Radio frequency power amplifier system and method for linearizing an output signal thereof

Abstract

To provide a radio frequency power amplifier system and a method for linearizing its output signal.SOLUTION: The present disclosure relates to a radio frequency power amplifier system (200) comprising a first input port (114) and a second input port (121). A radio frequency power amplifier system comprises a main amplifier (101) having an input (107) and an output (108), and a first auxiliary amplifier (102) and a second auxiliary amplifier (122) having respective inputs (109, 129) and outputs (110, 128). The radio frequency power amplifier system comprises an internal load (103) connected to the output (110) of the first auxiliary amplifier, and a feedback network (104) having an input end (111) connected to the output (110) of the first auxiliary amplifier and an output end (112) connected to the input (109) of the first auxiliary amplifier (102).SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] (Technical field of the invention) The inventive concepts relate to the field of radio frequency power amplifier systems, and in particular to devices that allow linearization of the relationship between their input and output signals and methods for performing such linearization. [Background technology]

[0002] BACKGROUND OF THE INVENTION It is well known that the output signal from a radio frequency power amplifier can be linearized by various techniques such as pre-distortion, feed-forward error correction, and feedback. These techniques are described in detail both in textbooks and in various scientific journals and white papers, for example, the white paper entitled "Linearizing High Power Amplifiers" by Allen Katz (Linear Technology Inc), and can be summarized as follows:

[0003] Predistortion techniques can be implemented as either analog or digital predistortion. In both methods, the behavior of a power amplifier is characterized and its response is compensated for by intentionally distorting the signal that drives the power amplifier. If the distortion applied to the input signal is carefully selected, the output signal from the power amplifier can be more linear. However, a major problem with such techniques lies in the fact that initial calibration and training are often required and that predistortion systems are often sensitive to drift, e.g., drift in temperature.

[0004] Another known technique is the feedback method, which can be described by the use of so-called Cartesian feedback. In a Cartesian feedback system, the response from the power amplifier is downconverted and compared to a baseband IQ signal. This approach eliminates problems related to drift and detailed characterization of the power amplifier's transfer function. The main disadvantages of Cartesian feedback systems are stability issues and the limited bandwidth that can be addressed while still maintaining stable operation. Cartesian feedback is therefore not suitable for systems in which the signal bandwidth is increased to several hundred MHz (e.g., the wireless communication standard 802.11ac, where the maximum bandwidth is 160 MHz). Cartesian feedback also adds circuit complexity and power dissipation.

[0005] Linear feedback is also used to linearize power amplifiers. Such techniques involve a feedback network from the power amplifier output to its input. This method gives better performance, but it is very difficult to achieve stability for all different load conditions.

[0006] While the techniques described above allow for a certain level of linearization, there is a need in the art that allows for improved linearization of power amplifier signals. Summary of the Invention [Means for solving the problem]

[0007] (Summary of the Invention) The aim of the inventive concept is to provide a technique that addresses at least some of the above concerns. This aim, as well as others that will become apparent hereinafter, are achieved by methods and devices as defined in the independent claims. Preferred embodiments are defined in the dependent claims.

[0008] According to a first aspect of the inventive concept, there is provided a radio frequency power amplifier system having first and second input ports arranged to receive first and second input signals, respectively. The radio frequency power amplifier system includes a main amplifier having an input and an output, and first and second auxiliary amplifiers having separate inputs and outputs. The radio frequency power amplifier system includes an internal load connected to the output of the first auxiliary amplifier, and a feedback network arranged to linearize the first input signal, the feedback network having an input connected to the output of the first auxiliary amplifier and an output connected to the input of the first auxiliary amplifier. The radio frequency power amplifier system also includes a feedforward amplifier having an input and an output.

[0009] The inputs of the main amplifier and the auxiliary amplifier are interconnected with the first input port at a common input node, the output of the second auxiliary amplifier and the second input port are interconnected with the input of the feedforward amplifier at a common node, and the outputs of the feedforward amplifier and the main amplifier are interconnected at a common output node.

[0010] The main amplifier is a replica of the first and second auxiliary amplifiers with increased gain, and the second auxiliary amplifier is a replica of the first auxiliary amplifier.

[0011] The ratio of the second input signal to the first input signal is the same as the ratio of the gain provided by the second auxiliary amplifier to the gain provided by the first auxiliary amplifier.

[0012] The second auxiliary amplifier, the second input port, and the feedforward amplifier together form a feedforward network that is arranged to at least partially remove deviations from the output of the main amplifier.

[0013] According to a second aspect of the inventive concept, there is provided a method for linearizing an output signal of a radio frequency power amplifier system according to the first aspect, the method comprising: The method includes obtaining deviations from an ideal response of a first auxiliary amplifier using a feedback network, isolating the deviations by removing a second input signal from the output of the second auxiliary amplifier, amplifying the isolated deviations using a feedforward amplifier, and adding the amplified deviations to the output of the main amplifier.

[0014] The radio frequency power amplifier system defined in the first aspect of the inventive concept may therefore be used to implement the method of the second aspect.

[0015] The concept of the present invention allows (i) indirect feedback to the main amplifier to be combined with (ii) a feedforward deviation to the output of the main amplifier, allowing the transfer function between the input signal and the output signal of the radio frequency power amplifier system to be improved.

[0016] With indirect feedback (i), a feedback network arranged around the first auxiliary amplifier is connected to an internal load for which the load conditions are known and stable. Such load conditions allow the feedback path to be stabilized, and therefore the signal processed by the first auxiliary amplifier to be linearized. The stable feedback path also allows an ideal response from the first auxiliary amplifier to a signal to be obtained. Because the loop gain of the feedback loop is typically not infinite, deviations from the ideal response are also obtained. Furthermore, the interconnection at the common input node allows the input signal of the main amplifier to be identical to the input signal of the first auxiliary amplifier. Because the main amplifier represents a replica of the first auxiliary amplifier, the response of the main amplifier to an input signal can be a replica of the response of the first auxiliary amplifier to the same input signal. Indirect feedback therefore allows the output signal of the main amplifier to be linearized (i.e., nearly undistorted), but carry amplified deviations from the ideal response of the first auxiliary amplifier depending on the finite loop gain of the feedback loop.

[0017] Regarding the feedforward deviation (ii), the interconnection at the common input node allows the input signal of the second auxiliary amplifier to be identical to the input signal of the first auxiliary amplifier. Because the second auxiliary amplifier is a replica of the first auxiliary amplifier, the output signal of the second auxiliary amplifier is similar to, increased from, or decreased from the output signal of the first auxiliary amplifier due to the finite loop gain of the feedback loop, allowing for a linearized output signal with a deviation similar to, increased from, or decreased from the ideal response of the first auxiliary amplifier. The feedforward network, consisting of the second auxiliary amplifier, the common node, and the feedforward amplifier and connected in parallel with the main amplifier, further allows the deviation to be isolated from the total output signal of the second auxiliary amplifier and amplified by the feedforward amplifier. Isolation of the deviation is made possible by subtraction of the signal provided by the second input port from the output signal of the second auxiliary amplifier via the common node of the feedforward network. The feedforward network then allows the amplified deviation to be added to the output signal of the main amplifier, resulting in complete or at least significant elimination of the deviation from the ideal response depending on the finite loop gain of the feedback loop.

[0018] The inventive concept thus allows for an improvement in the relationship between input and output signals in a radio frequency power amplifier. Such a concept allows for suppression of nonlinearities in the relationship over a wider frequency range of operation, by improving the elimination of distortions generated when amplifying the input signal and by completely or at least significantly eliminating deviations due to the finite loop gain of the feedback loop. Furthermore, the combination of indirect feedback and feedforward deviations allows the inventive concept to reduce its sensitivity to drift compared to other linearization techniques described in the prior art. The inventive concept allows for a reduction in power dissipation losses and is highly suitable for chip integration in both CMOS and bipolar technologies, for which implementation examples will be further described in the detailed description.

[0019] In this application, main amplifiers and auxiliary amplifiers may be understood to represent transconductance amplifiers, voltage amplifiers, current amplifiers, transimpedance amplifiers, and / or power amplifiers.

[0020] A feedforward amplifier may be understood to represent a current amplifier, a voltage amplifier, a transconductance amplifier, a transimpedance amplifier, and / or a power amplifier.

[0021] The gain of an amplifier can be understood as the factor by which a signal is increased from the input to the output of the amplifier. The gain of an amplifier represents the signal ratio or amplitude ratio between its output signal and its input signal. For example, if a main amplifier has a gain of nA, its output signal will represent its input signal increased by a factor of nA.

[0022] Loop gain can be understood as the gain provided by the feedback network multiplied by the gain in the forward amplifier path. For example, if an amplifier (forward amplifier) ​​provides a gain of A around which a feedback path providing a gain of β is connected, the loop gain of the feedback loop will be Aβ.

[0023] An ideal response or ideal case of an amplifier to an input signal can be understood as a perfectly linear behavior of the amplifier. Such an ideal response exhibits the absence of distortion and other nonlinearities between the input and output signals of the amplifier.

[0024] An amplifier replica may be understood to define an amplifier that responds similarly to the same input signal but provides a different gain to that signal. An amplifier replica may therefore define a relationship between the gain provided by one amplifier and the gain provided to an input signal by another amplifier. For example, a main amplifier of the inventive concept represents an enlarged replica of a first auxiliary amplifier in which the relationship between the gains of the amplifiers is by a factor n:1, i.e., if the gain of the first auxiliary amplifier is A, the gain provided by the main amplifier will be nA.

[0025] A node or common node may be understood as an interconnection between at least two of the inputs and outputs of an amplifier, and may be represented by at least one of a summer, a subtractor, or a general interconnection of conductors.

[0026] According to one embodiment, the main amplifier and the auxiliary amplifier may be configured to provide the same response to an input signal. Because the second auxiliary amplifier is a replica of the first auxiliary amplifier, and the main amplifier is also a replica of the first auxiliary amplifier, the responses that all amplifiers have to the same input signal may be similar in terms of linearity. Such linearity may be improved by a feedback loop because the inputs of all amplifiers are interconnected at a common input node inside the feedback loop. Deviations from the ideal response of the first auxiliary amplifier are also carried through the second auxiliary amplifier and the main amplifier because they are all driven by the same input signal, which depends on the finite loop gain of the feedback loop. However, the gain differs between the main amplifier and the auxiliary amplifier because the main amplifier is an enlarged replica of the first auxiliary amplifier and therefore provides a larger gain. The second auxiliary amplifier may also provide a different gain compared to the first auxiliary amplifier.

[0027] According to an embodiment, the first input port and the feedback network output of the radio frequency power amplifier system may be connected to a common input node via a subtractor. Such a subtractor allows a feedback signal to be subtracted from the signal at the first input port to suppress nonlinearities between the input signal and the output signal of the first auxiliary amplifier. Such a subtractor need not be a separate physical circuit, but may be a connection node to which the feedback signal is fed so as to be subtracted from the input signal at the first input port.

[0028] According to one embodiment, the common node where the output of the second auxiliary amplifier and the second input port are interconnected with the input of the feedforward amplifier may be a subtractor. Such a subtractor allows the input signal of the second input port to be subtracted from the output signal of the second auxiliary amplifier. Such subtraction allows deviations due to the finite loop gain of the feedback loop of the first auxiliary amplifier to be isolated from the total output signal of the second auxiliary amplifier. Such a subtractor therefore allows the input signal of the feedforward amplifier to represent only deviations that should be amplified and fed forward to the output of the main amplifier.

[0029] According to one embodiment, the ratio of the input signal provided by the second input port to the input signal provided by the first input port may be the same as the ratio of the gain provided by the second auxiliary amplifier to the gain provided by the first auxiliary amplifier. This embodiment is advantageous in that it ensures that deviations in the output signal of the second auxiliary amplifier can be isolated from the remainder of the output signal. Indeed, since the input signal of the second auxiliary amplifier is identical to the input signal of the first auxiliary amplifier, the signal provided by the second input port, which should be subtracted from the output signal of the second auxiliary amplifier, should be magnified relative to the signal provided by the first input port in the same manner as the magnification of the second auxiliary amplifier relative to the first auxiliary amplifier. In other words, if the second auxiliary amplifier has a gain ratio of 1:m relative to the first auxiliary amplifier, the input signal provided by the second input port should have a signal ratio of 1:m relative to the input signal provided by the first input port. For example, if the second auxiliary amplifier provides a gain of 0.5 to its input signal, the input signal provided by the second input port should represent half of the input signal provided by the first input port.

[0030] According to one embodiment, the feedforward amplifier may be configured to provide a gain such that its output signal has the same amplitude but opposite phase as the output signal of the main amplifier. This embodiment is advantageous in that the output signal of the feedforward amplifier represents an amplification of the deviations generated by the finite loop gain of the feedback network of the first auxiliary amplifier, which has the same amplitude but opposite phase as the deviations formed in the output signal of the main amplifier, and allows for complete or at least significant elimination of the deviations depending on the finite loop gain. Furthermore, the gain provided by the feedforward amplifier may depend on the gain provided by the second auxiliary amplifier. For example, if the gain provided by the second auxiliary amplifier is 0.5 and the gain provided by the main amplifier is 4, the gain provided by the feedforward amplifier should be 8. It should be noted that an output signal from the feedforward amplifier having an amplitude lower than the amplitude of the output signal of the main amplifier allows for partial elimination of the deviations formed in the output signal, thus still resulting in improved linearization.

[0031] According to one embodiment, the common output node where the outputs of the feedforward amplifier and the main amplifier are interconnected may be a summer. Such a summer allows an amplified deviation representing the output signal of the feedforward amplifier to be added (i.e., fed forward) to the output of the main amplifier. Such summation therefore allows complete or at least significant removal of deviations that depend on the finite loop gain of the feedback loop from the output signal of the main amplifier.

[0032] According to an embodiment, each of the auxiliary amplifiers may comprise a first transistor and a second transistor connected in series. The types of transistors used in the present invention may include, but are not limited to, metal-oxide-semiconductor field-effect transistors (MOSFETs) and / or bipolar junction transistors (BJTs). Such transistors allow modulation of an input signal into an amplified output signal. Furthermore, such transistors may allow modulation of power signals, voltage, and / or current signals.

[0033] According to an embodiment, the main amplifier may comprise a first transistor and a second transistor connected in series, and since the main amplifier represents a replica of the first auxiliary amplifier, it comprises first and second transistors (e.g., MOSFETs, BJTs, etc.) similar to the first auxiliary amplifier but with larger sizes, allowing for greater amplification of the input signal, i.e., providing greater gain.

[0034] According to an embodiment, the feedforward amplifier may represent an indirect feedback amplifier. Such an indirect feedback amplifier may comprise a feedback network that serves the same purpose as the feedback network arranged around the first auxiliary amplifier, i.e., that can reduce nonlinearities between the output signal and the input signal of the feedforward amplifier. Such feedback may be connected to a second internal load for which the load conditions are known and stable, thus enabling stability of the feedback. The feedback network of the feedforward amplifier thus allows deviations from the first auxiliary amplifier to be amplified and fed forward to the output of the main amplifier without any significant distortion potentially generated by the feedforward amplifier.

[0035] According to one embodiment, the radio frequency power amplifier system may be configured to receive input signals at first and second input ports, which may be provided from a driver circuit preceding the system. An alternative to this embodiment may be for the radio frequency power amplifier system to have only one input port. The input signal received from the driver circuit preceding the system at the single input port may then be equally supplied to the common input node and the common node.

[0036] According to certain embodiments, the output signal of a radio frequency power amplifier system may be fed to a load. The load and its conditions may dictate the load capability required from the radio frequency power amplifier system, i.e., the amplification required between the input and output signals of the system to power the load. Such a load may be represented by any suitable electronic device, such as an antenna, a filter, an external power amplifier, a loudspeaker, etc., that requires an amplified input signal to function.

[0037] According to an embodiment, the feedforward amplifier, the main amplifier, and the auxiliary amplifier may be configured on the same integrated circuit. "Integrated circuit" as used herein means an electronic circuit formed on a significantly small piece of semiconducting material (e.g., silicon). This embodiment is advantageous in that it allows the circuit components of the radio frequency power amplifier system to be significantly smaller in size, making them efficient for chip integration applications or the like. The present invention provides, for example, the following. (Item 1) A radio frequency power amplifier system (200), comprising: a first input port (114) and a second input port (121) arranged to receive a first and a second input signal, respectively; a main amplifier (101) having an input (107) and an output (108); a first auxiliary amplifier (102) and a second auxiliary amplifier (122) having respective inputs (109, 129) and outputs (110, 128); an internal load (103) connected to the output (110) of the first auxiliary amplifier (102); a feedback network (104) arranged to linearize the first input signal, the feedback network having an input (111) connected to the output (110) of the first auxiliary amplifier (102) and an output (112) connected to the input (109) of the first auxiliary amplifier (102); a feedforward amplifier (123) having an input (124) and an output (130); Equipped with the inputs (107, 109, 129) of the main amplifier (101) and the auxiliary amplifier (102, 122) are interconnected with the first input port (114) at a common input node (113); the output (128) of the second auxiliary amplifier (122) and the second input port (121) are interconnected with the input (124) of the feedforward amplifier (123) at a common node (127); the outputs (130, 108) of the feedforward amplifier (123) and the main amplifier (101) are interconnected at a common output node (125); the main amplifier (101) is a replica of the first and second auxiliary amplifiers (102, 122) with increased gain; the second auxiliary amplifier (122) is a replica of the first auxiliary amplifier (102); the ratio of the second input signal to the first input signal is the same as the ratio of the gain provided by the second auxiliary amplifier (122) to the gain provided by the first auxiliary amplifier (102); 1. A radio frequency power amplifier system, wherein the second auxiliary amplifier (122), the second input port (121), and the feed-forward amplifier (123) together form a feed-forward network (120) that is arranged to at least partially remove deviations from the output (108) of the main amplifier (101). (Item 2) Item 1. The radio frequency power amplifier system of item 1, wherein the main amplifier and the auxiliary amplifier are configured to provide identical responses to an input signal. (Item 3) 3. The radio frequency power amplifier system according to claim 1, wherein the first input port and the feedback network output are connected to the common input node via a subtractor (115). (Item 4) 10. The radio frequency power amplifier system of claim 9, wherein the common node is a subtractor. (Item 5) 10. The radio frequency power amplifier system of claim 1, wherein the feedforward amplifier (123) is configured to provide gain such that its output signal (130) has the same amplitude as the output signal (108) of the main amplifier (101) but has an opposite phase. (Item 6) 10. The radio frequency power amplifier system of claim 9, wherein the common output node is a summer. (Item 7) 10. The radio frequency power amplifier system of claim 9, wherein each of the auxiliary amplifiers comprises a first transistor (N1, N5) and a second transistor (N2, N6) connected in series. (Item 8) 2. The radio frequency power amplifier system of claim 1, wherein the main amplifier comprises a first transistor (N3) and a second transistor (N4) connected in series. (Item 9) 10. The radio frequency power amplifier system of claim 9, wherein the feedforward amplifier represents an indirect feedback amplifier. (Item 10) Item 3. The radio frequency power amplifier system of item 2, configured to receive input signals at the first and second input ports from a driver circuit preceding the radio frequency power amplifier system. (Item 11) 10. A radio frequency power amplifier system according to any one of the preceding items, configured to feed an output signal of the system to a load (105). (Item 12) 10. The radio frequency power amplifier system of claim 1, wherein the feedforward amplifier, the main amplifier, and the auxiliary amplifier are configured on the same integrated circuit. (Item 13) 1. A method for linearizing an output signal of a radio frequency power amplifier system, comprising: The radio frequency power amplifier system comprises: first and second input ports arranged to receive first and second input signals, respectively; a main amplifier having an input and an output; first and second auxiliary amplifiers having respective inputs and outputs; an internal load connected to the output of the first auxiliary amplifier; a feedback network arranged to linearize the first input signal, the feedback network having an input connected to the output of the first auxiliary amplifier and an output connected to the input of the first auxiliary amplifier; a feedforward amplifier having an input and an output; Equipped with the inputs of the main amplifier and the auxiliary amplifier are interconnected with the first input port at a common input node; the output of the second auxiliary amplifier and the second input port are interconnected with the input of the feedforward amplifier at a common node; the outputs of the feedforward amplifier and the main amplifier are interconnected at a common output node; the main amplifier is a replica of the first and second auxiliary amplifiers with increased gain; the second auxiliary amplifier is a replica of the first auxiliary amplifier; the ratio of the second input signal to the first input signal is the same as the ratio of the gain provided by the second auxiliary amplifier (122) to the gain provided by the first auxiliary amplifier (102); The method comprises: obtaining a deviation from an ideal response on the first auxiliary amplifier using the feedback network; isolating the deviation by removing the second input signal from the output of the second auxiliary amplifier; amplifying the isolated deviation using the feedforward amplifier; adding said amplified deviation to the output of said main amplifier; A method comprising:

[0038] The above and additional objects, features, and advantages of the present invention will be better understood through the following illustrative and non-limiting detailed embodiments of the present invention, with reference to the accompanying drawings. [Brief explanation of the drawings]

[0039] [Figure 1] FIG. 1 illustrates a block diagram of a radio frequency power amplifier system with indirect feedback. [Figure 2] FIG. 2 illustrates a block diagram of an embodiment of a radio frequency power amplifier system in accordance with the present invention in which indirect feedback is combined with feedforward deviation. [Figure 3] FIG. 3 illustrates an exemplary circuit of a CMOS-based implementation of the radio frequency power amplifier of FIG. [Figure 4] FIG. 4 illustrates an example circuit of the radio frequency power amplifier of FIG. 3, in which an example circuit of a feedforward amplifier is shown. [Figure 5] FIG. 5 illustrates a comparison of the linearity performance of a system such as that shown in FIG. 1 with an implementation of the present invention as shown in FIGS. DETAILED DESCRIPTION OF THE INVENTION

[0040] Detailed Description of the Embodiments FIG. 1 illustrates a block diagram of a radio frequency power amplifier system implementing an indirect feedback linearization technique. The block diagram of FIG. 1 shows a main amplifier 101 having an input 107 and an output 108, a first auxiliary amplifier 102 having an input 109 and an output 110, an internal load 103 connected to the output 110 of the first auxiliary amplifier, and a feedback network 104 having an input 111 connected to the output 110 of the first auxiliary amplifier 102 and an output 112 connected to the input 109 of the first auxiliary amplifier 102 via a subtractor 115. The inputs 107, 109 of the main and first auxiliary amplifiers 101, 102 are interconnected at a common input node 113. The power amplifier system 100 has a first input port 114 connected to the common input node 113 via a subtractor 115. The radio frequency power amplifier system 100 is connected to an external load 105, represented in FIG. 1 by an antenna.

[0041] The main amplifier 101 constitutes a replica of the first auxiliary amplifier 102. In many applications, the scaling factor between the main amplifier 101 and the first auxiliary amplifier 102 may range from approximately 2:1 to 8:1, however, other relationships may be provided as well. Such a scaling factor defines the relationship between the gain between the input and output signals of the main amplifier 101 and the corresponding gain of the first auxiliary amplifier 102. Thus, for example, with a 4:1 scaling factor, the gain of the main amplifier 101 is equal to four times the gain of the first auxiliary amplifier 102.

[0042] The radio frequency power amplifier system 100 of FIG. 1 operates as follows: an input signal is fed into the system at the first input port 114, from which a feedback signal, i.e., the output of the feedback network 104, is subtracted in the subtractor 115. Because the feedback network 104 is connected around the first auxiliary amplifier 102, it linearizes the first auxiliary amplifier 102 and causes little distortion of the output signal at its output 106. The operation of the feedback network 104 around the first auxiliary amplifier 102 can be stabilized because the internal load 103 is known and inaccessible from the outside. In other words, the load conditions are known and do not vary. Nonlinearities within the first auxiliary amplifier 102 are therefore suppressed due to the feedback network 104. The main amplifier 101 is then driven by the same input signal as the first auxiliary amplifier 102, because both of their inputs are interconnected at a common input node 113. Because the main amplifier 101 is a replica of the first auxiliary amplifier 102, it will respond to the same input signal in the same way as the first auxiliary amplifier 102 and will result in reduced nonlinearity between the input and output signals of the main amplifier 101. The output signal of the main amplifier 101 is then fed to the load 105 with increased linearity.

[0043] However, the finite loop gain provided by the feedback loop comprising the first auxiliary amplifier 102 and the feedback network 104 may cause the signal to deviate from the ideal response of the first auxiliary amplifier, which cannot be compensated for in this embodiment. Such deviations may be carried through the main amplifier 101 and thus amplified along with the rest of the linearized signal.

[0044] FIG. 2 shows a block diagram of a radio frequency power amplifier system 200, which comprises the radio frequency power amplifier system 100 of FIG. 1, with the addition of a feedforward network 120. The components and their functions described in FIG. 1 are therefore represented identically in FIG. 2. The feedforward network 120 of FIG. 2 comprises a second auxiliary amplifier 122 with an input 129 and an output 128. The second auxiliary amplifier 122 is a replica of the first auxiliary amplifier 102. Thus, both auxiliary amplifiers 102, 122 respond to an input signal in the same manner. The feedforward network 120 further comprises a feedforward amplifier 123 having an input 124 and an output 130. The input 129 of the second auxiliary amplifier 122 is interconnected with the inputs (107, 109) of the main amplifier 101 and the first auxiliary amplifier 102 at a common input node 113. The output 128 of the second auxiliary amplifier 122 and the second input port 121 are interconnected at a subtractor 127 with the input 124 of the feedforward amplifier 123, and the output 130 of the feedforward amplifier 123 and the output 108 of the main amplifier 101 are interconnected at a common output node 125, represented by a summer. The feedforward network 120 is therefore connected in parallel with the main amplifier 101.

[0045] The indirect feedback provided to the main amplifier 101 by the feedback network 104 arranged around the first auxiliary amplifier 102 functions in accordance with the operation of the radio frequency power amplifier system 100 described above with reference to FIG. 1. In addition to the indirect feedback, the feedforward network 120 of the radio frequency power amplifier system 200 operates as follows: the input signal of the second auxiliary amplifier 122 is identical to the input signal of the first auxiliary amplifier 102, which represents a linearized signal with deviations caused by the finite loop gain of the feedback loop. Because the second auxiliary amplifier 122 is a replica of the first auxiliary amplifier 102, the output signal of the second auxiliary amplifier 122 is similar to, increased or decreased from, the output signal of the first auxiliary amplifier 102. In addition, the input signal provided to the second input port 121 is amplified relative to the input signal provided to the first input port 114 by the same ratio as the ratio of the gain of the second auxiliary amplifier 122 to the gain of the first auxiliary amplifier 102. The input signal at the second input port 121 is subtracted in a subtractor 127 from the output signal of the second auxiliary amplifier 122, thus isolating the deviation from the ideal response of the first auxiliary amplifier 102 from the remainder of the output signal at the output 128. The input signal 124 of the feedforward amplifier 123 therefore represents only the deviation, which is then amplified by the feedforward amplifier 123 in such a way that it has the same amplitude as the deviation carried by the output 108 of the main amplifier, but with a 180° phase shift, i.e., opposite phase. A summer 125 then adds the amplified deviation to the output 108 of the main amplifier 101, resulting in a linearized output signal 126 from which the deviation from the ideal response of the power amplifier is completely or at least significantly removed. The output signal 126 is then supplied to the load 105.

[0046] It should be understood that another example of the radio frequency power amplifier system 200 can be such that the amplification of the deviation isolated by the feedforward amplifier 123 is in phase with the deviation implemented by the output 108 of the main amplifier 101, but the common output node 125 is represented by a subtractor. It should also be understood that another example of the radio frequency power amplifier 200 can be such that the input signal at the second input port 121 has a 180° phase shift compared to the input signal at the first input port 114, but the common node 127 is represented by a summer.

[0047] 2 can be further explained in mathematical terms. Considering that the gains provided by the first and second auxiliary amplifiers 102, 122 are the same and equal to A, the gain provided by the main amplifier 101 is nA (i.e., n times greater than the gain of the auxiliary amplifiers 102, 122), the gain provided by the feedforward amplifier 123 is −n (i.e., a gain of n with a 180° phase shift), and the gain provided by the feedback network 104 is β, then the signal S after the subtractor 115 is 113 can be expressed as follows:

number

number

number

number

number

number

[0048] Thus, in contrast to prior art linearization techniques, the signal processing performed using the added feedforward network 120 results in a better defined relationship between the output signal and the input signal, resulting in improved linearity.

[0049] Figure 3 shows an example of a CMOS-based implementation of the inventive concept. Figure 3 illustrates a differential implementation, consisting of an upper half 301 and a lower half 302, in which both halves are identical. Each half 301, 302 is driven by an input signal connected to a respective first and second input port, i.e., IN1+ and IN2+ for upper half 301 and IN1- and IN2- for lower half 302. The two input ports are driven by the same but opposite-phase currents i s2. The radio frequency power amplifier system 200 is fed by differential output ports OUT+ and OUT−, respectively, 304, 305, which are generated from a driver circuit preceding the radio frequency power amplifier system, and is connected to a transformer 306, which converts the differential output signal into a single-ended output signal OUT, which is supplied to a load 307. The different blocks of the radio frequency power amplifier system 200 illustrated in FIG. 2 are described below with respect to the upper half 301 of a CMOS-based implementation.

[0050] The main amplifier may include a capacitance C2 and two transistor devices, N3 and N4, each of which is an n-channel MOSFET or NMOS. The first auxiliary amplifier is implemented with a capacitance C1 and two transistor devices, N1 and N2, each of which is an NMOS, connected to an internal load consisting of a parallel resonant circuit 308. The first auxiliary amplifier is implemented in a similar manner to the main amplifier; i.e., the two amplifiers provide identical responses to the input signal. The feedback network may include a short circuit 309 from the drain of transistor N1 to input port IN1+. The two capacitances C1 and C2 can be considered short circuits at the frequency of operation and are not essential for the behavior of the radio frequency power amplifier system. The second auxiliary amplifier is implemented with a capacitance C3 and two transistor devices (NMOS), N5 and N6.

[0051] The second auxiliary amplifier is connected in parallel with the main amplifier, but in this implementation is not scaled to provide gain; i.e., the gain provided by the second auxiliary amplifier is the same as the gain in the first auxiliary amplifier. Since the auxiliary amplifiers are identical replicas in this implementation, transistor devices N5 and N6 should be equal in size to transistor devices N1 and N2, respectively. Thus, the current i in transistor devices N5 and N6 d2 is the current i in the transistor devices N1 and N2 due to indirect feedback. d1Furthermore, the feedforward amplifier 303 may be represented by a current amplifier and may provide gain so that its output signal may be of the same amplitude as the output signal of the main amplifier but have an opposite phase.

[0052] The operation of the CMOS implementation of the radio frequency power amplifier of the present invention can be explained using mathematical expressions as follows: s 304) generates a voltage at the first input port IN1+ and thereby a voltage at the gate of transistor device N2. The voltage at the gate of transistor device N2 flows through transistor devices N1 and N2, causing a current i d1 Similar to equation (2) in Figure 2, the current i d1 and input current i s The relationship between 304 and 306 can be expressed by the following equation:

number

number

number

number

number

number

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[0053] FIG. 4 shows an example circuit of a CMOS implementation of the radio frequency power amplifier of FIG. 3, with additional details regarding the circuitry of the feedforward amplifier 303. In FIG. 4, the feedforward amplifier 303 may be represented by a current amplifier and may include transistor devices (NMOS) N11 and N12, respectively, connected to a capacitance C11 and an internal load consisting of a parallel resonant circuit 310. The feedforward amplifier 303 therefore has a feedback network consisting of a short circuit 311 from the drain of transistor N11 to the input of the current amplifier 303. The feedforward amplifier 303 further includes a second capacitance C12 and a second pair of transistor devices (NMOS) N13 and N14, respectively. The example circuit of the amplifier 303 operates using an indirect feedback technique to linearize the relationship between the input and output signals of the feedforward amplifier 303. The feedback loop presented here also generates deviations from the ideal case due to its finite loop gain. However, this deviation will not be very noticeable because the input signal level to the feedforward amplifier is small compared to the maximum signal that the feedforward amplifier can handle. In this implementation, the input signal to the feedforward amplifier may represent a much weaker signal than the input signal to the main amplifier 101. Therefore, the deviation is negligible.

[0054] FIG. 5 shows a comparison of the linearity performance of the system shown in FIG. 1 with the implementation of the present invention as shown in FIGS. 3 and 4. According to an embodiment, the linearity performance is simulated using an 80 MHz 802.11ac input signal. The output power level in both simulations is +20 dBm, with the response of the radio frequency power amplifier of FIG. 1 shown in output spectrum 501 and the response of the implemented invention shown in output spectrum 502. As can be seen from these results, the EVM (error vector magnitude) performance of the implemented invention is improved by approximately 15 dB compared to the prior art, which represents the typical deviation caused by a loop gain of -10. In addition to the improved EVM, the present invention also provides an approximately 0.7 dB increase in gain compared to the prior art.

[0055] While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description is to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments.

[0056] Other variations of the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from an examination of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite articles "a" or "an" do not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

[Claim 1] The invention as described in the drawings of this application.

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