Power amplifier circuit

Abstract

A power amplifier circuit comprises an input connected to first (8) and second (9) active elements and an output and is configured for amplifying an input signal. The circuit further comprises a ring resonator (16) with first (14) and second (15) input ports to which the first (8) and second (9) active elements are connected. The ring resonator (16) comprises a single output port (19) connected to the output of the power amplifier circuit. The first (14) and second (15) input ports and the single output port (19) are connected at different angular positions on the ring resonator (16). These angular positions are such that the signals of the active elements (8, 9) constructively combine for providing the output signal. Furthermore, the first (8) and second (9) active elements have element output impedances which are substantially lower than a device output impedance of the output of the power amplifier circuit.

Description

[0001] P136988PC00 Title: Power amplifier circuit Field of the invention The present invention is directed at a power amplifier circuit, the circuit comprising an input for receiving an input signal and an output for providing an output signal, wherein the power amplifier circuit is configured for amplifying the input signal for providing the output signal. Background Industrial heating applications and plasma physics experiments require radio-frequency (RF) amplifiers capable of generating and delivering great amounts of RF power in media whose electric characteristics can vary dramatically. Achieving the necessary high power levels is done by combining the outputs of many amplifiers using dedicated combining networks. Losses in these combining networks must be kept as low as possible. Additionally, the greater the variation of the electric characteristics of the media, the more power is reflected back towards the amplifier. Protection systems which isolate the amplifiers from the reflected power are necessary, however they cannot provide perfect isolation. There is always some amount of reflected power which travels back to the amplifiers and influences their performance. Summary of the invention It is an object of the present invention to diminish the shortcomings of conventional power amplifiers, and to provide a power amplifier circuit that is efficient and is designed to be robust and well protected from reflected power from an arbitrary load connected thereto. To this end, there is provided herewith a power amplifier circuit as referred to above, wherein the power amplifier circuit further comprises at least two active elements for performing said amplification of the input signal. The input is thereby connected to a first active element of the at least two active elements and to a second active element of the at least two active elements. The power amplifier circuit further comprises a ring resonator including a first input port and a second input port. The first input port of the ring resonator connects to the first active element for receiving a first part of the output signal. The second input port of the ring resonator connects to the second active element for receiving a second part of the output signal. The ring resonator comprises a single output port connecting the ring resonator to the output of the power amplifier circuit. The first input port is provided at a first angular position on the ring. The second input port is provided at a second angular position on the ring different from the first angular position. The single output port is provided at a third angular position on the ring different from the first and the second angular position. In particular, the first, second and third angular position are such that the first part of the output signal and the second part of the output signal constructively combine for providing the output signal. Furthermore, each of the first active element and the second active element has an element output impedance which is substantially smaller than a device output impedance of the output of the power amplifier circuit. The present invention applies a modified rat-race coupler arrangement in order to diminish the effect of load side power reflections that are received in use at the output of the power amplifier circuit. In particular, the ring resonator of the present invention enables to decouple the output of the power amplifier circuit from the active elements. It enables to match to device output impedance of the power amplifier circuit to the load, while internally – between the input ports of the ring resonator and each of the active elements – reducing the element output impedance of the active elements. The latter effectively prevents a majority of reflections present in the resonator from entering the active elements. In addition, dimensioning of the ring resonator enable it to be designed for operation in a predefined bandwidth around a certain frequency, and to be impervious to signal outside this bandwidth. This prevents any disturbance signals outside the characteristic bandwidth of the resonator from reaching the active elements. Any reflections that may enter the ring resonator from the output port will mostly be dissipated in the ring resonator itself, due to the low output impedance of the first and the second active elements connected at the first and second input port of the ring resonator. So overall, the power amplifier design of the present invention is efficient and is designed to be robust and well protected from reflected power from an arbitrary load connected thereto. Although it may be applied to any desired RF application, the advantages of the proposed design enhance in those applications that typically suffer from load side reflections. Thus, for example in the earlier mentioned applications such as industrial heating or drying and plasma generation, the power amplifier circuit of the present invention may advantageously be applied. However, other fields of application may include for example power amplification in magnetic resonance imaging systems (MRI systems), radar systems or particle accelerators for example, to mention a few. In such applications, load side reflections can cause power loss, signal distortion, and potential damage to the amplifier. The proposed design of the invention enable to mitigate the effects of reflected power, and are thus crucial for improving performance and reliability in these fields. The term ‘device output impedance’ in the present document refers to the output impedance of the power amplifier as a whole, hence the output impedance of the output of the power amplifier circuit that will be connected to the load. Standardized, power amplifiers for RF power amplification are designed to have a typical output impedance of 50 ohms (50 Ω), which in the present invention also provides a preferred embodiment. Although, of course, the output impedance could have any other value if a different value is desired, given the application. The term ‘element output impedance’ applied in this document refers to the output impedance of each active element, i.e. each of the first active element connected at the first input port of the ring resonator and the second active element connected at the second input port of the ring resonator. These are the internal output impedances from each of the active elements towards input ports of the ring resonator. Although the wording will be understood by the skilled person without any expectable difficulty, for completeness it is to be mentioned here that the wording ‘constructive combination’ or variants thereof, as applied in the present document, should be interpreted to mean that the signals to which the wording applies combine in a manner such that due to constructive interference of the signals their amplitudes add up, thus effectively summing the signals to maximize the amplitude of the resulting signal. Conventional rat-race coupler typically include a ring resonator having four ports, each placed one-quarter wavelength away from each other around the top half of the ring. This configuration is also known to the skilled person as a ring hybrid. The present modified rat-race coupler design has three ports, including two input ports and an output port. The output port is placed at an angular position such that the first part of the output signal (i.e. the signal coming from the first active device) and the second part of the output signal (i.e. the signal coming from the first active device) constructively combine for providing the output signal. Multiple angular positions satisfy this requirement, and hence although there are preferred locations, the present invention should not be limited to be directed at a single third angular position for which this requirement is satisfied. This will be explained further below. Furthermore, throughout this document, reference is made to various angular positions. In geometric definitions of angular positions in degrees, the angular scale runs from 0 degrees to 360 degrees (full circle). However, in the field of microwave technology, it is common to express wavelengths (symbol λ) in terms of "electrical degrees". One full wavelength is 360 electrical degrees. The full circle across the midline of the resonator ring has a length (or circumference) of one-and- a-half times the wavelength λc of the center frequency fc of the bandwidth of the resonator ring, thus 1.5 * λc. Because one full wavelength λ equals 360 electrical degrees, the length (or circumference) of the midline of the resonator ring in terms of electrical degrees equals 540 electrical degrees (instead of 360 degrees in geometrical degrees). It is important to realize that the angular positions and angles on the resonator ring, in this document, are expressed as electrical degrees. The skilled person is well aware of the definition of electrical degrees. A degree is defined as 1 / 360thof a complete revolution, and for an alternating current a full period and thus a single full wave equals 360 degrees. In accordance with some embodiments, the element output impedance of each of the first active element and the second active element is smaller than 20% of the device output impedance of the output of the power amplifier circuit; preferably wherein the element output impedance is smaller than 10% of the device output impedance; more preferable wherein the element output impedance is smaller than 5% of the device output impedance. A low output impedance of a source (such as each of the active elements towards the ring resonator) in general, results in reflections from the load back to the output port to be minimized because the source can effectively absorb the reflected signals, reducing the impact thereof. The low output impedance at the first and second input port of the ring resonator, where the first and second active elements are respectively connected, thus results in minimal reflections towards the active elements. Towards the ring resonator, the low impedance at the first and second input ports can still result in reflections in the ring resonator, however there they have only little impact because the robust design of the ring resonator allows the reflections to safely dissipate without damage to sensitive components. On the other hand, on the output port of the ring resonator, it is important that the output impedance is well matched with the input impedance of the load, in order to prevent or reduce the occurrence of reflected signals as much as possible. The above embodiments thus enable to protect the power amplifier circuit from such reflections effectively. In some embodiments, the first active element and the second active element are configured for providing the first part of the output signal and the second part of the output signal to be in counter phase with each other, such that the momentary voltage of the first and the second part of the output signal is of opposite sign. These embodiments provide just one class of embodiments, as we will see further below. The counterphase signals (i.e. being half a wavelength (0.5*λ) out-of-phase) allow the active devices to present complementary impedances to the ring resonator, resulting in an active load-pulling effect. This enables to maintain optimal impedance matching across a wide range of load conditions. By presenting complementary impedances, these embodiments ensure that any variations in load impedance (i.e. at the output of the ring resonator) are transformed into smaller variations encountered by the active devices. This enhances the performance and efficiency the power amplifier circuit of the present invention. The counterphase signals coupled into the ring resonator via the first and second input ports at the first and second angular position respectively, constructively combines into a maximum output signal at the output port of the ring resonator, located at the third angular position. The identical but counterphase signals obtained from the active elements may be obtained in several different ways. In some embodiments, the first active element is connected to the input of the power amplifier circuit via a first conductive path, and the second active element is connected to the input of the power amplifier circuit via a second conductive path, wherein the first and the second conductive path are of different length such as to provide differential transmission lines having a length difference, the length difference providing a phase difference such that the input signal received by the first and the second active element is of opposite sign. This provides for a robust and elegant design for obtaining a half-wave phase difference between the input signals. Although for that reason, the use of differential transmission lines in this manner provides an attractive embodiment of the invention, other embodiments may provide characteristics or advantages that are beneficial to certain fields of application. For example, in some embodiments, the first active element is connected to the input of the power amplifier circuit via a first conductive path, and the second active element is connected to the input of the power amplifier circuit via a second conductive path, wherein at least one of the first conductive path, the second conductive path, or the power amplifier circuit, for providing a phase difference between the signals received by the first and second active element, comprises at least one element of a group comprising: a fixed or variable delay element, a passive or active phase shifting element, or a phase-locked loop. Some of these embodiments may for example provide benefits in terms of frequency bandwidth or enable to be tuned to different frequencies. In each of the above embodiments, the power amplifier circuit designs are suitable for providing, using the first active element and the second active element, the first part of the output signal and the second part of the output signal to be in counter phase with each other. In any of these, in accordance with yet further embodiments, an angular difference between the first and the second angular positions on the ring resonator equals θ, and wherein an angular difference between the first and the third angular positions on the ring resonator equals 3 / 2*θ. For example, the angle θ may equal 90 electrical degrees such as to match a length equal to λc / 4 with λc being the wavelength of the central frequency of the bandwidth of the ring resonator. This will result in a constructive combination of the two input signals at the angular position corresponding to 3 / 2*θ away from the first angular position where the first input signal is provided. In some embodiments, the first active element and the second active element are configured for providing the first part of the output signal and the second part of the output signal to be in-phase with each other, such that the momentary voltage of the first and the second part of the output signal is of equal sign. Although the additional active load-pulling effect may not be achieved when the signals coming from the first and second active elements are in-phase, the benefits of the present invention obtained using the ring resonator as matching network between the active elements and the output of the power amplifier circuit are still present in these embodiments. For these reasons, this class of embodiments is certainly still an improvement over the conventional power amplification devices used in the field. However, this class of embodiments also provides other advantages over differential configuration, i.e. the earlier described class of embodiments wherein the active elements provide their signals in counterphase to the ring resonator. In this configuration, the input impedance at the first and second input ports of the ring resonator is the complex conjugate of the impedance in the differential configuration. The inductive behavior observed in the differential configuration is inverted to capacitive behavior in the present embodiments wherein the devices are excited in-phase. These embodiments are therefore are particularly beneficial for applications where the load impedance is predominantly capacitive, because the amplifier can more effectively match the impedance of capacitive loads, improving power transfer and efficiency. In some of these embodiments, an angular difference between the first and the second angular positions on the ring resonator equals θ, and wherein an angular difference between the third angular position and one or both of the first and the second angular position on the ring resonator equals (3 / 2*θ+90º). In the abovementioned class of embodiments wherein the first and the second part of the output signal are applied to the ring resonator in-phase, constructive combination of the first and the second part of the output signal takes place in the ring resonator at different angular positions. Therefore, in some preferred embodiments of this class of embodiments, the third angular position preferably is located at an angular difference with both the first and the second angular position, wherein the angular difference is (3 / 2*θ+90º) – thus 90 electrical degrees plus one-and-a-half times the angular difference θ between the first and the second angular position, wherein θ equals a quarter wavelength – thus θ = λ / 4. In the present invention, the circumference of the midline through the ring resonator equals one-and-a-half times the wavelength λc of the center frequency fc of the ring resonator. Thus, in some embodiments the ring resonator may be designed with a predetermined circumference of its midline in order to design the ring resonator having a predetermined center frequency fc. In some embodiments of the power amplifier circuit of the present invention, the ring resonator comprises a width defined by an outer radius of the ring resonator minus an inner radius of the ring resonator. The width of the ring resonator determines the characteristic impedance (ZR) thereof. It is chosen in such a way that the ring acts as an impedance transformer under active load-pull conditions (e.g. see equations 4 and 6 in the description further below). The impedance transformer matches (or transforms) the optimum impedance that is presented to the active devices (ZA in these equations) to the reference impedance at the output port (ZL in these equations), which is ideally 50 Ohm, but will vary under mismatch. Other dimensions as well as the material of the ring resonator may be predesigned to provide the ring resonator with desired characteristics. For example, the thickness in the axial direction determines various characteristics of the impedance of the ring resonator, such as its resistance or its inductance. Also the material of which the ring resonator is made will significantly determine its electrical behavior. In some embodiments, the active elements are at least one element of a group comprising: transistors, such as bipolar junction transistors or field effect transistors; operational amplifiers; or vacuum tubes. As may be appreciated, many different variants of active elements may be applied in the present invention. In addition to the above, it is important to realize that the invention does not necessarily rely on the application of single elements applied as active elements. The active elements may also be provided by active circuits, each active circuit comprising multiple elements, wherein the circuits are configured for amplification of a signal offered to them in order to provide amplified signals to the ring resonator via the first and second input ports. Ideally, the active elements are identical or at least provide an identical output signal in response to identical input signals offered at the inputs of the active elements. In the power amplifier circuit of the present invention, these output signals will be offered to the ring resonator in- phase or in counter phase, as described above. In some embodiments, the device output impedance is fifty ohms – 50 Ω – and wherein the element output impedance is less than 5 ohms – 5 Ω –, preferably less than 3 ohms – 3 Ω –. For example, the element output impedance may be 2 ohms – 2 Ω –. Good results have been obtained with these values. The device output impedance is fifty ohms – 50 Ω – is in line with the industry standards, and hence provide excellent impedance matching for many applications. Brief description of the drawings The invention will further be elucidated by description of some specific embodiments thereof, making reference to the attached drawings. The detailed description provides examples of possible implementations of the invention, but is not to be regarded as describing the only embodiments falling under the scope. The scope of the invention is defined in the claims, and the description is to be regarded as illustrative without being restrictive on the invention. In the drawings: Figure 1 schematically illustrates schematically illustrates a power amplifier circuit in accordance with the concepts described herein; Figure 2 schematically .illustrates a ring resonator 16 applied as matching network in a power amplifier circuit in accordance with the concepts described herein; Figure 3 schematically illustrates an electronic scheme of the power amplifier circuit of figure 1; Figure 4 shows a symmetric variant of the ring resonator of figure 2, useable with active devices proving non-differential, thus in-phase signals; Figures 5A to 5D illustrate the S-parameters of the power amplifier circuit in accordance with the concepts described; Figure 6 shows a Smith diagram illustrating the achievable active impedances for the power amplifier circuit in accordance with the concepts described. Detailed description Figure 1 schematically illustrates a power amplifier circuit 1 in accordance with an embodiment of the present invention. The power amplifier circuit 1 comprises an input 4 for receiving an input signal and an output 5 for providing an output signal. The power amplifier circuit 1 is configured for amplifying the input signal received via the input 4 and for providing the amplified input signal at the output 5 as the output signal. The power amplifier circuit 1 further comprises two active elements 8 and 9, which in the illustrated embodiment are provided by a first transistor T1 (8) and a second transistor T2 (9). The active elements 8, 9 are connected to the input 4 via conductive paths 10, 11. The conductive paths 10 and 11 are of different length, which in the embodiment of figure 1 provides for differential transmission lines that establishes an phase difference between the signals that are respectively received by the first active element 8 and the second active element 9, the second active element 9 receiving a signal that is identical to the signal received by active element 8 but that is lagging in time such as to establish the desired phase difference. The phase difference, in the embodiment illustrated, equals a full multiple of half a wavelength such that the received signals at active elements 8 and 9 are in counter phase, thus a phase difference of 180 electrical degrees. The power amplifier circuit 1 further comprises a ring resonator 16 (sometimes referred to as ‘rat-race-coupler (RRC)’ or ‘ring hybrid’). The ring resonator 16 in the present invention comprises three ports, including a first input port 14, a second input port 15 and an output port 19. With reference to figure 2, the first input port 14 and second input port 15 are located at respectively a first angular position 20 and a second angular position 21. The angular difference between the first angular position 20 and the second angular position 21 will be referred to in the present document as θ and will be equal to a quarter wavelength λc (hence θ = λc / 4 = 90º (electrical degrees)), wherein λc is the wavelength of the center frequency fc of the bandwidth (BW) of the ring resonator 16. The total circumference of the midline through the ring resonator 16 equals one-and-a-half times the wavelength λc of the center frequency fc. In electrical degrees, this equals 540 electrical degrees (i.e.540º). If, for convenience, the second input port 15 at the second angular position 21 is defined to be at 0º, then in the clockwise direction the first angular position 20 indicating the location of the first input port 14 will be at θ = 90º (in fact θ = λc / 4 = 90º). The output port 19, in clockwise direction, is at a further one-and-a-half θ, thus 3 / 2*θ = 135º away from the first input port 20, hence as seen from the second input port 21 at the third angular position 22 at 225º. The locations indicated are ideal for active elements 8 and 9 operating in counterphase, because at the indicated location constructive interference between the signals coupled into the ring resonator 16 by the active elements 8 and 9 causes the two signals to be in phase such that the waves add up to double the amplitude, i.e. constructively combine. Figure 2, in dotted lines, also shows the locations of ports applied in a conventional RRC configuration. Output port 19 is located in between the conventional locations 15’ and 17’, where in a conventional RRC configuration the port 15’ is the second input port and port 17’ is a differential output port (although a differential port could be optional to the invention, it is not a port that is required in the present invention). The output port 19 is placed exactly halfway in between the conventional locations of ports 15’ and 17’ in the embodiment of figure 1. In the embodiment of figure 1, the power amplifier circuit of the present invention, also referred to as self-matching balanced power amplifier (SMBA or SMBPA), comprises two active elements 8 and 9. In the shown embodiment, the two active elements 8 and 9 may be transistors, for example a pair of power laterally diffused metal-oxide-semiconductor (LDMOS) transistors (for example of the type: Ampleon BLP9G0722-20G) and are biased for maximum power in class AB. The transistors are differentially driven and are connected to the resonator ring 16 of the power amplifier circuit 1 via a pair of feed lines and DC-blocking capacitors 24, 25. Regarding the specific materials and dimensions of the circuit board substrate on which the components of the power amplifier circuit 1 in figure 1 are built, please note that the printer circuit board (PCB) comprises a substrate with thickness of 20 mils (0.020 inches; approximately 0.508 millimeters) using RO4350 (a specific type of high-frequency laminate material manufactured by Rogers Corporation, 2225 W Chandler Blvd, Chandler, AZ 85224-6155, United States) with a 35µm copper layer. Figure 3 schematically illustrates the power amplifier circuit 1 of figure 1. The two balanced devices 8, 9 are represented as current sources 30, 31 which have equal magnitude Ib and are differentially excited, so that I1=Ib and I2= -Ib . The two current sources 30, 31 have the same output impedance, such that Zout,1=Zout,2=Zout. The current I3 is due to the output load 35 with impedance ZL. In this formulation, the currents are assumed to flow into the matching network 33 provided by the ring resonator 16. The dimensions of the power amplifier circuit 1 are chosen such that the two active devices 8 and 9 connected to the first input port 14 and the second input port 15 see an active input impedance ZA,1 = ZA,2 = ZA . In this manner, the power amplifier circuit 1 can present the optimum load impedance of the active devices 8, 9 (ZA = Z*out) for either maximum output power or efficiency, for example. The output port 19 preferably has a Z0 = 50 Ω impedance in accordance with industry standards, in order to provide excellent impedance matching with various loads. To illustrate the active matching using load-modulation, we first derive the generalised impedance matrix Z of the power amplifier circuit 1. Even-odd mode analysis cannot be directly applied, since the network is not symmetrical, so a different approach is used instead, which approach is described in the article by E. Levine and H. Matzner, “Calculating scattering parameters of Wilkinson power dividers via abcd matrices [speaker’s corner],”, IEEE Microwave Magazine, vol. 22, no. 11, pp. 82–88, 2021. The general 3-port Z-matrix is wherein ZR is the impedance of the ring resonator 16, and where

[0002] Using the analysis technique described in the article by A. N. Atanasov, M. S. Oude Alink, and F. E. van Vliet, “Active Load-Modulated Devices: A General PA Network Solution Identifying Highly Efficient Linearizer Systems,”, IEEE Transactions on Microwave Theory and Techniques, pp. 1–15, 2023, the output current at output port 19 is found to be and the active input impedance ZA seen at both the first and the second input ports 14 and 15, The expressions for the output current I3 and the active input impedance ZA,1,2 are generalised to account for the dimensions of the resonator ring 16 as well as the output load to be connected to the output 5 of the power amplifier circuit 1. In this manner ZA can be designed to match the optimum (with respect to power or efficiency) load impedance of the active devices. Figure 6 shows a Smith diagram illustrating the achievable active impedances for different values of θ and ZR. For increasing values of ZR, ZA is inductive between 0 < θ ≤ 135º , and capacitive between 135º < θ < 180º. When θ = 180º, ZA is real. When θ = 90 ◦ both expressions reduce to and And so, both current sources 30, 31 actively load modulate each other and create an active input impedance that acts as a scaled down quarter-wave transformer with a complex component, which can reach the edges of the Smith chart in figure 6. For example, if ZR = 12.5Ω and ZL = 50Ω, then a regular quarter- wave transformer will present a 3.125Ω input impedance, while the active input impedance of the power amplifier device in accordance with the present invention will be 0.8 + j6.25Ω. An alternative embodiment is illustrated in figure 4. Figure 4 only illustrates the resonator ring 16 of the power amplifier device. Characteristic about this class of embodiments is that at the first and second input ports 14 and 15, the active elements 8 and 9 provide their signals in-phase with each other. Thus, the signals received by the resonator ring 16’ in figure 4 at the first and second input ports 14 and 15 are identical and of equal sign. In this embodiment, in order to obtain constructive combination of signals at the output port 19’, the output port 19’ must be at a different angular position than in the situation illustrated in figure 2. As follows from figure 4, the angular position of output port 19’ is at exactly (3 / 2*θ + 90º) from both the first and second input ports 14 and 15. Thus, defining the second input port 15 again at angular position 0º the angular position of the output port 19’ will be at 315º. This yields a symmetrical configuration. As a result, in the embodiment of figure 4, the active input impedance is thereby changed into the complex conjugate of equation (4) above, inverting the inductive behaviour into a capacitive one. Additionally, large variations of ZL will result in small variations of ZA bound entirely within the Smith chart. This is in contrast to the conventional 90º hybrid balanced power amplifier, where the active input impedance seen by either active device can become negative as the output load mismatches beyond VSWR 2 : 1 (VSWR – voltage standing wave ratio), effectively halving its gain. The power generated by each current source and flowing into the first and second input ports 14 and 15 of the power amplifier circuit 1 is and is summed at port 19 such that The bandwidth (BW) of the power amplifier circuit 1, defined as a 10% drop in power, depends on θ and ZR, wherein ZR is the impedance of the ring resonator 16. Figures 5A to 5C show the S-parameters of the power amplifier circuit 1 for both simulation and measurement with a drain bias point of 28V and 180 mA per active device 8, 9. The bandwidth of the power amplifier circuit 1 is measured at 39MHz, while the simulated bandwidth is 36MHz, with center frequencies fc,1 and fc,2 at 2.10 gigahertz (GHz) and 2.14 gigahertz (GHz), respectively. During testing, also the measured power spectrum of the power amplifier circuit 1 has been analyzed, with the Pout at 2.1GHz measured to be 40dBm. The power of both the second harmonic at 4.2 GHz and the third harmonic at 6.3 GHz was measured to be 49dB lower, meaning the power amplifier circuit 1 also offers good harmonic suppression. Reflections at other frequencies were even roughly 65dB lower on average. In conclusion, The power amplifier circuit 1 is a balanced amplifier which uses active load modulation to allow two active elements 8 and 9 (in figure 1 transistors T1 and T2) to match each other. It applies a resonator ring 16 structure to achieve active output matching and power combining. In the above, a generalized impedance matrix of the ring 16 has been suggested, from which the active input impedances presented to both transistors were calculated, including the effects of output load mismatch. The proposed power amplifier circuit 1 of figure 1, along with a symmetric variant of figure 4, can reach complex impedances at the edge of the Smith chart of figure 6, where the optimal loads of active elements 8, 9 (such as transistors) tend to be, at the cost of reduced bandwidth. The maximum gain of the power amplifier circuit 1 was measured at 16.17dB with a maximum output power of 47.70dBm at 2.14GHz. The measured bandwidth is 39MHz. At the 3dB compression point the drain efficiency is 43.14%, and the power-added efficiency is 41.05%. The design also achieves good harmonic suppression, keeping the second and third harmonics 49dB lower than the fundamental. The power amplifier circuit 1 was measured with an active load-pull system and presented with load mismatch ranging from voltage standing wave ratio (VSWR) 10 : 1 to 40 : 1, far beyond the rating of the active elements 8, 9 provided by transistors. It was able to withstand and recover without damage to the devices, making it a very robust and mismatch tolerant design. As described above, the dimensions of the ring resonator 16 determine it’s frequency characteristics. For example, the circumference of the midline through the ring resonator equals one-and-a-half times the wavelength λc of the center frequency fc of the ring resonator. Thus, in some embodiments the ring resonator 16 can be designed having a predetermined circumference of its midline in order to provide the ring resonator 16 having a predetermined center frequency fc. In some embodiments of the power amplifier circuit 1, the ring resonator 16 comprises a width defined by an outer radius of the ring resonator minus an inner radius of the ring resonator. The width of the ring resonator 16 determines the characteristic impedance (ZR) thereof. It is chosen in such a way that the ring 16 acts as an impedance transformer under active load-pull conditions (e.g. see equations 4 and 6 above). The impedance transformer matches (or transforms) the optimum impedance that is presented to the active devices 8, 9 (ZA in these equations) to the reference impedance at the output port 19 (ZL in these equations), which is ideally 50 Ohm, but will vary under mismatch. . The present invention has been described in terms of some specific embodiments thereof. It will be appreciated that the embodiments shown in the drawings and described herein are intended for illustrated purposes only and are not by any manner or means intended to be restrictive on the invention. It is believed that the operation and construction of the present invention will be apparent from the foregoing description and drawings appended thereto. It will be clear to the skilled person that the invention is not limited to any embodiment herein described and that modifications are possible which should be considered within the scope of the appended claims. Also kinematic inversions are considered inherently disclosed and to be within the scope of the invention. Moreover, any of the components and elements of the various embodiments disclosed may be combined or may be incorporated in other embodiments where considered necessary, desired or preferred, without departing from the scope of the invention as defined in the claims. In the claims, any reference signs shall not be construed as limiting the claim. The term 'comprising' and ‘including’ when used in this description or the appended claims should not be construed in an exclusive or exhaustive sense but rather in an inclusive sense. Thus the expression ‘comprising’ as used herein does not exclude the presence of other elements or steps in addition to those listed in any claim. Furthermore, the words ‘a’ and ‘an’ shall not be construed as limited to ‘only one’, but instead are used to mean ‘at least one’, and do not exclude a plurality. Features that are not specifically or explicitly described or claimed may be additionally included in the structure of the invention within its scope. Any of the claimed or disclosed devices or portions thereof may be combined together or separated into further portions unless specifically stated otherwise, without departing from the claimed invention. Expressions such as: "means for ...” should be read as: "component configured for ..." or "member constructed to ..." and should be construed to include equivalents for the structures disclosed. The use of expressions like: "critical", "preferred", "especially preferred" etc. is not intended to limit the invention. Additions, deletions, and modifications within the purview of the skilled person may generally be made without departing from the spirit and scope of the invention, as is determined by the claims. The invention may be practiced otherwise then as specifically described herein, and is only limited by the appended claims.

Claims

Claims 1. Power amplifier circuit, the circuit comprising an input for receiving an input signal and an output for providing an output signal, wherein the power amplifier circuit is configured for amplifying the input signal for providing the output signal, the power amplifier circuit further comprising at least two active elements for performing said amplification of the input signal, wherein the input is connected to a first active element of the at least two active elements and to a second active element of the at least two active elements, wherein the power amplifier circuit further comprises a ring resonator, the ring resonator comprising a first input port and a second input port, wherein the first input port of the ring resonator connects to the first active element for receiving a first part of the output signal, wherein the second input port of the ring resonator connects to the second active element for receiving a second part of the output signal, and wherein the ring resonator comprises a single output port connecting the ring resonator to the output of the power amplifier circuit, wherein the first input port is provided at a first angular position on the ring, the second input port is provided at a second angular position on the ring different from the first angular position, and the single output port is provided at a third angular position on the ring different from the first and the second angular position, wherein the first, second and third angular position are such that the first part of the output signal and the second part of the output signal constructively combine for providing the output signal, and wherein each of the first active element and the second active element has an element output impedance which is substantially smaller than a device output impedance of the output of the power amplifier circuit.

2. Power amplifier circuit according to claim 1, wherein the element output impedance of each of the first active element and the second active element is smaller than 20% of the device output impedance of the output of the power amplifier circuit; preferably wherein the element output impedance is smaller than 10% of the device output impedance; more preferable wherein the element output impedance is smaller than 5% of the device output impedance.

3. Power amplifier circuit according to claim 1 or 2, wherein the first active element and the second active element are configured for providing the first part of the output signal and the second part of the output signal to be in counter phase with each other, such that the momentary voltage of the first and the second part of the output signal is of opposite sign.

4. Power amplifier circuit according to claim 3, wherein the first active element is connected to the input of the power amplifier circuit via a first conductive path, and wherein the second active element is connected to the input of the power amplifier circuit via a second conductive path, wherein at least one of: the first and the second conductive path are of different length such as to provide differential transmission lines having a length difference, the length difference providing a phase difference such that the input signal received by the first and the second active element is of opposite sign; or at least one of the first conductive path, the second conductive path, or the power amplifier circuit, for providing a phase difference between the signals received by the first and second active element, comprises at least one element of a group comprising: a fixed ort variable delay element, a passive or active phase shifting element, or a phase-locked loop.

5. Power amplifier circuit according to claim 3 or 4, wherein an angular difference between the first and the second angular positions on the ring resonator equals θ, and wherein an angular difference between the first and the third angular positions on the ring resonator equals 3 / 2*θ.

6. Power amplifier circuit according to claim 1 or 2, wherein the first active element and the second active element are configured for providing the first part of the output signal and the second part of the output signal to be in-phase with each other, such that the momentary voltage of the first and the second part of the output signal is of equal sign.

7. Power amplifier circuit according to claim 5, wherein an angular difference between the first and the second angular positions on the ring resonator equals θ, and wherein an angular difference between the third angular positionand one or both of the first and the second angular position on the ring resonator equals (3 / 2*θ+90º).

8. Power amplifier circuit according to any one or more of the preceding claims, wherein the ring resonator is provided having a predetermined circumference of its midline in order to provide the ring resonator having a predetermined center frequency fc.

9. Power amplifier circuit according to any one or more of the preceding claims, wherein the ring resonator comprises a width defined by an outer radius of the ring resonator minus an inner radius of the ring resonator, wherein the width of the resonator ring is selected such as to damp higher electronic modes above a fundamental mode of operation.

10. Power amplifier circuit according to any one or more of the preceding claims, wherein the active elements are at least one element of a group comprising: transistors, such as bipolar junction transistors or field effect transistors; operational amplifiers; or vacuum tubes.

11. Power amplifier circuit according to any one or more of the preceding claims, wherein the device output impedance is fifty ohms – 50 Ω – and wherein the element output impedance is less than 5 ohms – 5 Ω –, preferably less than 3 ohms – 3 Ω –.

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