# Patent

## Title
Radio-frequency transmitter, such as for broadcasting and cellular base stations

## Abstract

A transmitter that uses a digital pre-distortion (DPD) circuit to mitigate the effects of nonlinearity of a multistage or multi-branch power amplifier. The DPD circuit relies on two or more feedback signals received from an RF-output circuit of the transmitter to generate individually pre-distorted signals for the individual stages/branches of the power amplifier. The use of these individually pre-distorted signals advantageously enables the transmitter to achieve a more efficient suppression of inter-modulation-distortion products than that typically achieved with a comparable prior-art transmitter.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
The subject matter of this application is related to that of U.S. Patent Application Publication No. 2013/0063222, which is incorporated herein by reference in its entirety.
BACKGROUND
1. Field of the Invention
The present invention relates to equipment for telecommunication systems and, more specifically but not exclusively, to radio-frequency (RF) transmitters and power amplifiers, and passive RF circuits suitable for use therein.
2. Description of the Related Art
This section introduces aspects that may help facilitate a better understanding of the invention(s). Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
A recent trend in the telecommunications industry includes the introduction of wideband digital-modulation systems, such as the third generation (3G) cellular-system wideband code-division-multiple-access (WCDMA) and the fourth generation (4G) cellular-system orthogonal frequency-division multiple-access (OFDMA). This trend has had a profound effect on power-amplifier specifications because an RF power amplifier used in a wideband digital-modulation system needs to properly and efficiently handle a signal that has a fast-changing envelope, a high peak-to-average power ratio (PAPR), and a bandwidth that can be tens of megahertz. In addition, for cost reasons, a single power amplifier is usually configured to amplify multiple modulated carriers.
A typical RF power amplifier is inherently nonlinear, with its gain being a function of the output-power level. The gain usually decreases when the output power approaches the saturation level of the amplifier, and the phase of the gain can either increase or decrease, depending on the type of the active medium. Amplitude and/or phase distortions in the power amplifier tend to cause the generation of spurious spectral components often referred to as inter-modulation-distortion (IMD) products. IMD products are detrimental, for example, because they increase the level of interference between adjacent RF channels.
SUMMARY
Disclosed herein are various embodiments of a transmitter that uses a digital pre-distortion (DPD) circuit to mitigate the effects of nonlinearity of a multistage or multi-branch power amplifier, such as a Doherty power amplifier or a Chireix power amplifier. The DPD circuit relies on two or more feedback signals received from an RF-output circuit of the transmitter to generate individually pre-distorted signals for each stage/branch of the power amplifier. The use of these individually pre-distorted signals advantageously enables the transmitter to achieve a more efficient suppression of inter-modulation-distortion products than that typically achieved with a comparable prior-art transmitter.
According to one embodiment, provided is an apparatus having: a digital pre-distortion circuit configured to pre-distort a digital input signal to generate a first pre-distorted digital signal and a second pre-distorted digital signal different from the first pre-distorted signal; a first amplifier branch configured to generate a first amplified signal based on the first pre-distorted digital signal; a second amplifier branch configured to generate a second amplified signal based on the second pre-distorted digital signal; and a radio-frequency (RF) circuit configured to combine the first and second amplified signals to generate a combined signal. The RF circuit is further configured to generate first and second feedback signals based on at least two of the first amplified signal, the second amplified signal, and the combined signal. The digital pre-distortion circuit is configured to generate the first pre-distorted digital signal and the second pre-distorted digital signal based on said first and second feedback signals to counteract nonlinearity in the first and second amplifier branches.
According to another embodiment, provided is a signal-amplification method having the steps of: pre-distorting a digital input signal to generate a first pre-distorted digital signal and a second pre-distorted digital signal different from the first pre-distorted signal; generating a first amplified signal based on the first pre-distorted digital signal in a first amplifier branch; generating a second amplified signal based on the second pre-distorted digital signal in a second amplifier branch; combining the first and second amplified signals in a radio-frequency (RF) circuit to generate a combined signal; and generating first and second feedback signals based on at least two of the first amplified signal, the second amplified signal, and the combined signal. The step of pre-distorting comprises the sub-step of generating the first pre-distorted digital signal and the second pre-distorted digital signal based on said first and second feedback signals to counteract nonlinearity in the first and second amplifier branches.
According to yet another embodiment, provided is an apparatus having: means for pre-distorting a digital input signal to generate a first pre-distorted digital signal and a second pre-distorted digital signal different from the first pre-distorted signal; a first amplifier branch configured to generate a first amplified signal based on the first pre-distorted digital signal; a second amplifier branch configured to generate a second amplified signal based on the second pre-distorted digital signal; means for combining the first and second amplified signals to generate a combined signal; and means for generating first and second feedback signals based on at least two of the first amplified signal, the second amplified signal, and the combined signal. The means for pre-distorting is configured to generate the first pre-distorted digital signal and the second pre-distorted digital signal based on said first and second feedback signals to counteract nonlinearity in the first and second amplifier branches.



BRIEF DESCRIPTION OF THE DRAWINGS
Other aspects, features, and benefits of various embodiments of the invention will become more fully apparent, by way of example, from the following detailed description and the accompanying drawings, in which:
FIG. 1 shows a block diagram of a radio-frequency (RF) transmitter according to one embodiment of the invention;
FIG. 2 shows a block diagram of a transceiver that can be used in the RF transmitter of FIG. 1 according to one embodiment of the invention;
FIG. 3 shows a circuit diagram of an RF circuit that can be used in the RF transmitter of FIG. 1 according to one embodiment of the invention;
FIG. 4 shows a circuit diagram of an RF circuit that can be used in the RF transmitter of FIG. 1 according to another embodiment of the invention; and
FIG. 5 shows a top view of a microstrip circuit that can be used to implement the RF circuit of FIG. 4 according to one embodiment of the invention; and
FIG. 6 shows a circuit diagram of a two-stage amplifier according to one embodiment of the invention.



DETAILED DESCRIPTION
One method that can be used to linearize the nonlinear response of a radio-frequency (RF) power amplifier over its intended dynamic range is digital pre-distortion (DPD). DPD works in the digital domain and uses digital-signal-processing techniques to pre-distort a baseband signal before modulation, up-conversion, and amplification. With DPD, the power amplifier can be utilized substantially up to its saturation point while maintaining a sufficiently accurate linear relationship between the input and output signals. DPD is an attractive technique, e.g., because it can significantly increase the power efficiency of a power amplifier and be implemented using standard and/or inexpensive circuit components. A high degree of flexibility can be achieved if programmable hardware is used, such as digital signal processors (DSPs) and/or field-programmable gate arrays (FPGAs). In addition, DPD does not require significant changes in the schematics of the costly analog part (e.g., the RF-output circuit) of the corresponding transmitter and lends itself to various advantageous implementations in which the analog front end of the transmitter has a relatively small size, and the configurable digital part of the transmitter can be placed very close to the antenna.
FIG. 1 shows a block diagram of an RF transmitter 100 according to one embodiment of the invention. Transmitter 100 uses a Doherty amplification scheme to convert a digital input signal 102 into an analog RF-output signal 152. Output signal 152 is applied to an output load (e.g., an antenna) 160. A feedback path comprising a feedback-receiver (FBR) circuit 120 and a DPD circuit 110 enables transmitter 100 to apply digital pre-distortion to input signal 102, thereby suppressing IMD products in output signal 152.
The Doherty amplification scheme of transmitter 100 employs power amplifiers 1401 and 1402 connected in parallel as indicated in FIG. 1. Output signals 1421 and 1422 generated by amplifiers 1401 and 1402, respectively, are combined in an RF-output circuit 150 as further described below in reference to FIGS. 3-4 to produce output signal 152. Amplifier 1401 is configured to operate, e.g., as a class-B or a class-AB amplifier and is also referred to as a primary or carrier stage. Amplifier 1402 is configured to operate, e.g., as a class-C amplifier and is also referred to as an auxiliary or peak stage. A brief description of operating configurations corresponding to the pertinent amplifier classes can be found, e.g., in U.S. Pat. Nos. 7,498,876 and 7,928,799, both of which are incorporated herein by reference in their entirety.
Due to different configurations of amplifiers 1401 and 1402, only amplifier 1401 provides signal amplification when input signal 102 and therefore RF signals 1321 and 1322 are small. Amplifier 1402 remains turned off until RF signal 1322 reaches a certain threshold level. Near this threshold level, amplifier 1401 is close to saturation, and amplifier 1402 turns on to supply the output-signal portion that tends to be clipped off by the near-saturation operating regime of amplifier 1401. This complementary action of amplifiers 1401 and 1402 enables transmitter 100 to advantageously have relatively high power efficiency for a wide range of input-signal levels. A more detailed explanation of how high power efficiency can be achieved for amplifiers 1401 and 1402 is provided below in reference to FIG. 3.
In addition to producing output signal 152 by properly combining signals 1421 and 1422, RF-output circuit 150 is configured to generate feedback signals 1481-1483 based on signals 1421, 1422, and 152 and supply these feedback signals to FBR circuit 120. In one embodiment, feedback signal 1481 provided by RF-output circuit 150 to FBR circuit 120 is an attenuated copy of signal 1421; feedback signal 1482 is an attenuated copy of signal 1422; and feedback signal 1483 is an attenuated copy of signal 152. In various alternative embodiments, RF-output circuit 150 can be configured to provide to FBR circuit 120 only two of feedback signals 1481-1483 and/or generate each of feedback signals 1481-1483 based on a respective different linear combination of signals 1421, 1422, and 152.
In one embodiment, FBR circuit 120 comprises three feedback receivers (not explicitly shown in FIG. 1), each configured to process a corresponding one of feedback signals 1481-1483. The typical processing performed by a feedback receiver includes down-converting the corresponding feedback signal 148 to baseband and applying analog-to-digital conversion to the resulting analog baseband signal to generate a corresponding digital feedback signal 118. Digital feedback signals 1181-1183 generated by FBR circuit 120 correspond to analog feedback signals 1481-1483, respectively.
In general, the gain, efficiency, and AM-PM (amplitude-to-phase modulation) characteristics (e.g., the insertion-phase change as a function of signal amplitude) of a power amplifier, such as power amplifier 1401 or 1402, are all functions of both the output power and the load impedance. In a typical prior-art DPD scheme, the individual stimuli (input signals) for the carrier and peak stages of a Doherty power amplifier are generated by means of a 3-dB power splitter configured to split the RF signal that is generated based on a single pre-distorted digital signal generated by the DPD circuit. This means that the stimuli applied to the carrier and peak stages have a fixed phase relationship with one another. However, as already indicated above, the carrier and peak stages of a Doherty power amplifier are configured to operate in different regimes, which causes their output signals to generally have a phase mismatch between them. Moreover, this phase mismatch varies over time owing to the variations in the output-power level. Disadvantageously, this prior-art DPD scheme is not capable of equalizing the phase mismatch and relies mostly on amplitude pre-distortion for the suppression of IMD products.
This and other pertinent problems in the prior art are addressed in transmitter 100 by configuring DPD circuit 110 to generate individually pre-distorted digital signals 1121 and 1122 for amplifiers 1401 and 1402, respectively, based on multiple digital feedback signals, e.g., two or three of signals 1181-1183. These feedback signals provide sufficient information to enable the DPD circuit to implement both amplitude pre-distortion and phase equalization for amplifiers 1401 and 1402. In part due to a relatively small phase mismatch between output signals 1421 and 1422, transmitter 100 is able to better suppress IMD products in its output signal (i.e., signal 152) than a comparable prior-art transmitter.
In one configuration, DPD circuit 110 uses digital feedback signals 1181-1183 to adaptively pre-distort input signal 102 to generate individually pre-distorted digital signals 1121 and 1122. Pre-distorted digital signal 1121 is applied to a transmitter 1301, where it is first converted into a corresponding analog signal (not explicitly shown in FIG. 1). Transmitter 1301 then up-converts this analog signal from baseband to generate RF signal 1321. Pre-distorted digital signal 1122 is similarly processed in a transmitter 1302 to generate RF signal 1322. As already indicated above, RF signals 1321 and 1322 are the input signals (stimuli) that are applied to amplifiers 1401 and 1402, respectively.
DPD circuit 110 is configured to generate pre-distorted digital signal 1121 by applying a first nonlinear function to input signal 102, wherein the first nonlinear function creates an expanding nonlinearity that is an approximate inverse of the compressing nonlinearity of (e.g., compressive amplitude distortion in) amplifier 1401. DPD circuit 110 is further configured to generate pre-distorted digital signal 1122 by applying a second nonlinear function to input signal 102, wherein the second nonlinear function creates a nonlinearity that is an approximate inverse of the nonlinearity of amplifier 1402. As already indicated above, the first and second nonlinear functions usually differ from one another due to different operating configurations of amplifiers 1401 and 1402.
In various alternative configurations, DPD circuit 110 can similarly apply other types of first and/or second nonlinear functions to input signal 102 to generate pre-distorted digital signals 1121 and 1122. In general, the first and second nonlinear functions are constructed in an inter-related manner to cause the forward signal path comprising DPD circuit 110, transmitters 1301 and 1302, amplifiers 1401 and 1402, and RF-output circuit 150 to exhibit substantially linear signal-transfer characteristics. By substantially linear signal-transfer characteristics, it is meant that, with digital pre-distortion, the relationship between output signal 152 and input signal 102 can be approximated well by a constant gain, e.g., a complex or real gain value that does not depend on the input- (or output-) signal level within the intended dynamic range of transmitter 100. Description of representative DPD circuits and algorithms that can be used to implement DPD circuit 110 can be found, e.g., in U.S. Pat. Nos. 7,957,707, 7,904,033, 7,822,146, 7,782,979, 7,729,446, 7,606,324, 7,583,754, and 7,471,739, all of which are incorporated herein by reference in their entirety.
FIG. 2 shows a block diagram of a transceiver 200 that can be used in transmitter 100 (FIG. 1) according to one embodiment of the invention. Note that DPD circuit 110 shown in FIG. 2 is not part of transceiver 200. Digital-to-analog converters (DACs) 234 and an I-Q modulator 236 can be used to implement transmitter 1301 or transmitter 1302. Analog-to-digital converters (ADCs) 224 and an I-Q de-modulator 226 can be used to implement a portion of FBR circuit 120. Transceiver 200 also has a local-oscillator (LO) source 244 that is configured to supply a local-oscillator (carrier-frequency) signal 246 to I-Q modulator 236 and I-Q de-modulator 226. In a representative embodiment, transmitter 100 may have more than one instance of transceiver 200.
In operation, I-Q de-modulator 226 demodulates feedback signal 148 in a conventional manner by mixing it with LO signal 246. A resulting baseband signal 225 generated by I-Q de-modulator 226 has two components: an in-phase component 225I and a quadrature-phase component 225Q. Signals 225I and 225Q are analog signals that are converted into digital form by ADCs 224. The resulting digital signals Ifb and Qfb are components of the corresponding one of digital signals 1181-1183 (also see FIG. 1).
DPD circuit 110 uses digital signals Ifb and Qfb to determine the amount of distortion in the forward signal path of transmitter 100 (FIG. 1). For example, a symbol received by DPD circuit 110 via digital signals Ifb and Qfb can be combined with (e.g., summed with and/or subtracted from) one or more corresponding symbols received by the DPD circuit via the other one or more digital signals 118 (see FIG. 1). DPD circuit 110 then uses the corresponding original constellation symbol received via input signal 102 to determine the amount of pre-distortion that needs to be applied to the original I and Q components to counteract (e.g., cancel or significantly reduce) the distortion imposed by the forward signal path. The determined amount of pre-distortion can be partitioned, in any suitable manner, into a first portion and a second portion. The first portion, in form of the first nonlinear function, is applied to input signal 102 to generate pre-distorted digital signal 1121, while the second portion, in form of the second nonlinear function, is similarly applied to input signal 102 to generate pre-distorted digital signal 1122. Similar to signals 102 and 118, signal 112 is shown as having two components: an in-phase component labeled Ipd and a quadrature-phase component labeled Qpd. Note that FIG. 2 shows the generation of only one of pre-distorted digital signals 1121 and 1122. The other one of these signals can be similarly generated.
Components Ipd and Qpd of pre-distorted digital signal 112 are converted, in DACs 234, into analog signals 235I and 235Q, respectively. I-Q modulator 236 then uses analog signals 235I and 235Q to modulate LO signal 246. The resulting modulated carrier signal is RF signal 132 (also see FIG. 1). As already indicated above, signal 132 generated by I-Q modulator 236 can be one of signals 1321 and 1322 (see FIG. 1). The other one of these signals can be similarly generated.
FIG. 3 shows a circuit diagram of an RF circuit 300 that can be used as RF-output circuit 150 (FIG. 1) according to one embodiment of the invention. RF circuit 300 comprises a transcoupler 310, an impedance transformer 320, a directional coupler 330, and six terminals labeled A through F. The impedances indicated in FIG. 3 are exemplary and correspond to an implementation in which each of the external terminals is intended for being connected to a 50-Ohm line, driver, load, or terminator. One of ordinary skill in the art will understand how to change the various impedance values shown in FIG. 3 to match RF circuit 300 to an impedance value different from 50 Ohm.
In a representative configuration, terminals A-F can be connected as follows. Terminal A is configured to carry feedback signal 1481 (see FIGS. 1 and 2). Terminal B is configured to carry feedback signal 1482 (see FIGS. 1 and 2). Terminal C is configured to receive amplified signal 1421 (see FIG. 1). Terminal D is configured to receive amplified signal 1422 (see FIG. 1). Terminal E is configured to carry feedback signal 1483 (see FIGS. 1 and 2). Terminal F is configured to apply output signal 152 to load 160.
This representative configuration can be modified to produce several alternative configurations. For example, three different alternative configurations can be obtained by changing the connection of terminal A, B, or E from the above-indicated to a 50-Ohm terminator. In each of these three alternative configurations, RF circuit 300 will provide only two of feedback signals 1481-1483.
Transcoupler 310 is a four-terminal device having two parallel branches 312 and 314 that are located sufficiently close to each other for an RF signal propagating through branch 314 to electromagnetically couple into branch 312. Branch 314 comprises a quarter-wave impedance inverter disposed between terminals C and D. The signal coupling between branches 312 and 314 is relatively weak, e.g., about −30 dB, which ensures a minimal influence of branch 312 on the operation of the quarter-wave impedance inverter in branch 314.
In operation, the quarter-wave impedance inverter of branch 314 can be used to implement active load modulation for carrier stage 1401, for example, as follows (also see FIG. 1).
At low input-signal levels, peak stage 1402 is in the off state while carrier stage 1401 acts as a controlled current source. Peak stage 1402 (ideally) sees infinite impedance, and the impedance inverter of branch 314 causes carrier stage 1401 to see a higher than 50-Ohm impedance load. The higher impedance load causes carrier stage 1401 to reach near-saturation when its output current reaches only about one half of its nominal maximum value. When carrier stage 1401 is close to saturation, it advantageously works with nearly maximum power efficiency.
As soon as the input-signal level becomes sufficiently high to turn on peak stage 1402, the peak stage begins to apply additional current to terminal D. Peak stage 1402 now acts as a controlled current source, and carrier stage 1401 acts as a controlled voltage source. The additional current applied by peak stage 1402 to terminal D causes an increase in the output impedance seen by the quarter-wave impedance inverter of branch 314. Since the input and output impedances of a quarter-wave impedance inverter are related to one another as duals, the increase in the output impedance causes a corresponding decrease in the input impedance. Note that the input impedance of the quarter-wave impedance inverter of branch 314 is the load that is seen by carrier stage 1401. As the load of carrier stage 1401 decreases, the output current of the carrier stage increases, with the output voltage remaining close to the saturation level.
As the input-signal level increases further, the output impedance of the quarter-wave impedance inverter of branch 314 keeps increasing, and the effective load of carrier stage 1401 keeps decreasing. In this manner, the impedance inverter of branch 314 enables peak stage 1402 to modulate the load of carrier stage 1401 during high input-signal levels. The load modulation, in turn, keeps carrier stage 1401 operating in a regime that is characterized by advantageously high power efficiency.
Impedance transformer 320 comprises a length of transmission line that is one-quarter of a wavelength long and has an impedance of about 35 Ohm. (Note that, for a device that has a desired operating-frequency range, the quarter-wave length typically corresponds to the center frequency of that operating range.) Since impedance transformer 320 is terminated at 50 Ohm by directional coupler 330, it presents at terminal D an input impedance of 25 Ohm. The latter impedance matches the output impedance of two 50-Ohm transmission lines connected in parallel to terminal D.
Directional coupler 330 comprises branches 332 and 334. Branch 334 operates to present a fixed 50-Ohm termination to impedance transformer 320. The signal coupling between branches 332 and 334 is relatively weak, e.g., about −30 dB. Terminal G is connected to a 50-Ohm terminator 340. Terminal E outputs an attenuated copy of the RF signal presented at terminal H by impedance transformer 304.
Feedback signals SA, SB, and SE collected at terminals A, B, and E, respectively, of RF circuit 300 can be used to directly measure effective transfer functions T1 and T2 of stages 1401 and 1402, for example, based on Eqs. (1)-(3):




  
    
      
        
          T
          1
        
        =
        
          
            S
            A
          
          
            ap
            1
          
        
      
    
    
      
        (
        1
        )
      
    
  
  
    
      
        
          T
          2
        
        =
        
          
            S
            B
          
          
            ap
            2
          
        
      
    
    
      
        (
        2
        )
      
    
  
  
    
      
        
          
            
              T
              1
            
            ⁢
            
              p
              1
            
          
          +
          
            
              T
              2
            
            ⁢
            
              p
              2
            
          
        
        =
        
          
            S
            E
          
          c
        
      
    
    
      
        (
        3
        )
      
    
  




where a is a constant that represents the signal-coupling strength between branches 312 and 314; c is a constant that represents the signal-coupling strength between branches 332 and 334; and p1 and p2 represent pre-distorted signals 1121 and 1122, respectively. The measurement can be done on line or using an appropriate off-line calibration procedure. In principle, any two of Eqs. (1)-(3) are sufficient for the determination of transfer functions T1 and T2, provided that the coupling strengths are known. The use of all three equations enables the determination of the ratio (a/c) of the coupling strengths and, as such, can be used when only one of the two coupling strengths is known. After the individual transfer functions of stages 1401 and 1402 are determined, their reciprocals can be used in a relatively straightforward manner for amplitude pre-distortion and phase equalization.

FIG. 4 shows a circuit diagram of an RF circuit 400 that can be used as RF-output circuit 150 (FIG. 1) according to another embodiment of the invention. In terms of its intended function, RF circuit 400 is generally analogous to RF circuit 300 (FIG. 3). Therefore, the above-described connections of terminals A-F apply to RF circuit 400 as well as to RF circuit 300. However, one difference between RF circuits 300 and 400 is that the latter employs a transcoupler 450 that performs the functions that are similar to the above-described functions of both impedance transformer 320 and directional coupler 330 (see FIG. 3).
The use of transcoupler 450 can provide one or more of the following benefits/advantages:

    
    
        (1) A transmitter (e.g., transmitter 100, FIG. 1) employing RF circuit 400 can have a relatively high power efficiency, e.g., because transcoupler 450 has a lower insertion loss than a series consisting of impedance transformer 320 and directional coupler 330;
        (2) RF circuit 400 can have a relatively small size because transcoupler 450 occupies a relatively small area on a printed circuit board (PCB); and
        (3) The relatively small size of transcoupler 450 and the improved power efficiency of the corresponding power amplifier can be leveraged to reduce per-unit fabrication and operating costs.
    
    


As evident from the description of transcouplers 310 and 450 in FIGS. 3 and 4, a transcoupler is a circuit element that has two branches that can be referred to as the main branch and the auxiliary branch, respectively. The main branch has a length of about one quarter of the carrier wavelength and is configured to operate as an impedance inverter that presents a first impedance at the first end of the branch, said first impedance being proportional to an inverse of a second impedance presented to the branch at the second end of the branch. If the second impedance is a fixed impedance, then the first branch operates as a quarter-wave impedance transformer. Branches 314 and 454 are the main branches in transcouplers 310 and 450, respectively. The auxiliary branch is electromagnetically coupled to the main branch and configured to operate as a signal coupler that receives an attenuated copy of a signal from the main branch. Branches 312 and 452 are the auxiliary branches in transcouplers 310 and 450, respectively.
FIG. 5 shows a top view of a microstrip circuit 500 that can be used to implement RF circuit 400 (FIG. 4) according to one embodiment of the invention. Circuit 500 comprises a dielectric substrate 502 and two conducting layers attached to the opposite (e.g., top and bottom) sides of the dielectric substrate. Only the patterned top layer is visible in the view provided in FIG. 5. The various microstrip shapes of this patterned layer define the circuit elements of circuit 500. The bottom layer (typically referred to as the “ground plane”) is not visible in the view provided in FIG. 5. In a representative embodiment, the ground plane is not patterned and comprises a continuous layer of metal, such as copper.
Circuit 500 has seven terminals labeled A through G. The terminals labeled by the same letter in FIGS. 4 and 5 are functionally similar. Therefore, the terminals of circuit 500 can be electrically connected to external circuits, e.g., as already described above in reference to FIGS. 3 and 4.
Microstrips 512 and 514 are used to implement a transcoupler 510 that is analogous to transcoupler 310 (see FIGS. 3 and 4). Microstrip 514 is one-quarter of a wavelength long. Microstrip 512 is electromagnetically coupled to microstrip 514 using two interdigitated combs. One of the combs is electrically connected to microstrip 512 and is illustratively shown in FIG. 5 as having two fingers 516. The other comb is electrically connected to microstrip 514 and is illustratively shown in FIG. 5 as having four fingers 518. Two mitred bends 508 are used to electrically connect microstrip 512 to terminals A and B.
Microstrips 552 and 554 are used to implement a transcoupler 550 that is analogous to transcoupler 450 (see FIG. 4). Microstrip 554 is one-quarter of a wavelength long. Microstrip 552 is electromagnetically coupled to microstrip 554 using two interdigitated combs. One of the combs is attached to a side of microstrip 552 and is illustratively shown in FIG. 5 as having two fingers 556. The other comb is attached to a side of microstrip 554 and is illustratively shown in FIG. 5 as having four fingers 558. A mitred bend 508 is used to electrically connect microstrip 552 to terminal E. Microstrips 516 and 562 are used to electrically connect the ends of microstrip 554 to terminals D and F, respectively.
Circuit 500 also has an optional shunted stub 560 that is connected to transcoupler 550 as indicated in FIG. 5. Stub 560 is implemented using a microstrip 566 and a shunt 564 that is located at the distal end of that microstrip. Microstrip 566 is one-quarter of a wavelength long. Shunt 564 comprises one or more electrically conducting vias in dielectric substrate 502 that electrically connect the distal end of microstrip 566 to the ground plane, thereby short-circuiting stub 560. One function of stub 560 is to alter the effective impedance of transcoupler 550 compared to the impedance that the transcoupler would have without the stub. The altered impedance advantageously has an imaginary part that is substantially nulled and has a very weak frequency dependence within the pertinent spectral range around the nominal carrier frequency.
Note that each of microstrips 506, 512, 514, 552, and 562 has a first specified width, and each of microstrips 554 and 566 has a second specified width that is greater than the first. In a representative embodiment, the first and second widths are selected so that (i) each of the RF-transmission lines represented by microstrips 506, 512, 514, 552, and 562 has an impedance of 50 Ohm and (ii) each of the RF-transmission lines represented by microstrips 554 and 566 has an impedance of 35 Ohm. One skilled in the art will understand how to select other respective widths for these two sets of microstrips to obtain other impedance values.
FIG. 6 shows a circuit diagram of a two-stage/branch amplifier circuit 600 according to one embodiment of the invention. Circuit 600 can be used, e.g., to implement a Chireix amplification scheme. A circuit diagram of the corresponding transmitter can be obtained, e.g., by replacing amplifiers 1401 and 1402 and RF-output circuit 150 in transmitter 100 (FIG. 1) by circuit 600. More specifically, signals/lines 6321, 6322, 6481, 6482, and 652 in circuit 600 correspond to signals/lines 1321, 1322, 1481, 1482, and 152, respectively, in transmitter 100.
The Chireix amplification scheme of circuit 600 employs power amplifiers 6401 and 6402 connected in parallel as indicated in FIG. 6. Output signals 6421 and 6422 generated by amplifiers 6401 and 6402, respectively, are combined in an RF-output circuit 660 comprising output-matching circuits 6441 and 6442 and transcouplers 6501 and 6502. Amplifiers 6401 and 6402 are configured to generate signals 6421 and 6422 to be each other's complex conjugates and to have a constant envelope. After RF-output circuit 660 combines phase-modulated signals 6421 and 6422, output terminal D of that circuit has the corresponding amplitude-modulated signal 652.
Similar to RF-output circuits 300 and 400 (FIGS. 3 and 4), RF-output circuit 660 uses load-impedance modulation to achieve relatively high power efficiency for the amplifier stages. Transcoupler 6501 is configured to invert the load impedance, which is then transformed by output-matching circuit 6441 and presented to amplifier 6401. Transcoupler 6502 is similarly configured to invert the load impedance, which is then transformed by output-matching circuit 6442 and presented to amplifier 6402.
To properly convert phase-modulated signals 6421 and 6422 into amplitude-modulated signal 652, the deviation from the prescribed complex conjugate-phase relationship between the two amplifier branches in circuit 600 needs to be as small as possible. Circuit 600 helps to achieve this result by providing feedback signals 6481 and 6482 for use with an appropriate DPD circuit that can be similar to DPD circuit 110 of transmitter 100. Transcoupler 6501 is configured to generate feedback signal 6481. Transcoupler 6502 is similarly configured to generate feedback signal 6482. Based on these feedback signals, the corresponding DPD circuit can pre-distort signals 6321 and 6322 to achieve relatively accurate phase conjugation at terminal D.
In general, transcoupling elements analogous to transcouplers 450, 550, and 650 can be used in any multistage or multi-branch power-amplifier circuit configured to combine RF-output signals from two or more branches of amplifying elements and use feedback-based digital pre-distortion to linearize the overall transfer characteristics of the power amplifier, e.g., to suppress IMD products in its output signal. The above-described Doherty and Chireix amplification schemes are just two representative examples of such multistage power-amplifier circuits. From the description provided herein, one of ordinary skill in the art will be able to use transcoupling elements instead of conventional RF-circuit elements in various other circuits. Possible benefits/advantages of such use are already indicated above in reference to FIG. 4.
As used in this specification, the term “radio frequency” (RF) refers a rate of oscillation in the range of about 3 kHz to 300 GHz. This frequency may be the frequency of an electromagnetic wave or an alternating current in a circuit. This term should be construed to be inclusive of the frequencies used in wireless communication systems.
While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense.
Although RF circuit 400 (FIG. 4) has been described as being implemented using a microstrip technology, it can also be implemented using any other suitable technology, e.g., a coaxial technology or a planar (such as stripline, slotline, or planar-waveguide) technology. Other RF circuits disclosed in this specification can similarly be implemented using these technologies.
As used in the claims, the term “strip” should be construed to cover any conducting strip, such as a microstrip or a stripline, in the patterned layer of the corresponding planar circuit or printed circuit board.
In one embodiment, transcoupler 550 (FIG. 5) can be used in conjunction with one or more circuit elements that differ from directional coupler 510. For example, instead of being connected to microstrips 506 and 514, microstrip 554 of transcoupler 550 can be connected at the left (as viewed in FIG. 5) end thereof to a micro strip that has a width different from the width of microstrip 554 and the width of microstrip 562.
Various modifications of the described embodiments, as well as other embodiments of the invention, which are apparent to persons skilled in the art to which the invention pertains are deemed to lie within the principle and scope of the invention as expressed in the following claims.
The present invention may be implemented as circuit-based processes, including possible implementation on a single integrated circuit.
Unless explicitly stated otherwise, each numerical value and range should be interpreted as being approximate as if the word “about” or “approximately” preceded the value of the value or range.
The use of figure numbers and/or figure reference labels in the claims is intended to identify one or more possible embodiments of the claimed subject matter in order to facilitate the interpretation of the claims. Such use is not to be construed as necessarily limiting the scope of those claims to the embodiments shown in the corresponding figures.
Although the elements in the following method claims, if any, are recited in a particular sequence with corresponding labeling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those elements, those elements are not necessarily intended to be limited to being implemented in that particular sequence.
Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments necessarily mutually exclusive of other embodiments. The same applies to the term “implementation.”
Throughout the detailed description, the drawings, which are not to scale, are illustrative only and are used in order to explain, rather than limit the invention. The use of terms such as height, length, width, top, bottom, is strictly to facilitate the description of the invention and is not intended to limit the invention to a specific orientation. For example, height does not imply only a vertical rise limitation, but is used to identify one of the three dimensions of a three dimensional structure as shown in the figures. Such “height” would be vertical where the microstrips are horizontal but would be horizontal where the microstrips are vertical, and so on.
Also for purposes of this description, the terms “couple,” “coupling,” “coupled,” “connect,” “connecting,” or “connected” refer to any manner known in the art or later developed in which energy is allowed to be transferred between two or more elements, and the interposition of one or more additional elements is contemplated, although not required. Conversely, the terms “directly coupled,” “directly connected,” etc., imply the absence of such additional elements.
The present inventions may be embodied in other specific apparatus and/or methods. The described embodiments are to be considered in all respects as only illustrative and not restrictive. In particular, the scope of the invention is indicated by the appended claims rather than by the description and figures herein. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.

## Claims

1. An apparatus, comprising:
a digital pre-distortion circuit configured to pre-distort a digital input signal to generate a first pre-distorted digital signal and a second pre-distorted digital signal different from the first pre-distorted signal;
a first amplifier branch configured to generate a first amplified signal based on the first pre-distorted digital signal;
a second amplifier branch configured to generate a second amplified signal based on the second pre-distorted digital signal; and
a radio-frequency (RF) circuit configured to combine the first and second amplified signals to generate a combined signal, wherein:
the RF circuit is further configured to generate first and second feedback signals based on at least two of the first amplified signal, the second amplified signal, and the combined signal;
the digital pre-distortion circuit is configured to generate the first pre-distorted digital signal and the second pre-distorted digital signal based on said first and second feedback signals to counteract nonlinearity in the first and second amplifier branches: and
the RF circuit comprises a first transcoupler, said first transcoupler comprising:
a first branch connected between an output port of the first amplifier branch and an output port of the second amplifier branch; and
a second branch electromagnetically coupled to the first branch and having first and second terminals, wherein the first terminal is configured to carry the first feedback signal.

2. The apparatus of claim 1, wherein:
the first amplifier branch comprises:
a first transmitter configured to convert the first pre-distorted digital signal into a first RF signal; and
a first power amplifier configured to amplify the first RF signal to generate the first amplified signal;

the second amplifier branch comprises:
a second transmitter configured to convert the second pre-distorted digital signal into a second RF signal;
a second power amplifier configured to amplify the second RF signal to generate the second amplified signal.

3. The apparatus of claim 2, wherein the first power amplifier and the second power amplifier are configured to operate as a carrier stage and a peak stage, respectively, of a Doherty power amplifier.

4. The apparatus of claim 2, wherein the first power amplifier and the second power amplifier are configured to operate as respective phase-modulating stages of a Chireix power amplifier.

5. The apparatus of claim 1, wherein:
the first feedback signal is an attenuated copy of the first amplified signal; and
the second feedback signal is an attenuated copy of the second amplified signal.

6. The apparatus of claim 1, wherein:
the first feedback signal is an attenuated copy of the first amplified signal; and
the second feedback signal is an attenuated copy of the combined signal.

7. The apparatus of claim 1, wherein:
the RF circuit is further configured to generate a third feedback signal; and
each of the first, second, and third feedback signals is an attenuated copy of a respective one of the first amplified signal, the second amplified signal, and the combined signal.

8. The apparatus of claim 1, further comprising a feedback-receiver circuit configured to:
down-convert the first feedback signal to generate a first digital baseband signal;
down-convert the second feedback signal to generate a second digital baseband signal; and
apply said first and second digital baseband signals to the digital pre-distortion circuit, wherein the digital pre-distortion circuit is configured to generate the first pre-distorted digital signal and the second pre-distorted digital signal based on said first and second digital baseband signals.

9. The apparatus of claim 1, further comprising an antenna configured to emit an electromagnetic wave corresponding to the combined signal.

10. The apparatus of claim 1, wherein the digital pre-distortion circuit is configured to counteract said nonlinearity by counteracting compressive amplitude distortion in at least one of the first and second amplifier branches.

11. The apparatus of claim 10, wherein the digital pre-distortion circuit is further configured to counteract said nonlinearity by counteracting a phase mismatch between the first and second amplifier branches.

12. The apparatus of claim 1, wherein the digital pre-distortion circuit is configured to counteract said nonlinearity by counteracting a phase mismatch between the first and second amplifier branches.

13. The apparatus of claim 1, wherein the second terminal is configured to carry the second feedback signal.

14. The apparatus of claim 1, wherein:
the RF circuit further comprises an impedance transformer and a directional coupler connected in series with each other and the first branch of the first transcoupler; and
the directional coupler comprises:
a respective first branch connected between the impedance transformer and an output terminal; and
a respective second branch electromagnetically coupled to said respective first branch and having third and fourth terminals, wherein the third terminal is configured to carry the second feedback signal.

15. The apparatus of claim 14, wherein:
the RF circuit is further configured to generate a third feedback signal; and
the second terminal is configured to carry the third feedback signal.

16. The apparatus of claim 1, wherein the RF circuit further comprises a second transcoupler, said second transcoupler comprising:
a respective first branch connected in series with the first branch of the first transcoupler; and
a respective second branch electromagnetically coupled to said respective first branch and having third and fourth terminals, wherein the third terminal is configured to carry the second feedback signal.

17. The apparatus of claim 16, wherein:
the first branch of the second transcoupler has an impedance of about 35 Ohm; and
the first branch of the first transcoupler has an impedance of about 50 Ohm.

18. An apparatus, comprising:
a digital pre-distortion circuit configured to pre-distort a digital input signal to generate a first pre-distorted digital signal and a second pre-distorted digital signal different from the first pre-distorted signal;
a first amplifier branch configured to generate a first amplified signal based on the first pre-distorted digital signal;
a second amplifier branch configured to generate a second amplified signal based on the second pre-distorted digital signal; and
a radio-frequency (RF) circuit configured to combine the first and second amplified signals to generate a combined signal, wherein:
the RF circuit is further configured to generate first and second feedback signals based on at least two of the first amplified signal, the second amplified signal, and the combined signal; and
the digital pre-distortion circuit is configured to generate the first pre-distorted digital signal and the second pre-distorted digital signal based on said first and second feedback signals to counteract nonlinearity in the first and second amplifier branches; and

wherein the RF circuit comprises a first directional coupler, said first directional coupler comprising:
a first branch connected between an output port of the first amplifier branch and an output port of the second amplifier branch; and
a second branch electromagnetically coupled to the first branch and having first and second terminals, wherein the first feedback signal appears on the first terminal, wherein:
the RF circuit further comprises an impedance transformer and a second directional coupler connected in series with each other and the first branch of the first directional coupler; and
the second directional coupler comprises:
a respective first branch connected between the impedance transformer and an output terminal; and
a respective second branch electromagnetically coupled to said respective first branch and having third and fourth terminals, wherein the second feedback signal appears on the third terminal.

19. The apparatus of claim 18, wherein:
the RF circuit is further configured to generate a third feedback signal; and
the third feedback signal appears on the second terminal.

20. An apparatus, comprising:
a digital pre-distortion circuit configured to pre-distort a digital input signal to generate a first pre-distorted digital signal and a second pre-distorted digital signal different from the first pre-distorted signal;
a first amplifier branch configured to generate a first amplified signal based on the first pre-distorted digital signal;
a second amplifier branch configured to generate a second amplified signal based on the second pre-distorted digital signal; and
a radio-frequency (RF) circuit configured to combine the first and second amplified signals to generate a combined signal, wherein:
the RF circuit is further configured to generate first and second feedback signals based on at least two of the first amplified signal, the second amplified signal, and the combined signal; and
the digital pre-distortion circuit is configured to generate the first pre-distorted digital signal and the second pre-distorted digital signal based on said first and second feedback signals to counteract nonlinearity in the first and second amplifier branches; and

wherein the RF circuit comprises a first directional coupler, said first directional coupler comprising:
a first branch connected between an output port of the first amplifier branch and an output port of the second amplifier branch; and
a second branch electromagnetically coupled to the first branch and having first and second terminals, wherein the first feedback signal appears on the first terminal; and

wherein the RF circuit further comprises a transcoupler, said transcoupler comprising:
a respective first branch connected in series with the first branch of the first directional coupler; and
a respective second branch electromagnetically coupled to said respective first branch and having third and fourth terminals, wherein the second feedback signal appears on the third terminal.

21. The apparatus of claim 20, wherein:
the first branch of the transcoupler has an impedance of about 35 Ohm; and
the first branch of the first directional coupler has an impedance of about 50 Ohm.