier has a
high input impedance, the output of LNA 104 can be a voltage. Load
124 can be connected to voltage reference V.sub.DD 140. Whether
outputting a current or a voltage, the output of LNA 104 is taken
across the drain terminals of first transistor 122 and second
transistor 130.
[0024] In an LNA embodiment having a voltage output, load 124 can
be implemented with two inductors. The inductors offer a positive
reactance at RF frequencies to match, for example, a mixer,
amplifier, or parasitic capacitance coupled to the output of LNA
104 (not shown) that has a negative reactance. The equal and
opposite reactances will have a high impedance at the desired radio
frequency. A common source amplifier with a high impedance load
creates a voltage output.
[0025] In another embodiment having a current output, load 124 can
be implemented with two positive-channel field effect transistors
(PFET's), which can be used to create high impedance current
sources, which can be coupled to a mixer or amplifier presenting a
low input impedance. In other embodiments, load 124 can be
implemented with two inductors, which can be used as RF chokes
having a high impedance at radio frequency. Such inductors can be
coupled to a mixer or amplifier presenting a low input impedance.
Thus, in embodiments having a common source amplifier with either
type of high impedance load coupled to a low impedance mixer or
amplifier, a current output can be produced.
[0026] LNA 104 can include controller 134 for adjusting or setting
adjustable capacitors 120, 128, and 132, or for controlling source
selection switch 116. Controller 134 can include program memory 136
for storing and recalling software instructions and parameters
(e.g., software, microcode, firmware, or the like) used to operate
controller 134. Controller 134 can also include lookup table 138,
which can be used to store and recall data records that relate
adjustable capacitor settings to input frequencies (e.g., input
signal center frequencies or ranges of frequencies) that can be
amplified in LNA 104. Controller 134 is coupled to adjustable match
capacitor 120, adjustable gate-source capacitor 128, and adjustable
gate-source capacitor 132 for the purpose of controlling the
capacitance values of these adjustable capacitors. In one
embodiment, the control signals from controller 134 represent
binary data bits that correspond to respective capacitance
values.
[0027] In the embodiment shown in FIG. 1, LNA 104 has an
inductive-degeneration configuration, which has an inductor (e.g.,
L.sub.S 126) connected to the source of a transistor (e.g.,
transistor 122, 130) operating in the saturation region. In an
equivalent small-signal analysis, looking from terminals 144 and
146 (see FIGS. 1, 2, 6, and 7) into the gate(s) of one or more
transistors (e.g., transistors 122 and 130), source inductor 126
can create both a real and a reactive impedance at the gate of the
transistors. The real part is given by Equation 1, below:
Real ( Z SRC ) = gm * L S C GS , T Equation 1 ##EQU00001##
[0028] where gm is the transistor (i.e., MOSFET) transconductance,
and C.sub.GS,T is the total capacitance across the gate and source
terminals of either transistor 122 or transistor 130 (e.g., the
parallel combination of adjustable gate-source capacitor 128, 132
in parallel with the gate-source capacitance of transistor 122,
130). The imaginary part is given by Equation 2, below:
Imag ( Z SRC ) = .omega. * L S - 2 .omega. * C GS , T Equation 2
##EQU00002##
[0029] where .omega. is the angular frequency. As is known, in an
inductively-degenerated low-noise amplifier, the inductor adds very
little noise to the amplifier, and it also provides a real
impedance at the transistor gate, which can be used for input
matching.
[0030] FIG. 2 shows a high-level schematic diagram of a model of an
input impedance of LNA 104 in accordance with one or more
embodiments. As illustrated, signal source 110, which has a source
impedance 112, is coupled to the LNA input impedance model at first
and second terminals 106 and 108. Signal source 110 can be an
antenna and an associated matching network, which in combination
have an output impedance equivalent to source impedance 112. The
input impedance of LNA 104 can be modeled as a parallel equivalent
circuit that includes inductor 202 in parallel with adjustable
capacitor 204, which in turn are in parallel with adjustable
capacitor 206 and resistor 208. The value of adjustable capacitor
204 can be equal to the value of adjustable match capacitor 120
(see FIG. 1). Assuming that the input reactance of LNA 104 is
dominated by capacitance, the value of resistor 208 (i.e.,
R.sub.208), which is the parallel equivalent resistance of LNA 104,
can be determined from Equation 3, below:
R 208 = ( g m * L S ) ( C GS , T ) + 4 ( .omega. 2 * C GS , T * L S
* g m ) Equation 3 ##EQU00003##
[0031] where g.sub.m is the transconductance of the transistor,
.omega. is the center frequency of the source signal, C.sub.GS,T is
the total capacitance across the gate and source terminals of
either transistor 122 or transistor 130 (e.g., the parallel
combination of adjustable gate-source capacitor 128, 132 in
parallel with the gate-source capacitance of transistor 122, 130),
L.sub.S is the total inductance of source inductor 126.
[0032] The value of adjustable capacitor 206, again assuming the
input reactance of LNA 104 is dominated by capacitance, can be
described by Equation 4, below:
C 206 = C GS , T / 2 1 + ( .omega. * g m * L S / 2 ) 2 Equation 4
##EQU00004##
[0033] To obtain maximum power transfer from a source to a load,
the source impedance should equal the complex conjugate of the load
impedance. That is, the source should be conjugately matched to the
load. This can be expressed mathematically, in terms of admittance
and susceptance, as shown in Equation 5, below:
Y.sub.S+jB.sub.S=Y.sub.L-jB.sub.L Equation 5
[0034] In addition, for efficient power transfer, this conjugately
matched condition is required to avoid the reflection of energy
from the load back to the source (e.g., the rejection of an input
signal outside a sub-band). This is particularly true for
high-frequency systems, like radio frequency (RF) and microwave
systems, video systems, and the like.
[0035] Thus, the input matching network of LNA 104, which can be
considered the load for signal source 102, can be set so that:
R 208 = 1 Y s Equation 6 1 / ( j * w * L 202 ) + j * w * C 206 + j
* w * C 204 = - j * B s Equation 7 ##EQU00005##
[0036] FIG. 3 depicts an adjustable gate-source capacitor that can
be used in an LNA circuit as gate-source capacitor 128 or 132 (see
FIG. 1) in accordance with one or more embodiments. As shown, a
plurality of gate-source capacitor components 302, 304, 306, 308,
310, and 312, have a first terminal coupled to a first gate-source
capacitor terminal 314. In one embodiment, gate-source capacitor
components 302, 304, 306, 308, 310, and 312 can be implemented with
metal-insulator-metal (MIM) capacitors, which capacitors can be
built into integrated circuits using integrated circuit fabrication
techniques. Gate-source capacitor component 312 is shown with
dashed lines because there can be any number of gate-source
capacitor components. These gate-source capacitor components can be
selectively coupled to a second gate-source capacitor terminal 316
by switches connected in series with each gate-source capacitor
component. Such switches can be implemented with transistors, such
as transistors 318, 320, 322, 324, 326, and 328. These transistors
can each have a drain electrode coupled to a second terminal of the
gate-source capacitor component, a source electrode coupled to the
second gate-source capacitor terminal 316, and a gate electrode
coupled to a control signal 330, which control signals can be
provided by controller 134 in FIG. 1.
[0037] In one embodiment, the capacitance value of adjustable
gate-source capacitor 128 or 132 can be provided across first and
second gate-source capacitor terminals 314 and 316. The capacitance
value is adjusted by selecting one or more gate-source capacitor
components from the plurality of gate-source capacitor components,
and then connecting the selected combination of gate-source
capacitor components across first and second gate-source capacitor
terminals 314 and 316. In the embodiment shown in FIG. 3, a control
signal on the gate of the transistor will connect the selected
gate-source capacitor components, which will all be added together
to produce the adjustable gate-source capacitance (e.g.,
capacitance 128 or 132 in FIG. 1).
[0038] In another embodiment, the switches used for selectively
connecting gate-source capacitor components can be implemented with
so-called analog switches, which are electronic components that
behave in a way similar to a relay. The switching element of an
analog switch can be a transistor, such as a
metal-oxide-semiconductor field-effect transistor (MOSFET), which
is controlled by standard logic-level input.
[0039] In another embodiment, microelectromechanical systems (MEMS)
can be used to provide relays to selectively connect gate-source
capacitor components (e.g., 302, 304, 306, 308, 310, and 312) to
gate-source capacitor terminals 314 and 316. MEMS are very small
mechanical devices, which are often fabricated using modified
semiconductor fabrication technology, such as molding and plating,
wet and dry etching, electro discharge machining, and other
technologies capable of manufacturing very small devices.
[0040] In some embodiments, gate-source capacitor components can be
grouped into capacitor components associated with the selection of
a sub-band and capacitor components associated with the selection
of a frequency band. For example, gate-source capacitor components
332 can be used to select a sub-band for amplifying in LNA 104,
while gate-source capacitor component 334 may be used to select a
frequency band for amplifying. In one embodiment, gate-source
capacitor components 332 can be binary-weighted values (e.g., each
gate-source capacitor component value is two times the value of the
next smaller value) so the various gate-source capacitance values
in a range of values can be formed. Gate-source capacitor component
334 (and any other capacitor components in the group for selecting
a frequency band) can be selected in order to select a frequency
band that is not contiguous with frequency sub-bands that can be
selected using gate-source capacitor components 332. As an example,
gate-source capacitor component 334 can be switched in and out in
order to select frequency bands that are not contiguous, and that
are associated with different telecommunications standards.
[0041] FIG. 4 depicts an adjustable match capacitor that can be
used in an LNA circuit as adjustable match capacitor 120 (see FIG.
1) in accordance with one or more embodiments. As shown, a
plurality of first match capacitor components 402, 404, 406, and
408 have a first terminal coupled to a first match capacitor
terminal 410, and a plurality of second match capacitor components
412, 414, 416, and 418 have a second terminal coupled to a second
match capacitor terminal 420. In one embodiment, first match
capacitor components 402, 404, 406, and 408, and second match
capacitor components 412, 414, 416, and 418 can be implemented with
metal-insulator-metal (MIM) capacitors.
[0042] These match capacitor components (402-408 and 412-418) can
be selectively coupled, in first and second pairs, between first
match capacitor terminal 410 and second match capacitor terminal
420 by controlling a switch between the first and second match
capacitor components. In one embodiment, the switch can be
implemented with a transistor, such as transistors 422, 424, 426,
and 428. These transistors can have a drain electrode coupled to a
second terminal of the first match capacitor component, a source
electrode coupled to a first terminal of the second match capacitor
component, and a gate electrode coupled to a control signal 430,
which control signal can be provided by controller 134 in FIG. 1.
First and second match capacitor components 408 and 418, and
transistor 428 (i.e., switch), are shown with dashed lines because
there can be any number of match capacitor components.
[0043] Thus, in one embodiment, the capacitance value of adjustable
match capacitor 120 can be provided across first and second match
capacitor terminals 410 and 420. The capacitance value is adjusted
by selecting one or more match capacitor components (e.g., match
capacitor component pairs 402 & 412, 404 & 414, 406 &
416, and 408 & 418) from the plurality of match capacitor
components, and then connecting the selected combination of match
capacitor components across first and second match capacitor
terminals 410 and 420. In the embodiment shown in FIG. 4, a
high-level signal on the gate of the transistor will connect the
selected pair of first and second match capacitor components,
wherein the selected pairs will all be added together to produce
the adjustable match capacitance (e.g., capacitance 120 in FIG.
1).
[0044] To provide bias voltages to transistors 422, 424, 426, and
428, pairs of resistors can be connected to bias voltage V.sub.B
432 and to the drain and source of the transistors. For example, as
shown in FIG. 4, first bias resistor 434 and second bias resistor
436 can provide bias voltages for transistor 422, first bias
resistor 438 and second bias resistor 440 can provide bias voltages
for transistor 424, first bias resistor 442 and second bias
resistor 444 can provide bias voltages for transistor 426, and
first bias resistor 446 and second bias resistor 448 can provide
bias voltages for transistor 428. Note that if the switches for
connecting match capacitor components do not need a bias voltage,
resistors 434-448 are not needed. An example of switches that do
not need a bias voltage are MEMS switches, which are discussed
above.
[0045] Another alternative for implementing an adjustable capacitor
(e.g., an adjustable gate-source capacitor 128, 132, or an
adjustable match capacitor 120) is a voltage variable capacitor, or
a varactor. A varactor is a reverse-biased diode. The capacitance
of the diode can be changed by adjusting a DC bias voltage across
the diode. A voltage variable capacitor can be implemented with a
MOSFET, wherein the gate is one terminal of the capacitor, the
drain and source terminals are shorted together, and together they
comprise the second terminal of the capacitor. The capacitance of a
voltage variable capacitor can be adjusted by driving an array of
voltage variable capacitors with a multitude of binary signals to
create an assortment of capacitances, or it can be driven by an
analog bias voltage to produce a continuous range of capacitance
values.
[0046] As with the grouping of capacitors in FIG. 3, in some
embodiments, pairs of match capacitor components in FIG. 4 can be
grouped into capacitor components associated with the selection of
a sub-band, and capacitor components associated with the selection
of a frequency band. For example, pairs of match capacitor
components 450 can be used to select a sub-band for amplifying in
LNA 104, while match capacitor component 452 may be used to select
a frequency band for amplifying. In one embodiment, match capacitor
components 450 can have binary-weighted values (e.g., each match
capacitor component value is two times the value of the next
smaller value) so the various match capacitance values in a range
of values can be formed. Match capacitor component 452 (and any
other capacitor components in the group for selecting a frequency
band) can be selected in order to select a frequency band that is
not contiguous with frequency sub-bands that can be selected using
match capacitor components 450. As an example, match capacitor
component 452 can be switched in and out in order to select
frequency bands that are not contiguous, and that are associated
with different telecommunications standards.
[0047] In the operation of LNA system 100, components of LNA 104
can be tuned or adjusted so that the input impedance of LNA 104
conjugately matches the output impedance of source 102. As shown in
the embodiment of FIG. 1, adjustable match capacitor 120 and
adjustable gate-source capacitors 128 and 132 are coupled to
control signals provided by controller 134, wherein the control
signals can tune or adjust capacitance values.
[0048] In one embodiment, the coupling of source 102 to LNA 104 can
have an input reflection coefficient curve, which can be commonly
referred to as an S.sub.11 curve, that shows energy passes from
source 102 to LNA 104 in a narrow band of frequencies (e.g., a
sub-band of frequencies), and energy is reflected or rejected in
all other frequencies. FIG. 8 shows a plurality of S.sub.11 curves
802, 804, 806, 808, and 810, wherein S.sub.11 values 812 are
plotted against signal frequency 814 for selected settings of
adjustable match capacitor 120 and adjustable gate-source
capacitors 128 and 132. As shown in FIG. 8, LNA 104 can have an
S.sub.11, curve, such as curve 806, that has values less than a
desired minimum S.sub.11 value (e.g., S.sub.11,min 816) over a
sub-band of frequencies 820 within a frequency band 822, which band
extends from a low frequency, f.sub.L 824, to a high frequency,
f.sub.H 826. Such a frequency band 822 can be a range of
frequencies (e.g., ranging from f.sub.L to f.sub.H) that contains
adjacent channels or frequencies that a communication device can
receive according to a communications standard promulgated by a
government body or industry association. Examples of such frequency
bands include the frequency bands set forth in specifications
published by the 3.sup.rd Generation Partnership Project (3GPP), or
specifications published by American National Standards Institute
(ANSI), or standards published by the Telecommunications Industry
Association (TIA), or other similar standards. A sub-band 820 can
be defined as a portion of a frequency band 822 wherein frequency
band 822 contains two or more sub-bands 820. Note that the LNA
input can be modeled, from the perspective of terminals 106 and
108, as an equivalent RLC circuit, such as shown in FIG. 2, and
that the equivalent circuit, as well as LNA 104, has a Q factor,
wherein the Q factor of the LNA input has a value greater than a Q
factor needed to receive the entire frequency band 822 with less
than a predetermined input reflection coefficient (e.g., S.sub.11,
min 816 shown in FIG. 8).
[0049] Thus, FIG. 8 shows a plurality of sub-bands 820 within a
frequency band 822, wherein each sub-band is associated with an
S.sub.11 curve 802, 804, 806, 808, and 810, and each sub-band has a
corresponding sub-band center frequency, such as sub-band center
frequency f.sub.0 818 that corresponds to sub-band 820. Each
sub-band can be spaced apart from an adjacent sub-band so that
adjacent S.sub.11 curves intersect at points below S.sub.11,min 816
in order to ensure that LNA 104 can receive all frequencies in the
frequency band 822 by the appropriate selection of a sub-band.
Thus, by using the selection of sub-bands, LNA 104 can be
considered a wideband amplifier capable of amplifying signals over
a frequency band 822, and perhaps over two or more selectable
frequency bands (note that only one frequency band 822 is shown in
FIG. 8). The width of each S.sub.11 curve is determined by the
quality factor (i.e., the Q factor) of the equivalent parallel RLC
circuit (i.e., resistance-inductance-capacitance resonant circuit)
in FIG. 2. The quality factor depends on the amount of loss each
component adds to the equivalent RLC circuit. The quality factor of
each component can ensure that the minimum S.sub.11 values are met
over a frequency band or frequency sub-band.
[0050] In some embodiments, a sub-band 820 can include more than
one frequency channel, wherein the frequency channel is defined
according to the relevant specification. In another embodiment, a
sub-band may include a single channel. As a general principle, the
width of the S.sub.11 curve that defines the sub-band 820 is set to
receive one or more channels as defined by the specification, and
the curve is designed to overlap adjacent S.sub.11 curves so that
all channels in the frequency band can be amplified, which means
that the LNA is able to reflect or reject energy outside the
desired frequency range, thereby increasing the selectivity of the
amplifier and reducing the noise in the amplifier.
[0051] FIG. 5 shows high-level flowchart 500 depicting a process
that can be executed in LNA 104 in accordance with one or more
embodiments. As illustrated, flowchart 500 begins at 502, and
thereafter passes to 504 wherein the process selects a frequency
band for amplifying in the LNA. The frequency band can be a range
of frequencies selected in accordance with a communications
standard, wherein the frequency band includes a set of adjacent
frequencies or channels for receiving a signal. Typically such
frequency bands are discontinuous ranges of frequencies, wherein
such frequency bands are separated by a range of frequencies. In
one embodiment, the frequency band can be selected based upon a
user input (e.g., activated by a keyboard or button input, are
activated by voice command inputs, or the like), or selected based
upon the availability of a communication network (e.g., in response
to signal strength measurements of base stations signals, or the
like), or based on instructions from a communications system
controller (e.g., instructions from a base station, or the
like).
[0052] After selecting the frequency band, the process can tune or
adjust one or more adjustable gate-source capacitors, and tune or
adjust an adjustable match capacitor, for the selected frequency
band, as illustrated at 506. In one embodiment, tuning the one or
more adjustable gate-source capacitors can be implemented with
appropriate control signals from controller 134 (see FIG. 1),
wherein the signals (see signals 330 in FIG. 3) are used to tune or
adjust gate-source capacitors 128 and 132, wherein the adjustable
gate-source capacitors can be implemented according to the
configuration shown and FIG. 3. Adjusting gate-source capacitors
for a selected frequency band can include switching gate-source
capacitor component 334, which changes the gate-source capacitor
for tuning to a discontinuous frequency band. The value of the
gate-source capacitors can be calculated according to Equations 3
and 6, above.
[0053] Similarly, tuning the adjustable match capacitor can be
implemented using control signals 430 from controller 134 to tune
or adjust adjustable match capacitor 120. Adjusting the match
capacitor for a selected frequency band can include switching
gate-source capacitor component 452, which changes the match
capacitor for tuning to a discontinuous frequency band. The value
of the match capacitor can be calculated according to Equations 4
and 7, above. Under the control of instructions stored in program
memory 136, controller 134 can recall settings for the adjustable
capacitors from lookup table 138, wherein the recalled settings
correspond to the selected frequency band.
[0054] After tuning the adjustable capacitors for the selected
frequency band, the process can select a sub-band for amplifying,
as depicted at 508. In one embodiment, the sub-band selection can
be made according to a communications system standard or protocol
(e.g., initially turning to a control channel, or a channel used to
request service, or the like), or in response to instructions from
a base station or other communication system controller (e.g.,
instructions for receiving a call or other data, a handoff
instruction, or the like). Thus, the selection of a sub-band for
amplifying includes selecting a sub-band that contains the desired
channel.
[0055] Once the sub-band is selected, the process can tune or
adjust one or more adjustable gate-source capacitors to set a real
part of the load impedance substantially equal to a real part of
the source impedance (i.e., set R.sub.L=R.sub.S, or using an
admittance in a parallel equivalent circuit, set Y.sub.L=Y.sub.S)
at the selected sub-band center frequency, as depicted at 510. In
one embodiment, tuning the one or more adjustable gate-source
capacitors can be implemented with appropriate control signals 330
(see FIG. 3) from controller 134 (see FIG. 1), wherein the signals
are used to tune or adjust gate-source capacitors 128 and 132. In
one embodiment, a combination of binary-weighted gate-source
capacitor components 332 can be selected and connected to produce
the desired gate-source capacitance value according to data stored
in controller 134 for the selected sub-band.
[0056] Next, the process can tune or adjust an adjustable match
capacitor to set an imaginary part of the load impedance
substantially equal to a complex conjugate of the imaginary part of
the source impedance (i.e., set jX.sub.L=-jX.sub.S, or using a
susceptance in a parallel equivalent circuit, set
jB.sub.L=-jB.sub.S) at the selected sub-band center frequency, as
illustrated at 512. In one embodiment, tuning the adjustable match
capacitor can be implemented with appropriate control signals 430
(see FIG. 4) from controller 134 (see FIG. 1), wherein the signals
are used to tune or adjust match capacitor 120. In one embodiment,
a combination of binary-weighted match capacitor components 450 can
be selected and connected to produce the desired match capacitance
value according to data stored in controller 134 for the selected
sub-band.
[0057] Finally, the process amplifies the input signal to produce
an output signal, as depicted at 514. In one embodiment, the input
signal is received from source at terminals 106 and 108 (see FIG.
1), and LNA 104 produces an amplified output signal across the
terminals of load 124 (e.g., V.sub.OUT or I.sub.OUT). After
producing the amplified output signal, the process in the LNA ends,
as shown at 516. Although the process is shown having an end, the
process of amplifying at 514 can occur for as long as needed for
the selected frequency band and sub-band, and the process of
selecting bands and sub-bands can be repeated as necessary for
operation of the LNA in a communications system.
[0058] FIG. 6 depicts high-level block diagram of an LNA system 600
coupled to a source in accordance with one or more embodiments. As
a variation to the common-source configuration of the LNA shown and
FIG. 1, the embodiment of FIG. 6 is a cascode implementation of a
wide bandwidth inductively-degenerated low-noise amplifier. As
shown, LNA system 600 source 102 coupled to LNA 602 at terminals
106 and 108. Where components and FIG. 6 are similar to those
illustrated in FIG. 1 the same reference numerals have been used.
In the cascode implementation, transistors 604 and 606 have been
added, respectively, between transistors 122 and 130 and load 124.
The output of amplifier 602 is taken across the drain terminals of
transistors 604 and 606 to provide V.sub.OUT or I.sub.OUT (e.g., an
output voltage or an output current). The source terminals of
transistors 604 and 606 are connected to respective drain terminals
of transistors 122 and 130. The gate terminals of transistors 604
and 606 are connected to V.sub.CAS 608, which is a voltage to bias
the cascode transistors. The cascode devices improve the isolation
between the input of the amplifier and the output of the amplifier,
at the expense of requiring more supply headroom than a
common-source amplifier.
[0059] FIG. 7 depicts high-level block diagram of an LNA system 700
coupled to a source in accordance with one or more embodiments.
FIG. 7 illustrates a single-ended implementation of the
differential amplifier in FIG. 1. The principles of matching source
102 to LNA 702 is generally the same as in FIG. 1. In the
single-ended embodiment of FIG. 7 the output signal (e.g.,
V.sub.OUT or I.sub.OUT) is converted to a differential signal
within the amplifier by single-to-differential converter 704. Also,
inductor 706 connected to the source of transistor 122 does not
have a center tap. In a single-ended amplifier, the matching
networks operate in a manner substantially similar to the matching
network in a differential amplifier.
[0060] A low noise amplifier in accordance with one or more
embodiments provides an input impedance match for a source (e.g.,
for maximum power transfer) as well as amplifies the signal without
adding too much noise or distortion, while consuming minimal power.
By tuning a matching network between the source and the input of
the LNA to reflect energy outside of a selected frequency sub-band,
the selectivity of the LNA can be increased, and the noise in the
amplifier can be decreased.
[0061] Although the invention is described herein with reference to
specific embodiments, various modifications and changes can be made
without departing from the scope of the present invention as set
forth in the claims below. For example, while the techniques and
apparatus for conjugately matching the input impedance of the LNA
to the output impedance of the source may vary widely, one or more
embodiments can be used in a wireless receiver, or other similar
low-power signal amplifier. Accordingly, the specification and
figures are to be regarded in an illustrative rather than a
restrictive sense, and all such modifications are intended to be
included within the scope of the present invention. Any benefits,
advantages, or solutions to problems that are described herein with
regard to specific embodiments are not intended to be construed as
a critical, required, or essential feature or element of any or all
the claims.
[0062] Unless stated otherwise, terms such as "first" and "second"
are used to arbitrarily distinguish between the elements such terms
describe. Thus, these terms are not necessarily intended to
indicate temporal or other prioritization of such elements.
* * * * *