PHASE COMPENSATION FOR EFFICIENCY IMPROVEMENT FOR RF POWER AMPLIFIER

A method for amplifying an RF signal is provided. The method includes providing the RF signal to an input of a quadrature coupler. The method includes outputting, from the quadrature coupler, a carrier path signal and a peak path signal. The method includes amplifying the carrier path signal to provide an amplified carrier path signal and amplifying the peak path signal to provide an amplified peak path signal. The method includes generating a first neutralizing signal based on the carrier path signal and generating a second neutralizing signal based on the peak path signal. The method includes modifying the amplified peak path signal based on the first neutralizing signal to provide a neutralized peak path signal and modifying the amplified carrier path signal based on the second neutralizing signal to provide a neutralized carrier path signal. The method also includes combining the neutralized carrier path and peak path signals.

BACKGROUND

In wireless communication devices, a radio frequency (RF) front end module processes modulated RF signals that are received from an antenna or to be transmitted by an antenna. In the transmission path, an RF front end module often has an RF power amplifier having one or more stages of power amplification that amplify the power of the RF signals to a level suitable for transmission. The performance of an RF power amplifier can be evaluated by a variety of metrics, such as gain, efficiency, linearity, and harmonic leakage.

SUMMARY

The subject matter disclosed herein relates to techniques for improving the efficiency, e.g., power added efficiency (PAE), of a RF power amplifier, such as a Doherty power amplifier. An RF power amplifier implemented according to the disclosed techniques utilizes neutralization paths between a carrier path and a peak path of a Doherty amplifier to offset parasitic capacitance in the circuit and thereby achieve high gain at the output stage. The increased output stage gain can improve the overall PAE of the RF power amplifier such that fewer driver stages are needed. The reduced number of driver stages can lead to a reduced size of the RF power amplifier circuit.

In general, in some aspects, the subject matter of the present disclosure can be embodied in an electronic circuit, such as a power amplifier circuit of an RF front end circuit. The electronic circuit includes: a quadrature coupler arranged to receive an RF signal and configured to output a carrier path signal and a peak path signal; a carrier amplification circuit arranged to receive the carrier path signal and configured to generate an amplified carrier path signal; a peak amplification circuit arranged to receive the peak path signal and configured to generate an amplified peak path signal; a first neutralization circuit arranged between an input of the carrier amplification circuit and an output of the peak amplification circuit; a second neutralization circuit electrically arranged between an input of the peak amplification circuit and an output of the carrier amplification circuit; and a combiner circuit. The first neutralization circuit is configured to generate a first neutralizing signal and modify the amplified peak path signal based on the first neutralizing signal to obtain a neutralized peak path signal. The second neutralization circuit is configured to generate a second neutralizing signal and modify the amplified carrier path signal based on the second neutralizing signal to obtain a neutralized carrier path signal. The combiner circuit is configured to combine the neutralized carrier path signal and the neutralized peak path signal.

In some implementations, the quadrature coupler is configured to offset the carrier path signal from the peak path signal by +90° in phase, the first neutralization circuit includes a +90° phase shifter, and the second neutralization circuit includes a −90° phase shifter. Alternatively, the quadrature coupler is configured to offset the carrier path signal from the peak path signal by −90° in phase, the first neutralization circuit includes a −90° phase shifter, and the second neutralization circuit includes a +90° phase shifter.

In some implementations, the first neutralization circuit includes a first neutralization capacitor, and the second neutralization circuit includes a second neutralization capacitor.

In some implementations, a first capacitance value of the first neutralization capacitor and a second capacitance value of the second neutralization capacitor are determined according to at least one of: a frequency of the RF signal, a device size of the electronic circuit, an amplification gain of the electronic circuit, or a stability of the electronic circuit.

In some implementations, the combiner circuit includes a quarter-wave length phase shifter connected in series with either the peak amplification circuit or the carrier amplification circuit according to a type of combination, the type including at least one of voltage combination or current combination.

In some implementations, the electronic circuit further includes a driver circuit. The driver circuit includes an input matching circuit and at least one stage of power amplification. The input matching circuit is arranged to receive an input signal. The driver circuit is configured to generate the RF signal by amplifying the input signal using the at least one stage of power amplification.

In some implementations, wherein each stage of the at least one stage of power amplification includes a corresponding power amplifier and a corresponding interstage matching circuit connected in series.

In some implementations, the carrier amplification circuit includes one or more carrier amplification bipolar junction transistors (BJTs), and the peak amplification circuit includes one or more peak amplification BJTs.

In some implementations, the electronic circuit further includes an output matching circuit electrically coupled the combiner circuit.

In some aspects, the subject matter of the present disclosure can be embodied in a method for amplifying an RF signal. The method includes providing the RF signal to an input of a quadrature coupler. The method includes outputting, from the quadrature coupler, a carrier path signal and a peak path signal. The method includes amplifying the carrier path signal to provide an amplified carrier path signal and amplifying the peak path signal to provide an amplified peak path signal. The method includes generating a first neutralizing signal based on the carrier path signal and generating a second neutralizing signal based on the peak path signal. The method includes modifying the amplified peak path signal based on the first neutralizing signal to provide a neutralized peak path signal and modifying the amplified carrier path signal based on the second neutralizing signal to provide a neutralized carrier path signal. The method also includes combining the neutralized carrier path and peak path signals.

In some implementations, the carrier path signal leads the peak path signal by 90°. Generating the first neutralizing signal includes connecting a +90° phase shifter and a first neutralization capacitor in series to obtain a first neutralization circuit, and providing the carrier path signal to the first neutralization circuit to obtain the first neutralizing signal. Generating the second neutralizing signal includes connecting a −90° phase shifter and a second neutralization capacitor in series to obtain a second neutralization circuit, and providing the peak path signal to the second neutralization circuit to obtain the second neutralizing signal. Modifying the amplified peak path signal includes coupling the first neutralizing signal to the amplified peak path signal. Modifying the amplified carrier path signal includes coupling the second neutralizing signal to the amplified carrier path signal.

In some implementations, the peak path signal leads the carrier path signal by 90°. Generating the first neutralizing signal includes connecting a −90° phase shifter and a first neutralization capacitor in series to obtain a first neutralization circuit, and providing the carrier path signal to the first neutralization circuit to obtain the first neutralizing signal. Generating the second neutralizing signal includes connecting a +90° phase shifter and a second neutralization capacitor in series to obtain a second neutralization circuit, and providing the peak path signal to the second neutralization circuit to obtain the second neutralizing signal. Modifying the amplified peak path signal includes coupling the first neutralizing signal to the amplified peak path signal. Modifying the amplified carrier path signal includes coupling the second neutralizing signal to the amplified carrier path signal.

In some implementations, the method further includes determining a first capacitance value of the first neutralization capacitor and a second capacitance value of the second neutralization capacitor according to at least one of: a frequency of the RF signal, a device size, an amplification gain, or a circuit stability.

In some implementations, combining the neutralized carrier path signal and the neutralized peak path signal includes: phase-shifting, using a quarter-wave length phase shifter, either the neutralized carrier path signal or the neutralized peak path signal according to a type of combination, the type including at least one of voltage combination or current combination.

In some implementations, the method further includes: receiving an input signal; amplifying the input signal using a driver circuit including at least one stage of power amplification; and obtaining the RF signal from the driver circuit.

In some implementations, the method further includes: obtaining a combined RF signal; and providing the combined RF signal to an output matching circuit.

DETAILED DESCRIPTION

For RF power amplifiers with multiple stages of power amplification, the last stage is an output stage, while the other stages are collectively referred to as driver stages. The output stage is usually where a large amount of amplification (e.g., gain) is achieved, while each of the driver stages, due to circuit design constraints, usually contributes less to the overall gain. For each stage of amplification, higher gain usually means higher efficiency of power amplification. In many applications of mobile technologies, such as mobile phones, it is desirable to have high power amplification efficiency because of limited power supply.

Many RF power amplifiers operate at a power level often referred to as the backoff power. The backoff power of an RF power amplifier is a power level below the saturation point of the RF power amplifier. Having the RF power amplifier operate at the backoff power instead of at the saturation point can help keep the RF power amplifier operating in the linear range even at occasions when the power of the input signal reaches a level (e.g., a peak level) that is higher than the average power level. However, the efficiency of an RF power amplifier tends to decrease when the RF power amplifier operates at a lower power level than the saturation point. Some communication technologies use signals whose power has high peak-to-average ratio, e.g., 10 dB for orthogonal frequency-division multiplexing (OFDM) signals. In these scenarios, the backoff power can be considerably lower than the saturation point, which leads to considerable decrease in average efficiency.

Doherty amplifiers can be effective in increasing power amplification efficiency at the backoff power, in particular at the output stage. A Doherty amplifier divides an RF signal into two paths with a phase difference of 90°, uses a main amplifier (also known as a carrier amplifier) and a peak amplifier to separately amplify the divided signals, and combine the two paths after amplification. While Doherty amplifiers can increase power amplification efficiency at the backoff power thanks to load modulation provided by the peak amplifier, Doherty amplifiers can experience extra power loss and reduced efficiency at the peak power due to parasitic capacitance associated with the transistors in each path.

As described in detail below, implementations of this disclosure utilize neutralization paths to modify the amplified signals on the carrier path and the peak path by effectively reducing or canceling the parasitic capacitance on each path. As a result of such neutralization, RF power amplifiers implemented according to this disclosure can, in some cases, have increased gain at the output stage, and, consequently, increased power efficiency of one or more stages of the power amplifier combined. With the increased output stage gain, the number of driver stages can, in some implementations, be reduced, resulting in smaller circuit size and lower manufacturing complexity and cost.

FIG.1is a schematic diagram of an example wireless communication system100including a wireless device110capable of communicating with one or more wireless communication networks. The one or more wireless communication networks with which the wireless device110is capable of communicating can include but is not limited to one or more cellular or wireless wide area networks (WWANs), one or more wireless local area networks (WLANs), one or more wireless personal area networks (WPANs), or a combination thereof.

In the example ofFIG.1, the wireless device110is communicating with at least one WWAN by way of at least one base station120and at least one WLAN by way of at least one access point130. The at least one base station120can support bi-directional communication with wireless devices that are within its corresponding area of coverage122. Similarly, the at least one access point130can support bi-directional communication with wireless devices that are within its corresponding area of coverage132.

In some implementations, the at least one WWAN with which the at least one base station120is associated can be a fifth generation (5G) network among other generations and types of networks. In these implementations, the at least one base station120can be a 5G base station that employs orthogonal frequency-division multiplexing (OFDM) and/or non-OFDM and a transmission time interval (TTI) shorter than 1 ms (e.g. 100 or 200 microseconds), to communicate with wireless devices, such as wireless device110. For example, the at least one base station120can take the form of one of several devices, such as a base transceiver station (BTS), a Node-B (NodeB), an evolved NodeB (eNB), a next (fifth) generation (NR) NodeB (gNB), a Home NodeB, a Home eNodeB, a site controller, an access point, a wireless router, a server, router, switch, or other processing entity with a wired or wireless network.

System100can use multiple channel access functionality, including for example schemes in which the at least one base station120and the wireless device110are configured to implement the Long Term Evolution wireless communication standard (LTE), LTE Advanced (LTE-A), and/or LTE Multimedia Broadcast Multicast Service (MBMS). In other implementations, the at least one base station120and wireless device110are configured to implement UMTS, HSPA, or HSPA+ standards and protocols. Of course, other multiple access schemes and wireless protocols can be utilized. In some examples, one or more such access schemes and wireless protocols can correspond to standards that impose RF power amplifier linearity requirements.

In addition, and as shown inFIG.1, the wireless device110is configured to communicate with one or more personal area network (PAN) devices/systems130(e.g., Bluetooth® or radio frequency identification (RFID) systems and devices) over one or more WPANs. The one or more PAN devices/systems130can support either one-way or bi-directional communication with wireless devices that are within its corresponding area of coverage142.

To communicate with one or both of the at least one base station120and the access point130, the wireless device110can include singular or multiple transmitter and receiver components similar or equivalent to one or more of those described in further detail below with reference toFIG.2to support multiple communications with different types of access points, base stations, and other wireless communication devices.

AlthoughFIG.1illustrates one example of a communication system, various changes can be made toFIG.1. For example, the communication system100could include any number of wireless devices, base stations, access points, networks, or other components in any suitable configuration.

FIG.2is a block diagram that illustrates example details of the wireless device110that can implement the subject matter according to this disclosure. The wireless device110can, for example, be a mobile telephone, but can be other devices in further examples such as a desktop computer, laptop computer, tablet, hand-held computing device, automobile computing device and/or other computing devices. As shown in the figure, the wireless device110is shown as including at least one transmitter210, at least one receiver220, memory230, at least one processor240, and at least one input/output device260. Here, only one transmitter and only one receiver are shown, but in many implementations, multiple transmitters and receivers are included to support multiple communications of different types at the same time. Each transmitter may employ the innovations of the present disclosure.

The processor240can implement various processing operations of the wireless device110. For example, the processor240can perform signal generation, signal coding, signal analysis, data processing, power control, input/output processing, or any other functionality enabling the wireless device110to operate in a communication system, such as system100(FIG.1). The processor240can include any suitable processing or computing device configured to perform one or more operations. For example, the processor240can include a microprocessor, microcontroller, digital signal processor, field programmable gate array, or application specific integrated circuit, or a combination of these devices.

The transmitter210can be configured to modulate data or other content, filter and amplify outgoing RF signals for transmission by at least one antenna250A. In some implementations, the transmitter210can also be configured to amplify, filter and upconvert baseband or intermediate frequency (IF) signals to RFs signals before such signals are provided to the antenna250A for transmission. The transmitter210can include any suitable structure for generating RF signals for wireless transmission. Additional aspects of the transmitter210are described in further detail below with reference to components212-218as depicted inFIG.2.

The receiver220can be configured to demodulate data or other content received in incoming RF signals by at least one antenna250B. In some implementations, the receiver220can also be configured to amplify, filter and frequency down convert RF signals received via the antenna250B either to IF or baseband frequency signals prior to conversion to digital form and processing. The receiver220can include any suitable structure for processing signals received wirelessly.

Each of the antennas250A and250B can include any suitable structure for transmitting and/or receiving wireless RF signals. In some implementations, the antennas250A and250B can be implemented by way of a single antenna that can be used for both transmitting and receiving RF signals.

One or multiple transmitters210, one or multiple receivers220, and one or multiple antennas250could be used in the wireless device110. For example, in one implementation, device110includes at least three transmitters210and at least three receivers220for communicating via at least a personal area network such as Bluetooth®, a Wi-Fi network such as an IEEE 802.11 based network, and a cellular network. Each transmitter210may employ the concepts of the present disclosure. Although shown as separate blocks or components, at least one transmitter210and at least one receiver220could be combined into a transceiver. Each transceiver may employ the concepts of the present disclosure. Accordingly, rather than showing a separate block for the transmitter210and a separate block for the receiver220inFIG.2, a single block for a transceiver could have been shown.

The wireless device110further includes one or more input/output devices260. The input/output devices260facilitate interaction with a user. Each input/output device260includes any suitable structure for providing information to or receiving information from a user, such as a speaker, microphone, keypad, keyboard, display, and/or touch screen.

In addition, the wireless device110includes at least one memory230. The memory230stores instructions and data used, generated, and/or collected by the wireless device110. For example, the memory230could store software or firmware instructions executed by the processor(s)240and data used to reduce or eliminate interference in incoming signals. Each memory230includes any suitable volatile and/or non-volatile storage and retrieval device(s). Any suitable type of memory may be used, such as random access memory (RAM), read only memory (ROM), hard disk, optical disc, subscriber identity module (SIM) card, memory stick, secure digital (SD) memory card, and the like.

In some implementations, the transmitter210can include signal processing circuitry212, modulation circuitry214, and RF front end circuitry218. The signal processing circuitry212may include one or more circuits that are configured to process signals received as input (e.g. from processor240). For example, the signal processing circuitry212may include a digital-to-analog converter (D/A), which converts a digital input (e.g. a digital signal from processor240) into an analog signal, which is then provided to a low pass filter, which filters the analog signal and provides the filtered analog signal to the modulation circuitry214. The modulation circuitry214, in addition to receiving the filtered analog signal from the signal processing circuitry212, can, in some implementations, also receive a signal from a local oscillator216for modulating or adjusting the frequency of the analog signal, e.g., from a first frequency to a second frequency that is higher than the first frequency. For instance, the modulation circuitry214can include a mixer that frequency up-converts the filtered analog signal from a relatively low frequency (e.g. baseband frequency, or an IF that is offset from the baseband frequency) to a relatively high frequency RF signal. Thus, a signal from the local oscillator216is used as a carrier signal in transmitter210. Moreover, as shown inFIG.2, transmitter210includes RF front end circuitry218, which can include, e.g., amplification and filtering circuits that amplify and filter, respectively, the RF signal. The RF front end circuitry can also include a power amplifier that is configured to provide sufficient amplification of the signal to meet transmission requirements, as may be specified by wireless communication standards. Examples of such standards include those set forth by the 3rd Generation Partnership Project (3GPP), which is a group that develops standards for cellular telecommunications technologies, including radio access, core network, and service capabilities.

The RF signal amplified by the power amplifier may be filtered again by at least one additional filter downstream of the power amplifier before being provided as an output of the transmitter210to the at least one antenna250A for wireless transmission. Such filter or filters can alternatively be provided upstream from the power amplifier in which case the output of the power amplifier is provided to the at least one antenna250A for wireless transmission.

FIG.3is a graph300showing an example of a simulated relationship between an output stage gain and PAE of a two-stage amplifier. The simulation assumes that the total gain achieved by the driver stages and the output stage is 32 dB, with the output power being at 29 dbm. The simulation assumes that the driver stages and the output stage consume 400 mW and 3000 mW, respectively, of direct current (DC) power supply.

As shown on the graph300, the PAE of the amplifier is positively correlated to the output stage gain. For example, with the output stage gain increasing from 10 dB to 15 dB, the PAE increases from about 16.76% to about 20.40%. Such positive correlation indicates that the efficiency can be achieved by increasing the output stage gain.

FIG.4is a schematic diagram of an example circuit400of a power amplification stage, according to some implementations. The circuit400receives an input RF signal IN and outputs an output RF signal OUT. The circuit400is formed using one or more BJTs Q1, one or more resistors R1, and one or more capacitors C1. R1and C1together form a bias circuit that provides a bias current to the collector of Q1for amplification.

FIG.5Ais a schematic diagram of an example RF power amplifier circuit500A, according to some implementations. The RF power amplifier circuit500A can be implemented in, e.g., the RF front end circuitry218ofFIG.2. The RF power amplifier circuit500A can be a Doherty power amplifier, which can have a carrier path output at node A and a peaking path output at node B. The relative phase difference between A and B can be +/−90°. For the purpose of neutralization (described below in detail), one or more phase shifters can be introduced in neutralization paths to accomplish a phase difference of 180°.

The RF power amplifier circuit500A is configured to receive an input RF signal from the input port501(which can be coupled to an input port of the RF front end circuitry218), amplify the RF signal, and output the amplified RF signal via the output port570. To achieve the amplification, the RF power amplifier circuit500A has a driver circuit510with one or more driver stages, and an output stage arranged between the driver circuit510and the output port570.

Each driver stage includes a power amplifier512with an output coupled to an interstage matching circuit513. The power amplifier512can have a BJT-based structure similar to that of the circuit400. Alternatively or additionally, the power amplifier512can have a different structure, such as a Field-Effect Transistor (FET)-based structure, with different components from the circuit400. The interstage matching circuit513provides impedance matching between two consecutive driver stages and between the last driver stage and the output stage. The driver circuit510also has an input matching circuit511that provides impedance matching between the input port501and the first driver stage. The interstage matching circuits513and the input matching circuit511together facilitate the propagation of RF signals from the input port501through the driver circuit510to the output stage.

The output stage of the RF power amplifier circuit500A includes a quadrature coupler520, a carrier amplification circuit530, a peak amplification circuit532, and a combiner circuit550A. The quadrature coupler520, carrier amplification circuit530, peak amplification circuit532, and combiner circuit550A, along with the connections therebetween, form a basic structure of a Doherty amplifier. The quadrature coupler520receives an RF signal512from the driver circuit510and splits the RF signal512into a carrier path signal521and a peak path signal522. In addition, the quadrature coupler520offsets the carrier path signal521from the peak path signal522by −90° in phase. In other words, the carrier path signal521lags behind the peak path signal522by 90° in phase. In some other implementations, the quadrature coupler520can instead offset the carrier path signal521from the peak path signal522by +90° in phase, causing the carrier path signal521to lead the peak path signal522by 90° in phase.

The carrier path signal521and the peak path signal522are input to the carrier amplification circuit530and the peak amplification circuit532, respectively, for amplification. Each of the carrier amplification circuit530and the peak amplification circuit532can have a BJT-based structure similar to that of the circuit400. Alternatively or additionally, each of the carrier amplification circuit530and the peak amplification circuit532can have a different structure, such as a Field-Effect Transistor (FET)-based structure, with different components from the circuit400.

The combiner circuit550A combines the amplification outputs from the carrier amplification circuit530and the peak amplification circuit532and provide the combination outcome to the output port570. The combination provided by the combiner circuit550A can be considered of a type of current combining, where the current from the carrier path and the current from the peak path are added (e.g., via an RF coupler) at node C. An output matching circuit560can be arranged between the combiner circuit550A and the output port570to provide impedance matching between the two.

The combiner circuit550A can include a quarter-wave length phase shifter552(e.g., a transmission line with impedance set to provide a +90° phase shift) connected in series with the carrier amplification circuit530. For example, in implementations where the carrier path signal521lags behind the peak path signal522by 90° in phase, the quarter-wave length phase shifter552can be connected in series with the carrier amplification circuit530to compensate for the −90° phase difference imposed by the quadrature coupler520between the carrier path signal521and the peak path signal522. With the phase compensation, the currents on the two paths are again in phase when combined at node C.

The output stage of the RF power amplifier circuit500A further includes two neutralization circuits. A first neutralization circuit is arranged between an input523of the carrier amplification circuit530and an output526of the peak amplification circuit532. A second neutralization circuit is arranged between an input524of the peak amplification circuit532and an output525of the carrier amplification circuit530. The first neutralization circuit includes a first neutralization capacitor541and a first phase shifter543, connected in series. The second neutralization circuit includes a second neutralization capacitor542and a second phase shifter544, connected in series. AlthoughFIG.5Ashows that a branch of the carrier path signal521flows through the first neutralization capacitor541first and the first phase shifter543second, the RF power amplifier circuit500A contemplates swapping the positions of the first neutralization capacitor541and the first phase shifter543such that the branch of the carrier path signal521flows through the first phase shifter543first and the first neutralization capacitor541second. Likewise, althoughFIG.5Ashows that a branch of the peak path signal522flows through the second neutralization capacitor542first and the second phase shifter544second, the RF power amplifier circuit500A contemplates swapping the positions of the second neutralization capacitor542and second first phase shifter544such that the branch of the peak path signal522flows through the second phase shifter544first and second first neutralization capacitor542second.

In implementations where the carrier path signal521lags behind the peak path signal522by 90° in phase, the first phase shifter543is a −90° phase shifter and the second phase shifter544is a +90° phase shifter. As such, when a branch of the carrier path signal521flows through the first neutralization circuit, the first phase shifter543imposes an additional −90° phase shift to the branch. As a result, the first neutralization circuit generates a first neutralizing signal548that differs from the peak path signal522by 180° in phase. Likewise, when a branch of the peak path signal522flows through the second neutralization circuit, the second phase shifter544imposes an additional +90° phase shift to the branch. As a result, the second neutralization circuit generates a second neutralizing signal546that differs from the carrier path signal521by 180° in phase.

In alternative implementations where the carrier path signal521leads the peak path signal522by 90° in phase, the first phase shifter543is a +90° phase shifter and the second phase shifter544is a −90° phase shifter. With the phase shift provided by the first and second phase shifters543and544, the first and second neutralizing signals548and546in these implementations also differ from the peak path signal522and the carrier path signal521, respectively, by 180° in phase.

The first and second neutralizing signals548and546are provided to the peak path and the carrier path, respectively, to neutralize (e.g., cancel or reduce) the parasitic capacitance associated with amplifying transistors of the carrier amplification circuit530and the peak amplification circuit532. Taking the first neutralizing signal548as an example, the first neutralizing signal548is coupled with an amplified peak path signal output by the peak amplification circuit532at output526. The coupling can modify the amplified peak path signal to generate a neutralized peak path signal528. Similarly, the second neutralizing signal546is coupled with and thereby modifies an amplified carrier path signal to generate a neutralized carrier path signal527.

Compared with Doherty amplifiers that do not have neutralization circuits, a Doherty RF power amplifier modified in accordance with circuit500A can, by way of modifying the amplified carrier path and peak path signals, neutralize the parasitic capacitance associated with amplifying transistors of the carrier amplification circuit530and the peak amplification circuit532on each path. The neutralization can be attributed to the impedance introduced by the neutralization capacitors and the phase shifters of the neutralization circuits.

The tuning of the capacitance of the first and second neutralization capacitors541and542can consider a number of factors or constraints, such as the frequency of the RF signal515, the size of RF power amplifier circuit500A (or, separately, the size of the output stage), the gain of the RF power amplifier circuit500A (or, separately, the target gain of the output stage), and the stability of the RF power amplifier circuit500A. Similarly, the tuning of the characteristic impedance of the first and second phase shifters543and544can consider a number of factors or constraints with a goal of achieving the needed neutralization effect while reducing unwanted impact to Doherty load modulation.

FIG.5Bis a schematic diagram of another example RF power amplifier circuit500B, according to some implementations. The RF power amplifier circuit500B can be substantially the same as the RF power amplifier circuit500A except for having a combiner circuit550B that is different from the combiner circuit550A. For the sake of brevity, description of the RF power amplifier circuit500B is only focused on the combiner circuit550B, while the description and numbering of other components of the RF power amplifier circuit500B, which can be the same as those of the RF power amplifier circuit500A, are omitted.

Different from the combiner circuit550A that combines currents from the carrier path and the peak path, the combiner circuit550B uses a RF transformer554for voltage combining. In voltage combining, two signals with a phase difference of 180° are input to the “+” and “−” input ports of the transformer554, whose output port, in some implementations, can be coupled to a matching circuit and further to amplifier output. This type of voltage combining also uses a quarter-wave length phase shifter552(e.g., a transmission line with optimized impedance set to provide a +90° phase shift). For example, in implementations where the carrier path signal lags behind the peak path signal by 90° in phase, the quarter-wave length phase shifter552can be connected in series with the peak amplification circuit such that the signals at the “+” and “−” input ports of the transformer554are 180° different in phase.

FIG.6shows two graphs600A and600B that show a simulated comparison of Doherty load modulation (“Doherty action”) performance with and without neutralization circuits, according to some implementations. The load modulation can be evaluated based on the real parts (e.g., resistive component) of effective impedance Zmand Zpon the carrier path and the peak path, respectively. Graph600A shows the ideal Doherty load modulation performance without neutralization circuits, while Graph600B shows the Doherty load modulation performance with neutralization circuits. Both graphs assume the input voltage to the Doherty amplifier is 1.2 V.

As shown in the graphs600A and600B, Doherty load modulation can be maintained with proper tuning of Zmand Zp, which can be adjusted by tuning the impedance of the neutralization circuits. This comparison shows that implementations described herein can be applied to Doherty power amplifiers without significant compromises to the proper functioning thereof.

FIG.7is a graph700showing a simulated relationship between the effect of parasitic capacitance neutralization and the impedance of the phase shifter in a neutralization circuit, according to some implementations. The vertical axis shows the percentage of the parasitic capacitance that is cancelled by the neutralization circuit, and the horizontal axis shows the impedance of the phase shifter in the neutralization circuit. As shown in the graph700, the neutralization circuit performs better when the phase shifter has low impedance. The relationship shown in the graph700can be used to design neutralization circuits, such as those shown inFIGS.5A and5B.

FIG.8is a flowchart of an example method for amplifying an RF signal, according to some implementations. It would be understood that the method800can be performed, for example, by any suitable system, environment, software, hardware, or a combination of systems, environments, software, and hardware, as appropriate. In some implementations, various steps of the method800can be run in parallel, in combination, in loops, or in any order. The method800can be embodied in a circuit similar to circuits500A and500B ofFIGS.5A and5B. For example, some signals and circuit components can be similar to the corresponding signals and circuit components of circuits500A and500B. Alternatively or additionally, some operations of the method800can be performed by one or more circuit components of circuits500A and500B.

At802, the method800involves providing the RF signal to an input of a quadrature coupler (e.g., the quadrature coupler520).

At804, the method800involves outputting, from the quadrature coupler, a carrier path signal (e.g., the carrier path signal521) and a peak path signal (e.g., the peak path signal522).

At806, the method800involves amplifying (e.g., using the carrier amplification circuit530) the carrier path signal to provide an amplified carrier path signal.

At808, the method800involves amplifying (e.g., using the peak amplification circuit532) the peak path signal to provide an amplified peak path signal.

At810, the method800involves generating a first neutralizing signal (e.g., the first neutralizing signal548) based on the carrier path signal.

At812, the method800involves generating a second neutralizing signal (e.g., the second neutralizing signal546) based on the peak path signal.

At814, the method800involves modifying the amplified peak path signal based on the first neutralizing signal to provide a neutralized peak path signal (e.g., the neutralized peak path signal528).

At816, the method800involves modifying the amplified carrier path signal based on the second neutralizing signal to provide a neutralized carrier path signal (e.g., the neutralized carrier path signal527).

At818, the method800involves combining (e.g., using the combiner circuits550A or550B) the neutralized carrier path signal and the neutralized peak path signal.

While features described above are primarily implemented by wireless devices, these features can likewise be implemented by access nodes, base stations, or other types fixed or portable wireless communication equipment and/or infrastructure. For example, a base station in communication with a cellular phone can have RF front end circuitry that implements the above-described features with respect to thermally adjustable DC bias circuit.