MULTI-BAND HIGH-EFFICIENCY DOHERTY AMPLIFIER
BACKGROUND
Wireless communication standards are continuously changing
in order to adapt to the high volume data transfer required today
between consumers. As a result, operators of wireless communication
systems struggle with extra costs that result from upgrading the
wireless system or the complete replacement of the already deployed
sites. Also, the continuously changing of wireless communication
standards disturbs base station vendors in their product strategy and
portfolio. Multi-Standard or Multi-Band Radio Base Station is a
solution that may reduce the deployment cost for the operators as well
as the production cost for the network infrastructure component
vendors. Particularly, a transceiver power amplifier is one of the most
critical components in a wireless base station. Today's Power
Amplifier is not only required to be highly efficient in order to keep the
base station Operating Expense (OPEX) low but must be also
broadband enough to operate in a Multi-band system.
Because of its simplicity, the Doherty Power Amplifier (PA) is the
most efficient architecture that is used today to efficiently transmit
high peak to average ratio modulated signals. However, the Doherty
PA is inherently narrowband and its performance is significantly
reduced when used in a multi-band transceiver. In fact, the radio
frequency bandwidth (BW) of the Doherty PA is not only limited by the
individual stage BW (Main stage and Peak stage) and the output
quarter-line combiner, but also the BW of the Doherty PA is limited by
the inherent phase linearity of the transmission lines that constitute
the matching networks and/ or the offset lines used at the output of
the main and peak stages.
For example, the conventional Doherty amplifier uses two
amplifier stages, the main amplifier and the peak amplifier, with their
outputs interconnected by a quarter wavelength impedance inverter.
The Doherty amplifier maintains its maximum efficiency at power
back-off with the help of the peak amplifier and the quarterwavelength
transformer. At power back-off, the peak amplifier is
intentionally kept OFF to allow the saturation of the main amplifier by
doubling its load impedance through the impedance transformer. The
BW of the Doherty amplifier is related to the BW of the individual
stages and to the output quarter wavelength transformer BW. Also, at
average operating power, the Doherty BW is narrower than expected
even with using broadband main and peaking stages. This is due to
the fact that the matching network transmission line phase and/ or
the offset lines phase are inherently linear over frequency, which
inherently limits the BW of the Doherty Amplifier.
This significantly degrades the efficiency and the power at the
frequency band edges. The same applies for the Peaking OFF
impedance, which is real and high at only one frequency (good
isolation only at mid-band). Hence, this drawback limits the Doherty
high efficiency operating BW to a max of 5-7 %. The effect of the
output quarter wavelength inverter on the BW starts to be significant
for BW higher than 10%. To be able to be used in a multi- standard
transceiver, the Doherty power amplifier is required to be tunable over
the whole frequency bands of interest.
SUMMARY
The present invention relates to a Multi-Band Doherty amplifier.
Embodiments of the present invention provide an amplifying structure
including a main amplifier configured to amplify a first signal, a peak
amplifier configured to amplify a second signal, a tunable impedance
inverter configured to perform impedance inversion to modulate a load
impedance of the main amplifier, and a combining node configured to
receive the amplified second signal from the peak amplifier and an
output of the tunable impedance inverter. The tunable impedance
inverter includes a tuner configured to tune the impedance inversion
over at least one broad frequency band. The tuner is (i) at least one
capacitor, (i) at least one varactor, or (ii) at least one open stub
shunted by a diode.
The amplifying structure further includes power splitter
configured to receive an input signal having the at least one frequency
band and split the input signal into the first signal and the second
signal.
The tunable impedance inverter may further include a circulator
connected to the tuner configured to split the amplified first signal
into incident signals and reflected signals, where the tuner adjusts a
phase of the reflected signals for destructive combining at an input of
the tunable impedance inverter. Alternatively, the tunable impedance
inverter includes a first hybrid coupler connected to the tuner. The
amplifying structure may further include a controller configured to
control the tunable impedance inverter by adjusting values of the
tuner.
According to another embodiment of the present invention, the
amplifying structure includes a main amplifier configured to amplify a
first signal, a peak amplifier configured to amplify a second signal, a
tunable impedance inverter configured to perform impedance
inversion to modulate a load impedance of the main amplifier, where
the tunable impedance inverter includes a circulator configured to
split the amplified first signal into incident signals and reflected
signals and a tuner configured to adjust a phase of the reflected
signals at an input of the impedance inverter. The structure also
includes a combining node configured to receive the amplified second
signal from the peak amplifier and an output of the tunable
impedance inverter. The tuner is a capacitor, varactor, or an open
stub shunted by a diode.
The circulator includes a first port connected to an output
terminal of the main amplifier, a second port connected to the
combining node, and a third port connected to the tuner. The
amplifying structure may further include a controller configured to
control the tunable impedance inverter by adjusting values of the
tuner.
In another embodiment of the present invention, the amplifying
structure includes a main amplifier configured to amplify a first signal,
a peak amplifier configured to amplify a second signal, a tunable
impedance inverter configured to perform impedance inversion to
modulate a load impedance of the main amplifier, where the tunable
impedance inverter includes a first hybrid coupler and a tuner, the
tuner being configured to tune the impedance inversion over at least
one broad frequency band, and a combining node configured to
receive the amplified second signal from the peak amplifier and an
output of the first hybrid coupler. The tuner includes two tuning
elements and each of the two tuning elements is connected to a
respective output port of the first hybrid coupler. The two tuning
elements are (i) capacitors, (ii) varactors, or (iii) open stubs shunted by
diodes.
The amplifying structure further includes a controller
configured to control the tunable impedance inverter by adjusting
values of the tuner. Also, the amplifying structure further includes a
first offset line and a second offset line. The first offset line and the
second offset line provides connectivity between the first hybrid
coupler and the main amplifier, if selected. The first offset line has a
length tuned for a first frequency band and the second offset line has
a length tuned for a second frequency band.
One of the first offset line and the second offset line is selected
based on manually moving a DC block capacitor of the main amplifier.
Alternatively, a first selector is configured to select between the first
offset line and the second offset line.
The amplifying structure further includes a third offset line and
a fourth offset line. The third offset line and the fourth offset line
provides connectivity between the first hybrid coupler and the peak
amplifier, if selected. The third offset line has a length tuned for the
first frequency band and the fourth offset line has a length tuned for
the second frequency band.
One of the third offset line and the fourth offset line is selected
based on manually moving a DC block capacitor of the peak amplifier.
Alternatively, a second selector is configured to select between the
third offset line and the fourth offset line.
The amplifying structure may further include a second hybrid
coupler connected between the main amplifier and the first hybrid
coupler, and a third hybrid coupler connected between the combining
node and the peak amplifier. The output ports of the first and second
hybrid couplers are connected to tuning elements. The tuning
elements of the second and third hybrid couplers are (i) capacitors, (ii)
varactors, or (iii) open stubs shunted by diodes.
In another embodiment of the present invention, a Doherty
amplifier includes a tunable impedance inverter configured to tune
impedance inversion to a center frequency of a signal for amplification,
and a controller configured to control the tunable impedance inverter.
BRIEF DESCRIPTION OF THE DRAWINGS
Example embodiments will become more fully understood from
the detailed description given herein below and the accompanying
drawings, wherein like elements are represented by like reference
numerals, which are given by way of illustration only and thus are not
limiting of the present invention, and wherein:
FIG. 1 illustrates a Doherty amplifier structure according to an
embodiment of the present invention;
FIG. 2 illustrates a Doherty amplifier structure according to
another embodiment of the present invention;
FIG. 3 illustrates a variation of the Doherty amplifier structure
of FIG. 2 according to an embodiment of the present invention; and
FIG. 4 illustrates a variation of the Doherty amplifier structure
of FIGS. 2 and 3 according to another embodiment of the present
invention.
DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS
Various embodiments of the present invention will now be
described more fully with reference to the accompanying drawings.
Like elements on the drawings are labeled by like reference numerals.
As used herein, the singular forms "a", "an", and "the" are
intended to include the plural forms as well, unless the context clearly
indicates otherwise. It will be further understood that the terms
"comprises", "comprising,", "includes" and/or "including", when used
herein, specify the presence of stated features, integers, steps,
operations, elements, and/or components, but do not preclude the
presence or addition of one or more other features, integers, steps,
operations, elements, components, and/ or groups thereof.
The present invention will now be described with reference to
the attached figures. Various structures, systems and devices are
schematically depicted in the drawings for purposes of explanation
only and so as not to obscure the present invention with details that
are well known to those skilled in the art. Nevertheless, the attached
drawings are included to describe and explain illustrative examples of
the present invention. The words and phrases used herein should be
understood and interpreted to have a meaning consistent with the
understanding of those words and phrases by those skilled in the
relevant art. To the extent that a term or phrase is intended to have a
special meaning, i.e., a meaning other than that understood by skilled
artisans, such a special definition will be expressly set forth in the
specification that directly and unequivocally provides the special
definition for the term or phrase.
Embodiments of the present invention overcome the deficiencies
of the conventional Doherty amplifiers when used in broadband
applications by providing a tunable impedance inverter. The tunable
impedance inverters of the present invention are configured to perform
impedance inversion to modulate a load impedance of the main
amplifier. The tunable impedance inverter includes a tuner
configured to dynamically tune the impedance inversion over at least
one broad frequency band. For instance, the tunable impedance
inverter tunes the impedance inversion to a center frequency of a
signal for amplification. The tunable aspect of the impedance inverter
may be controlled by a controller to allow for dynamic adjustments
dependent on a frequency of an input signal, which may range over of
a broadband signal and/ or multiband signal.
The Doherty amplifiers of the present invention may be
embodied in a base station in a wireless communication system that
provides wireless connectivity to a number of end uses. The Doherty
amplifiers may amplify signals to be transmitted to the end uses.
Further, the Doherty amplifiers of the present invention may be
embodied in other types of devices such as W-CDMA, UMTS, LTE or
WiMAX base stations, base transceiver stations, base station routers,
WiFi access points, or any other device that provides the radio
baseband functions for data and/ or voice connectivity between a
network and one or more end users. The end users may include but
are not limited to end user (EU) equipment, fixed or mobile subscriber
units, receivers, cellular telephones, personal digital assistants (PDA),
personal computers, or any other type of user device capable of
operating in a wireless environment.
The Doherty amplifiers of the present invention include a
tunable circulator-based impedance inverter or a tunable hybrid
coupler operating as an impedance inverter that includes tuning
elements. These embodiments are discussed with reference to FIGS.
1-4 of the present application.
FIG. 1 illustrates a Doherty amplifier structure according to an
embodiment of the present application. The Doherty amplifier
structure includes a power splitter 105 configured to split an input
signal into a first signal and a second signal, a main amplifier 110 for
amplifying the first signal, a peak amplifier 115 for selectively
amplifying the second signal, a phase compensator 125 for shifting a
phase of the second signal, a tunable impedance inverter 120
configured to perform impedance inversion, a quarter- wavelength
transformer 130 for combining the output of the main amplifier 110
and the peak amplifier 115, and a load impedance 135.
The power splitter 105 may be a 3dB power splitter, for example,
and includes at least three ports. The 3dB power splitter generally
divides the input signal into first and second signals having equal
amplitudes. In addition, other types of power splitters apart from the
3dB power splitter may be used within the Doherty amplifier of FIG. 1.
A first port of the power splitter 105 receives an input signal. The
input signal maybe a radio-frequency (RF) signal having a frequency
over a broad frequency band (e.g., 860-960 MHz). A second port of
the power splitter 105 is connected to an input of the main amplifier
110, and a third port of the power splitter 105 is connected to an
input of the peak amplifier 115 via the phase compensator 125. The
phase compensator 125 is connected between the third port of the
power splitter 105 and the input of the peak amplifier 115 in order to
compensate for the phase change introduced by the tunable
impedance inverter 120. The phase compensator 125 may be a
circulator.
After the input signal is split into the first and second signals
and the second signal passes through the phase compensator 125, the
first and second signals are then amplified, respectively, by the main
amplifier 110, or the combination of the main amplifier 110 and the
peak amplifier 115, as discussed below.
For instance, the peak amplifier 115 is selectively operable to
operate at selected times in combination with the main amplifier 110.
That is, the peak amplifier 115 may b e kept off until power
requirements call for a higher power output from the main amplifier
110, at which time the peak amplifier 115 is turned on and operates
to increase the power output of the main amplifier 110. In other
words, the peak amplifier 115 amplifies the second signal if the signal
strength of the second signal is above a threshold level. The term
"selectively operable" indicates the amplifier operational state changes
in response to the input signal. Otherwise, if the signal strength of
the second signal is below the threshold level, the peak amplifier 115
is turned OFF (e.g., open circuit), and only the main amplifier 110
operates to amplify the first signal. Once the signal strength of the
second signal increases above the threshold level, the peak amplifier
115 is turned on, and the load impedance for the main amplifier 110
gradually decreases to the designed level.
The output of the main amplifier 110 and the peak amplifier
115 are connected at a combining node 116 via the tunable
impedance inverter 120. If the peak amplifier 115 and the main
amplifier 110 are operating, the combining node 116 receives the
amplified second signal from the peak amplifier 115 and the output of
the tunable impedance inverter 120. If only the main amplifier 110 is
operating (as discussed above), the combining node 116 receives the
signal from the output of the tunable impedance inverter 120.
The tunable impedance inverter 120 receives the amplified first
signal from the main amplifier 110 and performs impedance inversion
to modulate a load impedance of the main amplifier 110. Impedance
inversion in quarter-wavelength transformers (e.g., conventional
Doherty amplifiers) is accomplished by the destructive combining of
reflected signals on the input side of the quarter-wavelength
transformer and constructive combining of transmitted signals on the
output side of the quarter- wavelength transformer. However, for
signals that deviate from the center frequency that the conventional
Doherty amplifier was designed for, the destructive and constructive
combining becomes less perfect, and the impedance inversion
properties of the transformer degrade (thus limiting the bandwidth of
operation). Furthermore, changing either impedance or the length of
the transformer cannot be accomplished dynamically.
In contrast, embodiments of the present invention provide a
tunable impedance inverter to tune the impedance inversion over a
broad frequency range. In one embodiment, as shown in FIG. 1, the
tunable impedance inverter 120 includes a circulator 122 and a tuner
which uses capacitors as the tuner 12 1. The tuner 12 1 may include
any type of capacitors including but not limited to fixed capacitors and
variable capacitors such as a digital capacitors, for example. In
addition, varactors (diode) may be used in place of the fixed or variable
capacitors. Values of the capacitors tune the tunable impedance
inverter 120 in a targeted operating bandwidth. The values of the
tuner 12 1 may be selected manually for every frequency band of
operation. In addition, variable capacitors (e.g., a digital capacitors)
may be used as the tuning elements 121 in order to dynamically tune
the tunable impedance inverter 120 by varying the capacitor bias.
The circulator 122 provides separate paths for incident and
reflected waves of the first signal, which allows for phase adjustment
of reflected waves to be independent to that of the incident waves.
Incident waves are propagating from the output of the main amplifier
110 (source) towards the load impedance. On the other hand, the
reflected waves are propagating from the load impedance towards to
the output of the main amplifier 110 . In a quarter-wavelength
transformer, the incident and the reflected waves bounce between the
two interfaces and both the incident and reflected waves share the
same propagation path and thus undergo the same phase shift.
The circulator 122 includes at least three ports. The first port of
the circulator 122 is connected to the output of the main amplifier
110, the second port is connected to the combining node 116, and the
third port is connected to the tuning element 12 1. The proper
alignment of the reflections at the input side of the tunable impedance
inverter 120 provide for correct operation of the Doherty amplifier of
FIG. 1. This is accomplished by the capacitor-based tuner 12 1 that
terminates the third port of the circulator 122. The values of the
capacitors within the tuner 12 1 set the phase of the reflected waves
which sets the frequency at which the impedance inversion is
performed. In the case of using fixed capacitors within the tuner 12 1,
a fixed capacitor values for every frequency band of operation will
allow a static tuning of the impedance inverter. In the case of using
variable capacitors such the digital capacitors as the tuner 12 1, the
digital capacitors allows a dynamic tuning of the impedance inversion.
Because the third port of the circulator 122 is terminated with a tuner
12 1 consisting of fixed value capacitors or variable capacitors, the
Doherty amplifier of FIG. 1 respectively allows for static or dynamic
phase tuning of the reflected waves in order to achieve destructive
combining of the reflected waves at the input side of the tunable
impedance inverter 120, and thus obtain proper impedance inversion
at the frequency band of interest. This impedance inversion operation
can be performed over the entire frequency bandwidth of the
circulator 122.
In the case of using a fixed capacitor values as the tuner 12 1,
the optimum capacitor values that correspond to the target operating
frequency band are added manually in the Power Amplifier board. In
the case of using variable capacitors as the tuner 12 1, the optimum
capacitor values may be dynamically adjusted by the controller 123.
For instance, the controller 123 is configured to control the variable
capacitors in the tuner 121 by adjusting capacitor values in order to
allow for dynamic phase tuning of the reflected waves. The controller
123 includes a processor and memory, where the memory stores
frequencies that map to different capacitance values. The controller
123 transmits control signals to the variable capacitors and sets the
optimum capacitor values suitable for the center frequency of the
frequency band of interest.
The signal from the output of the tunable impedance inverter
120 and the amplified second signal from the peak amplifier 115 are
combined via the quarter-wavelength transformer 130, and the
combination is supplied to the load impedance 135.
FIG. 2 illustrates a Doherty amplifier structure according to
another embodiment of the present invention. Similar to the Doherty
amplifier structure of FIG. 1, the Doherty amplifier structure of FIG. 2
includes the main amplifier 110 for amplifying the first signal, the
peak amplifier 115 for amplifying the second signal, the quarterwavelength
transformer 130 for transforming the combining node
impedance (e.g., 25 ) to the load impedance 135 of 50. These
components operate in a similar manner as discussed above.
However, the Doherty amplifier structure of FIG. 2 uses a hybrid
coupler 205 as a power splitter for splitting the input signal into the
first signal and the second signal. Among other types of hybrid
couplers, the hybrid coupler 205 may be a broadband 3dB hybrid
coupler, for example. The hybrid coupler 205 includes four ports. An
input port of the hybrid coupler 205 receives the input signal, an
isolation port is coupled to a load 202 (e.g., 50), a first output port is
connected to the input of the main amplifier 110, and a second output
port is connected to the input of the peaking amplifier 115.
Similar to the Doherty amplifier structure of FIG. 1, the main
amplifier 110 amplifies the first signal and the peak amplifier
amplifies the second signal. In this embodiment, a hybrid coupler 22 1
operates as the tunable impedance inverter 220 to modulate the load
of the main amplifier 110, which is discussed later in the specification.
The output of the main amplifier 110 is connected to an input
port of the tunable impedance inverter 220 via a first offset line circuit
2 15-1 , and the output of the peak amplifier 115 is connected to an
isolation port of the tunable impedance inverter 220 via a second
offset line circuit 2 15-2. The first offset line circuit 2 15-1 and the
second offset line circuit 2 15-2 allows the Doherty amplifier circuit of
FIG. 2 to handle a wider bandwidth and/or a dual frequency band.
For Multiband Doherty application, multiple offset lines are required.
According to an embodiment, a first offset line 2 17-1 in parallel
with a second offset line 2 17-2 provides connectivity between the main
amplifier 110 and the tunable impedance inverter 220. For instance,
one of the first offset line 2 17-1 and the second offset line 2 17-2 is
selected to provide connectivity between the output of the main
amplifier 110 and the tunable impedance inverter 220. The first offset
line 2 17-1 has a length tuned for a first frequency band, while the
second offset line 2 17-2 is tuned for a second frequency band. The
first frequency band may be different from the second frequency band.
Similarly, a third offset line 2 17-3 in parallel with a fourth offset line
2 17-4 provides connectivity between the peak amplifier 115 and the
tunable impedance inverter 220. For instance, one of the third offset
line 2 17-3 and the fourth offset line 2 17-4 is selected to provide
connectivity between the output of the peak amplifier 115 and the
tunable impedance inverter 220. The third offset line 2 17-3 has a
length tuned for the first frequency band, while the fourth offset line
2 17-4 is tuned for the second frequency band, or vice versa.
In another embodiment, the first offset line 2 17-1 is tuned for
one portion of the frequency broadband (e.g., higher portion), and the
second offset line 2 17-2 is tuned for another portion of the frequency
broadband (e.g., a lower portion). The third offset line 2 17-3 is tuned
for one portion of the frequency bandwidth, which is the same tuned
bandwidth for either the first offset line 2 17-1 or the second offset line
2 17-2, and the fourth offset line 2 17-4 is tuned for another portion of
the frequency bandwidth, which is the same tuned bandwidth for the
other offset line different from the third offset line 2 17-3.
Switching between the first offset line 2 17-1 and the second
offset line 2 17-2 and between the third offset line 2 17-3 and the
fourth offset line 2 17-4 is performed manually by moving an output
DC block capacitor or automatically by selector circuits. FIG. 2
illustrates an embodiment of automatically switching between the
offset lines 2 17, which is described below. FIG. 3 illustrates an
embodiment of manually switching between the offset lines 2 17,
which is described later in the specification.
As shown in FIG. 2, the first offset line circuit 2 15-1 includes a
first selector 2 16-1 to automatically select between the first offset line
2 17-1 and the second offset line 2 17-2. For example, the first selector
2 16-1 automatically selects either the first offset line 2 17-1 or the
second offset line 2 17-2 to provide connectivity between the main
amplifier 110 and the tunable impedance inverter 220 depending on
the frequency of the input signal.
Similarly, the second offset line circuit 2 15-2 includes a second
selector 2 16-2 to switch between a third offset line 2 17-3 and a fourth
offset line 2 17-4. The second selector 2 16-2 automatically selects
either the third offset line 2 17-3 or the fourth offset line 2 17-4 to
provide connectivity between the peak amplifier 115 and the tunable
impedance inverter 220 depending on the frequency of the input
signal.
The first and the second selectors 2 16 may include high power
switching PIN diodes having low series ON resistor (low insertion loss),
and a low reverse capacitor (high isolation during the OFF state of the
diode) arranged in a manner to allow switching between the first offset
line 2 17-1 and the second offset line 2 17-2, or switching between the
third offset line 2 17-3 and the fourth offset line 2 17-4.
FIG. 3 illustrates a variation of the Doherty amplifier structure
of FIG. 2 according to an embodiment of the present application. The
Doherty amplifier structure of FIG. 3 is the same as the amplifier
structure of FIG. 2 except the first and second selectors 2 16 are
removed, and DC block capacitors 2 19 are illustrated. For instance,
one of the first offset line 2 17-1 and the second offset line 2 17-2 is
selected based on manually moving the DC block capacitors 2 19-1
and 2 19-2 on the selected offset line. Also, one of the third offset line
2 17-3 and the fourth offset line 2 17-4 is selected based on manually
moving the DC block capacitors 2 19-3 and 2 19-4 on the selected
offset line. In other words, switching between two offset lines may be
performed manually by positioning the output DC block capacitors
2 19 on the selected offset line.
Referring to FIGS. 2 or 3, the tunable impedance inverter 220
receives signals from the first offset line circuit 2 15-1 and/or the
second offset line circuit 215-2, and performs impedance inversion to
modulate a load impedance of the main amplifier, as discussed below.
The tunable impedance inverter 220 includes the hybrid coupler 22 1
and at least two capacitors 222 operating a s a tuner to tune the
impedance inversion over at least one broad frequency band. The two
capacitors may be digital capacitors, fixed capacitors, or varactors as
described with reference to FIG. 1. In an alternative embodiment, the
tunable impedance inverter 220 may include connecting the output
ports of the hybrid coupler 22 1 to shunt PIN Diodes with the same
values in conjunction with open stubs.
By terminating two outputs of the hybrid coupler 22 1 by the
same fixed capacitor value, the same digital capacitors, the same
varactors or the same shunt PIN Diodes in conjunction with open
stubs, the 90 degree phase of the hybrid coupler 22 1 can be ensured
over the entire frequency band. In fact, terminating the two output
ports of the hybrid coupler 22 1 by a capacitor (open circuit) reflects all
the incident waves coming from the input of the hybrid coupler 22 1
toward its fourth isolated port which becomes its main output port.
Moreover, terminating this isolated port with a 25 Ohm load (instead
of 50 Ohm) double the impedance of the input port of the hybrid
coupler 22 1, which makes the resulted circuit reacting exactly as an
impedance inverter. Hence, the 2xRL (100 Ohm) termination of the
Main stage is provided with a minimum insertion loss (2 x IL of the
coupler) when the peak amplifier 115 is OFF.
The value of the capacitors 222 sets the phase of the reflected
wave a t the desired frequency band which makes the resulted
impedance transformer tunable over frequency. The resulted tunable
impedance inverter 220 is able to operate over a multiple of
frequencies. The tunable impedance inverter 220 is connected
between the output side of the first offset line circuit 2 15-1 and a
combining node 116 located between the output side of the second
offset line circuit 2 15-2 and the quarter-wavelength transformer 130.
Similar to the tunable impedance inverter 120 of FIG. 1, the hybrid
coupler 22 1 is configured to perform impedance inversion to modulate
a load impedance of the main amplifier 110.
The hybrid coupler 22 1 includes four ports. An input port of
the hybrid coupler 22 1 is connected to the output side of the first
offset line circuit 2 15-1 , an isolation port is connected to the
combining node 116, and a first output port is connected to a first
variable capacitor 222-1 , and a second output port is connected to a
second capacitor 222-2. The first variable capacitor 222-1 and the
second capacitor 222-2 operate as a tuner to tune the impedance
inversion over a broad frequency band. As indicated above, both the
first and second output ports of the hybrid coupler 22 1 are terminated
by the same fixed capacitor value. By adjusting the values of the first
and second capacitors 222, the impedance inversion is tuned over a
broad frequency band. Furthermore, a controller 223 may control the
adjustment of values for the first and second variable capacitors 222
in a manner similar to the controller 123 of FIG. 1. For instance, the
controller 223 includes the frequency-capacitance value map as
described above. The controller 223 controls the capacitance values of
the first variable capacitor 222-1 and the second variable capacitor
222-2 based on the frequency-capacitance value map and the
frequency of the input signal. The values of the first and second
variable capacitors are typically the same with respect to each other,
but vary together in order to control the impedance inversion.
Similar to FIG. 1, the output of the tunable impedance inverter
220 and the amplified second signal from the peak amplifier 115 are
combined via the quarter-wavelength transformer 130, and the
combination is supplied to the load impedance 135.
FIG. 4 illustrates a variation of the Doherty amplifier structure
of FIG. 2 according to an embodiment of the present application. The
Doherty amplifier structure of FIG. 4 is the same as FIGS. 2 and 3
except a first hybrid coupler 3 15-1 is used in place of the first offset
line circuit 2 15-1 and a second hybrid coupler 3 15-2 is used in place
of the second offset line circuit 2 15-2.
The first hybrid coupler 3 15-1 includes at least four ports. An
input port of the first hybrid coupler 315-1 is connected to the output
of the main amplifier 110, an isolation port is connected to the input
port of the tunable impedance inverter 220, and first and second
output ports are terminated via a third capacitor 222-3 and a fourth
capacitor 222-4. The second hybrid coupler 3 15-2 is similar to the
first hybrid coupler 3 15-1 . For instance, the second hybrid coupler
315-2 includes at least four ports. An isolation port of the second
hybrid coupler 3 15-2 is connected to the output of the peak amplifier
115, an input port is connected to the node 116, and first and second
output ports are terminated via a third variable capacitor 222-3 and a
fourth variable capacitor 222-4.
In this embodiment, because the isolation ports of the hybrid
couplers 3 15-1 and 3 15-2 are terminated with 50 Ohm impedances,
the hybrid couplers 315-1 and 315-2 are not performing impedance
transformation. Rather, the hybrid couplers 3 15-1 and 3 15-2 are
performing a phase shifting operation. The same as for the hybrid
coupler 22 1, the phase shift introduced is related to the capacitor
values that load the 2 output ports of the two hybrid couplers 3 15-1
and 315-2. Hence, the hybrid couplers 315-1 and 315-2 react as
tunable offset lines that will allow moving the load of the main
amplifier 110 into a highest efficiency area on the 2 :1 VSWR circle,
while tuning a phase of the peak amplifier 115 to provide a good
OPEN circuit, respectively.
Also, the capacitors 222 of the first and second hybrid couplers
3 15 may be controlled by the controller 323, which operates in a
similar manner with respect to the controller 223 of FIG. 2. Besides
controlling the values of the first and second capacitors 222 of the
tunable impedance inverter 220 in a manner described above, the
controller 323 also controls the capacitance values of the capacitors
222 of the first and second hybrid couplers 3 15. Because the rest of
the components of FIG. 4 operate in a similar manner as described
with reference to FIGS. 2 and 3, the details of these components are
omitted for the sake of brevity.
What is claimed:
1. An amplifying structure, comprising:
a main amplifier (1 10) configured to amplify a first signal;
a peak amplifier (1 15) configured to amplify a second signal;
a tunable impedance inverter (120, 220) configured to perform
impedance inversion to modulate a load impedance of the main
amplifier, the tunable impedance inverter including a tuner (12 1, 222)
configured to tune the impedance inversion over at least one broad
frequency band; and
a combining node (1 16) configured to receive the amplified
second signal from the peak amplifier and an output of the tunable
impedance inverter.
2. The amplifying structure of claim 1, further comprising:
power splitter (105) configured to receive an input signal having
the at least one frequency band and split the input signal into the first
signal and the second signal.
3. The amplifying structure of claim 1, wherein the tunable
impedance inverter further includes:
a circulator (122) connected to the tuner configured to split the
amplified first signal into incident signals and reflected signals,
wherein the tuner adjusts a phase of the reflected signals for
destructive combining at an input of the tunable impedance inverter.
4. The amplifying structure of claim 1, further comprising:
a controller (123, 223, 323) configured to control the tunable
impedance inverter by adjusting values of the tuner.
5. The amplifying structure of claim 1, wherein the tuner is (i) at least
one capacitor, (i) at least one varactor, or (ii) at least one open stub
shunted by a diode.
6. The amplifying structure of claim 1, wherein the tunable
impedance inverter includes a first hybrid coupler (22 1) connected to
the tuner.
7. An amplifying structure, comprising:
a main amplifier (1 10) configured to amplify a first signal;
a peak amplifier (1 15) configured to amplify a second signal;
and
a tunable impedance inverter (120, 220) configured to perform
impedance inversion to modulate a load impedance of the main
amplifier, the tunable impedance inverter including a circulator (12 1)
configured to split the amplified first signal into incident signals and
reflected signals and a tuner (122) configured to adjust a phase of the
reflected signals at an input of the impedance inverter; and
a combining node (1 16) configured to receive the amplified
second signal from the peak amplifier and an output of the tunable
impedance inverter.
8. The amplifying structure of claim 7, wherein the tuner is a
capacitor, varactor, or an open stub shunted by a diode.
9. The amplifying structure of claim 7, wherein the circulator
includes:
a first port connected to an output terminal of the main
amplifier;
a second port connected to the combining node; and
a third port connected to the tuner.
10. The amplifying structure of claim 7, further comprising:
a controller (123) configured to control the tunable impedance
inverter by adjusting values of the tuner.