Aim

The present invention relates to power amplifiers, in particular to broadband power amplifier circuits with high efficiency over a broad bandwidth.

Background of the Invention

Gallium Nitride (GaN) has enabled the creation of power amplifiers (PAs) with a high efficiency and output power. This has made GaN an attractive option in many new applications. For example, GaN monolithic microwave integrated circuit (MMIC) PAs will be extensively used in cellular base-stations for fifth generation wireless networks (5G), offering size reduction and enhanced system integration. Such applications impose strict requirements on the linearity and efficiency of PAs over a broad bandwidth.

However, complex-modulated signals with non-constant envelope are used In modem communications to efficiently employ the available frequency spectrum. These signals feature large peak-to-average ratio (PAPR) that degrades average efficiency of conventional PA structures. Therefore, special architectures such as Doherty PA, Outphasing PA, and envelope tracking PA are employed to improve the average efficiency. However, these architectures suffer from severe limitations including limited bandwidth and degraded linearity. In addition, in conventional PAs the efficiency degrades when the PA is operated in back-off from the peak power, because active device consumes almost the same power from supply but produces less output power.

Moreover, broadband PAs normally have a low efficiency, for example, in distributed amplifiers, where optimum load impedance cannot be easily provided to transistors, and power transferred to the drain line termination degrades efficiency of the PA. In most of the harmonic tuned PAs, high efficiency can only be obtained over a narrow bandwidth. Furthermore, hybrid circuits, because of their large size, cannot be used in specific applications of integrated circuit PAs.

The present invention aims to at least ameliorate the aforementioned disadvantages by providing circuit network developed to provide optimum load impedance for integrated circuit broadband PAs.

Summary

According to a first aspect of the present invention, there is provided, a broadband power amplifier circuit, said amplifier circuit comprising: an active element for receiving an impedance matched signal and for amplifying the impedance matched signal to supply an amplified signal; and an output matching network having a load Impedance and coupled to the active element for receiving the amplified signal; and wherein the Impedance of the output matching network is configured to match to the optimum load impedance of the active element.

The output matching network may comprise two reactance elements. In order to match the impedance of the output matching network to the optimum load impedance of the active element, the two reactance elements may take specific values at the centre of the frequency band. These values may be determined by the loaded quality factor of the output matching network and the optimum load impedance of the active element. This may be achieved by designing or selecting elements of the output matching network with inductors and capacitors such that they take specific values of inductance and capacitance. The Inductors and capacitors may be arranged in parallel or in series, or in a combination thereof. In an embodiment, the output matching network may transform a load impedance into the optimum load resistance of the transistor over a frequency bandwidth. The frequency bandwidth has a lower and upper limit. A reactive load impedance may then be in the second-harmonic bandwidth to maximize efficiency of the power amplifier.

Typically, the optimum load impedance may be greater than the load impedance. This may be the case for GaN transistors. Neglecting loss of passive components, at the centre of the frequency band, which may be equal to the root of the product of the upper and lower frequency limit, optimum load resistance may be equal to the product of the reactance of one of the reactance elements and a loaded quality tector of the network, whilst the reactance of the second reactance element is equal to the product of the loaded quality factor of the network and the load resistance. The loaded quality factor may be equal to the root of the ratio of the optimum and load resistance minus 1 .

In embodiments, having a first inductor and a second inductor in paralel to a capacitor and a further capacitor in series, this can act as an open-circuit at a frequency equal to inverse root of the product of the two inductors, and as a short-circuit at a frequency equal to the root of the sum of the two capacitors. It can be appreciated that by choosing a resonance frequency of the inductor-capacitor circuit equal to twice the frequency of the capacitance, an almost reactive impedance composed of L||C maybe is achieved at a second-harmonic band, which can achieve a high efficiency. Moreover, the frequency of the reactance of the short circuit frequency can be adjusted within the frequency of the frequency of the capacitance and the open circuit frequency to control bandwidth.

The broadband power amplifier circuit further comprises an input matching network having a source impedance that receives an input signal and supplies the impedance matched signal to the active element.

In some embodiments, the input matching network matches the source impedance to the optimum load impedance of the active element.

The active element may be a transistor.

In a preferred embodiment, the amplifier circuit may be implemented on Gallium Nitride (GaN) on silicon carbide (SIC). This can reduce the chip area of the circuit allowing an overall size reduction and enhanced system integration.

In one embodiment, the output matching network may be an embedded minimum inductor band-pass filter configured to match the load impedance to the impedance of the active element for a continuous-mode operation of the power amplifier. In some embodiments, load impedance may be substantially resistive, and close to optimum load resistance of the active device, in the fundamental harmonic frequency band, and substantially reactive in the second harmonic frequency band. This may be beneficial to achieve high efficiency of the power amplifier.

In other embodiments, the output matching network is configured to match the impedance of the active element over fundamental and second harmonic frequency bands of the active element. In addition, the band-pass filter may be further configured to suppress transmission of harmonic frequencies towards the load resistance. A large out-of-band attenuation can help to improve efficiency by providing reactive load impedance in the second-harmonic bandwidth. A simple network topology, composed of only four circuit elements, may aid enabling of implementation of the band-pass filter with low loss and compact chip area.

According to another embodiment, to improve the efficiency of the amplifier further, the input matching network may be configured to match the source impedance to the optimum source impedance of the active element over the fundamental frequency band.

In other embodiments, the power amplifier may be configured as a monolithic microwave integrated circuit. This can reduce the chip area of the circuit allowing an overall size reduction and enhanced system integration.

In some embodiments, the active element may comprise: a main amplifier for supplying the amplified signal at a first set of input signal conditions; and an auxiliary amplifier for also supplying the amplified signal at a second set of input signal conditions. A width of the auxiliary amplifier may typically by larger than a width of the main amplifier.

The main amplifier may comprise one or more active elements or unit cells, and the auxiliary amplifier may be electrically connected in parallel with the main amplifier and additionally may comprise at least two active elements or unit cells. The number n of active elements or unit cells In the auxiliary amplifier may be given by n = 2 ^k -1 , where k is the number of control bits.

These active elements or unit cells may have separate gate bias controls and are turned on an off depending on the power received from the input matching network. In some embodiments, substantially all power from the input matching network and/or the input asymmetric quadrature coupler may be delivered to the main amplifier at low bias voltages, and may be shared substantially equally between the first and the auxiliary amplifier at high bias voltages. This configuration may Improve the gain and efficiency of the power amplifier at back-off.

To improve the efficiency further, the output matching network and/or the auxiliary output matching network may be configured to match the load impedance to the optimum impedance of the active element over fundamental, second and third harmonic frequency bands. Optimum load impedance may be achieved when the load impedance is the optimum load resistance, short-circuit, and open-circuit, respectively, at the fundamental, second and the third harmonic frequency bands.

In some embodiments, the amplifier may further comprise an input power divider, optionaly an input asymmetric quadrature coupler, for dividing the power of the Input signal between the main and auxiliary amplifier. In some embodiments the coupling may be determined by an input asymmetric quadrature coupling coefficient of the input asymmetric quadrature coupler

In other embodiments, the input matching network may be coupled to the main amplifier, and wherein the amplifier further comprises an auxiliary Input matching network coupled to the auxiliary amplifier such that the input matching network supplies part of the power from the input asymmetric quadrature coupler to the main amplifier and the auxiliary input matching network supplies the remaining power from the input asymmetric quadrature coupler to the auxiliary amplifier.

Furthermore, to achieve a high efficiency, a harmonic output matching network may be used for this PA to provide harmonic load impedances for the class-F operation. This network can provide the optimum load resistance at the fundamental frequency, a short-circuit impedance at the second harmonic, and an open- circuit at the third harmonic.

The network typically features low loss and compact chip area that are essential for Integrated circuit implementation of the PA. The network may absorb the parasitic drain-source capacitance of the transistors and the drain bias feed as its constituent elements. These features can enable folly integrated implementation of the PA.

The Input power divider network typically exploits voltage-dependency of the transistors' gates-source capacitance to adaptively divide the input power between the main and auxiliary cells. For a Gallium Nitride (GaN) monolithic microwave Integrated circuit (MMIC) process used for Implementation of the PA circuit, the gates-source capacitance of the transistors typically decreases approximately by a factor of two when their gate bias voltage reduces from ON to OFF state. At low input power levels, all auxilary cells are OFF, and their input capacitance is smaller than that of the main amplifier (C _aux < C _main )· The input impedance of the main amplifier is then smaller, and more power is delivered to the input of the main amplifier. This can improve backoff gain and efficiency of the PA. At high Input power levels, the main and auxiliary cells have the same gate bias voltage, and hence the same gate-source capacitance (C _aux = C _main ). Thus, the input power is divided equally between the main and auxiliary amplifiers. This network can also provide optimum load impedance in the fundamental, second-harmonic, and third- harmonic frequency bands. The network typically features low loss and compact chip area that are desirable requirements for integrated circuit realizations. The multi-harmonic matching network may be composed of three resonators that their resonant frequencies determine broadband behaviour of the load impedance presented to the transistor. The network may absorb parasitic drain-source capacitance of the transistor and provides drain bias feed.

In an embodiment, an input asymmetric quadrature coupler may be provided for dividing the power of the input signal between the main and auxiliary amplifier.

The passive network may comprise an additional Input matching network such that Input matching network supplies part of the power from the input asymmetric quadrature coupler to the main amplifier and the additional Input matching network supplies the remaining power from the Input asymmetric quadrature coupler to the additional amplifier. Furthermore, an output power combiner, optionally an asymmetric quadrature coupler, for combining the output power of the amplified signal from the main amplifier and the auxiliary amplifier may be provided. The output power of the output asymmetric quadrature coupler may be determined by an output asymmetric quadrature coupling coefficient of the output asymmetric quadrature coupler The passive network may also comprise an auxiliary output matching network, such that the output matching network matches the load impedance to the impedance of the main amplifier and the auxiliary output matching network matches the load impedance to the impedance of the auxiliary amplifier. This arrangement may allow load modulation over a broad bandwidth. The main amplifier may be biased at class-B, and the auxiliary amplifier biased at class-C.

As described above, the input asymmetric quadrature hybrid operates as the input power divider, and the output asymmetric quadrature hybrid operates as the output power combiner. The amplifiers may be matched to the source and load impedances, normally 50 ohms, using the input and output matching networks. The size of auxiliary amplifier is typically larger than that of the main amplifier, by a factor that determines the output power back-off where efficiency is maximized.

The coupling coefficient of the input hybrid may be chosen such that a larger portion of the input power be applied to the auxiliary amplifier. The output hybrid may be designed to combine output power of the two amplifiers based on a specific weighting. Its coupling coefficient can determine these weights. The bias of the auxiliary amplifier may be selected based on the desired back-off level.

To improve efficiency of the PA, the output matching networks of the main and auxiliary amplifiers can be designed such that they present optimum load impedances to the transistors at fundamental and harmonic frequencies. A broadband operation may then be achieved thanks to the use of amplifiers with broadband impedance matching networks and hybrid couplers implemented as broadband Lange couplers.

In embodiments, the power amplifier operates in a continuous class-F mode, and the passive network may be a multi-harmonic output matching network, comprising said network and parasitic capacitances of the active devices.

According to an example of the present invention, there is provided a broadband power amplifier circuit for enhancing back-off output power efficiency, said amplifier circuit comprising: an active element for receiving an input signal and for amplifying the input signal to supply an amplified signal, said active element comprising a main amplifier and an auxiliary amplifier; an input asymmetric quadrature coupler for dividing the power of the input signal between the main and auxiliary amplifier; and an output asymmetric quadrature coupler for combining the output power of the amplified signal from the main amplifier and the auxiliary amplifier.

The main and the auxilary amplifiers may each comprise an input matching network, a transistor, and an output matching network; said input matching network coupled to the input asymmetric quadrature coupler, the output matching network coupled to the output asymmetric quadrature coupler; and the transistor coupled to the both the input matching network and the output matching network.

According to another example of the present invention, there is provided a method for providing load modulation in a broadband power amplifier, said method comprising: receiving an input signal at an input matching network, said input matching network having a source impedance; matching the source impedance of the input matching network to an impedance of an active element; supplying an output signal from the input matching element to the active element; amplifying the output signal via the active element; matching the load impedance of an output matching network to the impedance of the active element; and supplying the amplified signal to the output matching network, said output matching network having a load impedance.

These and other aspects of the disclosure will be apparent from, and elucidated with reference to, the embodiments described hereinafter.

Brief description of Drawings

Embodiments will be described, by way of example only, with reference to the drawings, in which figure 1 shows a power amplifier circuit (PA) according to an embodiment of the present disclosure; figure 2a shows an output matching network for use in the PA of figure 1 ; figure 2b shows an equivalent circuit of figure 2a; figures 3a and 3b shows a response of real and imaginary load impedance across frequency; figure 4 shows an embodiment of an input matching network for use in the PA of figure 1 ; figure 5 shows a network for integration into the circuit of figure 2a; figures 6a and 6b show harmonics at differing frequencies; figure 7 shows harmonic frequency bands; figure 8 shows a chip micrograph architecture of an equivalent circuit to figure 1 ; figures 9 and 10 show CW measurement results; figure 11 shows a measured output signal constellation and spectrum for a 64-QAM signal; figure 12 shows EVM and average PAE versus average output power for different modulation bandwidths; figure 13 shows a PA circuit based on a modified balanced amplifier structure; figure 14 shows equivalent circuits used for analysis of the unbalanced PA (14a Is an output circuit and 14b Is an input circuit); figure 15 shows simplified models of sub PA's input-output characteristics figure 16 shows output power back-off level versus the width ratio of the transistors K and coupling coefficient of the output coupler C _o . figure 17 shows efficiency of the unbalanced PA versus normalised output power; figures 18a and 18b shows gain of the unbalanced PA versus normalised output power figures 19a and 19b show the effects of the input coupler coupling coefficient on the efficiency and gain of the unbalanced PA of figure 13; figure 20 shows a circuit used to derive scattering parameters of the unbalanced PA of figure 13; figure 21 shows a schematic of the implemented unbalanced PA; figures 22a and 22b shows simulated performance of the meandered Lange couplers realized in a GaN-on-SiC process. figure 23 shows a chip micrograph of the PA circuit; and figure 24 shows circuit simulation results of a proof of concept PA circuit using the proposed structure of figure

1; figure 25 shows plots of drain efficiency and large signal gain; figure 26 shows the output power and drain efficiency at several input power levels; figure 27 shows a PA architecture composed of two amplifiers operating in parallel; figures 28 to 30 shows scattering parameters versus frequency for the circuit of figure 27; and figures 31 and 32 show signal simulations. It should be noted that the figures are diagrammatic and not drawn to scale. Relative dimensions and proportions of parts of these Figures have been shown exaggerated or reduced in size, for the sake of clarity and convenience in the drawings. The same reference signs are generally used to refer to corresponding or similar feature in modified and different embodiments.

Detailed description of Drawings

Figure 1 shows a power amplifier circuit (PA) 1 comprising an input matching network 2, an active element 4, and an output matching network 6. The input matching network 2 is coupled to the active element 4. The input matching network 2 Is configured to receive an input signal 3, and generate an impedance matched signal 5. The impedance matched signal 5 is then received by the active element 4, which amplifies the signal 5. The amplified signal 7 is then received by an output matching network 6, which then produces an output signal 8.

Figure 2a shows one embodiment of the output matching network 6 of the present invention. The output matching network 6 Is minimum inductor band-pass filter (BPF) which can provide wide bandwidth and large out-of-band attenuation with smaller inductors compared to standard filter structures. In particular, the structure shown broadly comprises two reactance elements, χ _ρ (ω) and X _s (ω). Each reactance element comprises capacitors and inductors (one or more of each) configured as shown in Figure 2a.

The output matching network 6 transforms the load impedance R _L into the optimum load resistance of the transistor R _opt over the bandwidth ω _L ≤ ω ≤ ω _Η , where ω _L and ω _Η , respectively, is the lower and upper limit of the bandwidth. It should provide a reactive load impedance in the second-harmonic bandwidth to maximize efficiency of the PA 1. It is assumed that R _opt > R _L . which is usually the case for GaN transistors . Neglecting loss of passive components, it can be shown that the following conditions should be satisfied at the center of the frequency band, , to achieve the optimum load resistance where is the loaded quality factor of the network. Assuming C ₁ = C _ds . using the above with plus sign and Χρ(ω) = L ₁ ω/(1 - L ₁ C ₁ ω ² ), L ₁ is derived as

Where is the loaded quality factor of the network. Assuming C ₁ = C _ds , using (1) with plus sign and Χρ(ω) = L ₁ ω/(1 - L ₁ C ₁ ω ² ), L ₁ is derived as

Using the circuit in Fig. 2(b), reactance Χ,(ω) is derived as which acts as an open-circuit at and as a short-circuit at By choosing ω ₀ = 2ω _c , an almost reactive impedance composed of Is achieved at the second-harmonic band, which Is required to achieve a high efficiency. Moreover, us can be adjusted within ωc < ωs < ωο to control bandwidth. With chosen ω ₀ and ω _s . (2) with minus sign and (4) can be used to determine C3, C2, and L2 as follows

Efficiency of the output matching network 6 is an important metric in integrated circuit PAs 1. Using the equivalent circuit shown in Fig. 2(b), and assuming that resistances representing losses meet the conditions R _p » R _opt and R _s « R _L. it can be shown that the efficiency is derived as

Using Fig. 2 and eq (3)-(7), the efficiency can be derived in terms of the circuit parameters. where Q _L1 and Q _L2 denote quality factors of the inductors. The efficiency degrades for higher impedance transformation ratio R _opt /R _L. , and hence higher Qo, while it can be improved using inductors with larger quality factor. The efficiency is also dependent on the process parameter R _opt C _ds , the center frequency ωc, and the series reactance's open- and short-circuit frequencies, ω _o and ω _s . We use this design approach for a broadband 2-4 GHz PA 1 in a 0.25-μm GaN-on-SiC technology. The device is composed of two parallel transistors with 6*125-μm width and 28 V supply voltage to achieve 37 dBm output power over the bandwidth. Using load-pull simulations, R _opt and C _ds are derived as 55 Ω and 1 pF, respectively. The low Impedance transformation ratio of 1 :1 leads to a small Q _o , of 0.32. The single-section network of Fig. 2 is therefore used as output matching network 6 of the PA 1. Using ω ₀ = 2ω ₀ and ω _s /ω _c as a design parameter, the circuit elements are derived from eq (3) and (5)-(7). From the circuit frequency response (not shown due to limited space), ω _s /ω _c is chosen as 1.2 to achieve a broadband optimum load resistance. Using ep (8)-10), the efficiency of the output matching network 6 is derived as indicating that Q _L1 is more critical for efficiency. As the inductor Li should also meet a minimum width based on electromigration current density limit (16 μm in this process), it is realized as a meandered microstrip transmission line, while a spiral inductor structure with small chip area is chosen for L ₂ . In this design, Q _L1 ≈ 18 and Q _L2 ≈ 15, leading to η ₀ ≈ 88.7% (0.52 dB Insertion loss).

In Fig. 3a and 3b, load impedance presented to intrinsic drain of the transistor and insertion loss of the designed output matching network 6 are shown versus frequency. In 2-4 GHz, the real part of the load impedance is almost flat and equal to R _opt , while in the second-harmonic band it rolls off to zero. The imaginary part of the load Impedance Is zero at the center of the frequency band, GHz, as expected from the theory. In the second-harmonic band, load impedance is located inside high-efficiency area of Smith chart determined using load-pull simulations. Insertion loss of the output matching network 8 Is 0.46-0.93 dB In the target 2-4 GHz frequency band. The simulated minimum insertion loss is close to the predicted theoretical insertion loss, while this increases toward edges of the frequency band as a result of imperfect impedance matching.

The design of the input matching network 2 Is less critical than that of the output matching network 6. The output power and efficiency are not as sensitive to source impedance mismatch, while a moderate insertion loss can be exploited to improve bandwidth and gain flatness of the PA 1. Figure 4 shows an exemplary embodiment of the input matching network 2. As shown, the input matching network 2 comprises two reactance elements, Χ _ρ (ω) and Χ _s (ω). Each reactance element comprises capacitors and inductors (one or more of each), and in the embodiment shown each comprise two capacitors and an inductor. This circuit transforms the source impedance into the complex optimum source Impedance with R _S,opt < R _s .

The short-circuit frequency of the parallel reactance can be adjusted to achieve a reactive source impedance at the second-harmonic frequency band, ω _s, P ≈ 2ω _c . Furthermore, the source impedance contributes to the nonlinear mechanism that transforms the transistor's input capacitance nonlinearity into AM-PM distortion. It is shown that AM-PM is substantially increased when the input matching network 2 has a high quality factor. The input matching network 2 Is designed to provide a moderate Input matching S11 < -8 dB, but a low quality factor of 0.4— 0.8 in the 2-4 GHz bandwidth.

The continuous class-F mode is described by the following optimum load impedance at fundamental, second-, and third-harmonic frequencies where R _opt = 2V _dc /l _max is the optimum load resistance, V _dc and Imax denote the drain dc voltage and the maximum current, and -1≤ γ ≤1.

In practice, it is difficult to meet all these conditions, especially in an MMIC PA where loss of passive components prohibits the use of high-order matching circuits. For example, eq (1) and (2) Indicate that a constant ratio should be maintained between reactive parts of the second-harmonic and fundamental impedances, l.e., X _L (2f)/X _L (f) ≈ -1 .59, while eq (3) Indicates that an open-circuit Impedance Is required at the third harmonic. These conditions, however, cannot be easily satisfied over a broad bandwidth. Furthermore, nonlinearity of the transistor’s parasitic capacitances and output resistance limit accuracy of this model [6]. In order to develop practical design criteria, we use the optimum load impedance at fundamental frequency as per eq (1), while harmonic load-pull simulations are used to deter- mine the optimum load impedances at harmonic frequencies.

In the following harmonic load-pull simulations, a 025-μm GaN-on-SiC process from WIN Semiconductors is used. The transistor Is driven 2-3dB into gain compression, to deliver about 37 dBm output power with 24 dBm input power at 5 GHz. The input source impedance is optimized at the fundamental frequency and short- circuited at higher order harmonics. The transistor parameters R _opt and C _out are roughly extracted as 70 Ω and 0.5 pF, respectively. The second- and third-harmonic load-pull simulation results are, respectively, shown in Figs. 1 and 2. The impedances are given at the extrinsic drain of the transistor. The results are given for γ = 0, while further simulations indicate that the harmonic load-pull contours do not significantly change with γ.

To enable the continuous class-F mode operation, an harmonic matching network shall provide optimal load impedance in the fundamental and harmonic frequency bands. Unfortunately, a traditional single-frequency multi-harmonic matching network cannot support these conditions over a broad bandwidth. For MMIC implementation, this network should have a simple architecture to reduce loss and save chip area. Moreover, the network should absorb the drain-source parasitic capacitance of the transistor and include a parallel inductor to provide the drain bias path. As an example, simple network for an integrated GaN PA has been previously proposed in, but it is applicable only to class-J mode.

In this work, a network 10 is proposed, shown in Fig. 5, that is capable of meeting the continuous class-F mode conditions over a wide bandwidth for MMIC PAs. This network 10 can be integrated Into the circuit shown In Fig 2a, replacing circuit 6 Broadband frequency response of the load impedance at fundamental and harmonics can be controlled via resonant frequencies of the three resonators (ω _1,2,3 ). We present an intuitive analysis to provide insights into the circuit operation. Then, the circuit can be optimized in a circuit simulator to achieve the desired broadband response in the presence of loss and parasitics of the passive elements.

The input Impedance of the circuit In Fig. 5 Is derived as where Χ _ρ (ω) and Χ _s (ω) are respectively given by

By equating the respective real and Imaginary parts of (12) and (15) at center of the band ω ₀ = 2ττf ₀ , it can be shown that where G _L (f ₀ ) and BL(f ₀ ) are real and Imaginary parts of the optimum fundamental load admittance 1/Z _L (f ₀ ). We assume that the resonant frequency of the first resonator ω ₁ is placed at ω ₀ < ω ₁ < 2ω ₀ . Therefore, the resonator operates as an inductive reactance at the fundamental and a capacitive reactance at harmonics frequencies. It can be shown that this choice leads to the desired frequency response behavior at harmonic bands. Moreover, we choose C ₁ = C _out , and hence using eq (16), the inductor L ₁ is derived as

The input impedance of the network should be reactive at harmonics to achieve high efficiency. Using eq (15), it is shown that the condition R _in (nω ₀ ) « Χ _in (nω ₀ ) is simplified as

This condition is automatically satisfied at sufficiently high frequencies, e.g., the third harmonic, as Χ _n (nω ₀ ) ^≈ 1/nω ₀ C1 becomes much smaller than R _L . At the second harmonic, we can choose ω ₃ ≈ 2ω ₀ , to obtain a large X _s (2ω ₀ ). At the third harmonic, an inductive impedance is required (see Fig. 6a and 6b), which can be obtained by adjusting the resonant frequency ω ₂ and circuit elements to make Χ _s (3ω ₀ ) inductive.

The layout structure of the harmonic matching network has significant effects on performance of the broadband integrated PA. In addition to the optimum load impedance conditions, other issues, including the losses and parasitics of passive elements, electromigration current density limit of transmission lines, and chip area should also be considered in the layout design. Properties of the double-metal transmission lines on the 100-μm SiC substrate are summarized in Table I below. The line width should be chosen based on these trade-offs.

To proceed, we present the design of a PA 1 operating In 4- 6 GHz using the developed technique. In the proposed circuit 10 of Fig. 5, inductor L, should tolerate a dc current of 300 mA at the maximum output power. Moreover, simulations indicate that the layout of this inductor is critical in the PA’s 1 broadband performance. The other inductors are not burdened by such current density constraint and so their widths are optimized based on insertion loss and chip area. The inductors L _1,2,3 are implemented as meandered microstrip transmission lines with the width of 30, 15, and 30 μm, respectively. The circuit elements are roughly estimated as L _1,2,3 = [1.4 0.6 1.1] nH and C _1,2,3 = [0.5 0.5 0.3] pF. The resonant frequencies of the resonators are derived as f _1,2,3 = [6.0 9.1 8.7] GHz, indicating that the conditions set on these resonant frequencies are maintained as f ₀ <f ₁ <2f ₀ ,f ₂ ≈ 2f ₀ , and f ₃ ≈ 2f ₀ . The insertion loss of the matching network Is 0.6-1.1 dB over the bandwidth.

The input impedance of the harmonic matching network 10 in the fundamental (intrinsic drain) and harmonics (extrinsic drain) frequency bands is shown in figure 7. It is noted that load impedance in the fundamental band is almost placed on the constant resistance arc of the Smith chart, as theoretically expected from (1), while traces of the load impedance in the second- and third-harmonic bands are close to the high-efficiency areas shown in Figs. 6a and 6b. The PA circuit schematic 1 and chip micrograph 12 are shown in Fig. 8.

CW measurement results are shown in Figs. 9 and 10. The chip is fabricated in a 0.25-μm GaN-on-SiC process from WIN Semiconductors. The PA 1 is biased at V _GS = -2.4 V and V _DS = 28 V, consuming 17 mA of bias current. The gate- source bias voltage is chosen such that soft gain compression of the PA 1 be minimized. Measured drain efficiency (DE), power- added efficiency (PAE), and gain versus output power are shown in Fig. 9. The PA 1 achieves maximum output power of 36.1 dBm and maximum DE/PAE of 51/48%, with associated power gain of 12.2 dB. In Fig. 10, output power and DE/PAE are shown versus frequency, at the fixed input power of 24dBm. The PA 1 achieves 33.9-36.1 dBm output power, 42-51% DE, 38-48% PAE, and 10-12.2 dB power gain in 4.0-6.0 GHz. The output power 1-dB bandwidth is 3.6-5.6 GHz.

The modulated signal measurements are performed by using R&S SMW200A vector signal generator and R&S FSW43 vector signal analyzer. In Fig. 11 , measured output signal constellation and spectrum are shown for a 64- QAM signal with 100 MHz modulation bandwidth and 8 dB PAPR, at 5.0 GHz. An error vector magnitude (EVM) of -32.4 dB (2.4%) is achieved at average output power of 30.2dBm. Also, adjacent channel leakage ratio (ACLR) of the output spectrum Is -37.8 dBc for the lower channel and -38.5dBc for the upper channel. In Fig. 12, EVM and average PAE are given versus average output power for different modulation bandwidths. The EVM degrades with an increased modulation bandwidth. For EVM < -28 dB, average output power of 32.1/32.0/30.2 dBm and average PAE of 39/38/32% are achieved for 50/100/200 MHz modulation bandwidth. The effect of higher modulation bandwidth is more pronounced when a more stringent EVM condition should be satisfied.

In Table II shown below, performance of the designed PA Is compared with broadband GaN MMIC PAs. The PA achieves a high efficiency over 4-8 GHz (40.8% fractional bandwidth), while it features a small chip area using the proposed harmonic matching network. Moreover, a low EVM of -32 dB is obtained for a 64-QAM signal with 100 MHz modulation bandwidth (BW _m ), which is essential for 5G applications.

A PA circuit 13 based on a modified balanced amplifier structure is shown in figure 13. This PA architecture can provide load modulation over a broad bandwidth. The PA shown in the figure 13 is composed of a main amplifier 14 biased at class-B, an auxiliary amplifier 16 biased at class-C, an asymmetry quadrature hybrid 18 operating as the input power divider, and an asymmetry quadrature hybrid 20 operating as the output power combiner. The amplifiers 14, 16 are matched to the source and load impedances, normally 50 ohms, using input 2 and output matching 10 networks. The size of auxiliary amplifier 16 is larger than that of the main amplifier 14, by a factor that determines the output power back-off where effidency Is maximized. The coupling coefficient of the input hybrid is chosen such that a larger portion of the input power be applied to the auxiliary amplifier 16. The output hybrid 20 is designed to combine output power of the two amplifiers 14, 16 based on a specific weighting. Its coupling coeffident determines these weights. The bias of the auxiliary amplifier 16 is seleded based on the desired back-off level.

Back-Off Efficiency Enhancement

A diredional coupler can be described by the folowing matrix of scattering parameters: where a and b are related to the coupling coeffident of the coupler and 0 < C < 1 as a = C and Figure 14 shows equivalent drcuits used for analysis of the unbalanced PA. Using the circuit of figure 14 (14(a) is an output drcuit and 14(b) in an input drcuit) and assuming that output impedance of the main and auxiliary sub PAs 14, 16 Is matched to the load impedance and the output coupler 20, i.e., Z _o,m = Z _o,a = Z _o = R _L , the output voltage is derived as leading to It should be noted that because the input hybrid coupler 18 provides 90 phase shift between the input voltages of the main and auxiliary amplifiers, there Is also a 90 phase difference between their output voltages, l.e., these can be considered as For small output coupling coefficients, while for large coupling coefficients, Therefore, a moderate coupling coefficient should be chosen to achieve proper back-off efficiency enhancement. The power delivered to the load can be determined using equation 25 as where respectively denote output power of the main and auxiliary sub PAs 14, 16. A part of the power generated by the main and auxiliary sub PAs 14, 16 is delivered to the isolated port 19. Using figure 14, voltage of the Isolated port 19 can be obtained as resulting In

If the output voltage ratio of the main and auxiliary PAs 14,16 can be maintained as then V _iso = 0 and the power delivered to the isolated port 19 becomes zero. However, this is not a requisite to achieve a high efficiency at back-off where V _0,a = 0. The power delivered to the Isolated port 19 Is derived as

It is noted that P _out + P _iso = P _o,m + P _0,a as expected from power conservation. The P _iso should be minimized to improve the efficiency of the output power combiner 20, e.g., through using a small coupling coefficient C _o .

The Input-output characteristics of the two sub PAs 14, 16 can be modelled as shown In figure 15. It Is assumed that the main sub PA 14 is biased at class-B and features a linear power gain of G _p and saturated output power level of P _sat . The Input saturation power level Is given by P _sat / G _p . The auxiliary sub PA 16 Is biased at class-C with input tum-on power level of P _on,a . The auxiliary sub PA 16 can turn on either before or after saturation of the main sub PA 14. We choose the PA parameters such that the onset of the main sub PA 14 saturation coincides with the turnon of the auxiliary sub PA 16, and define associated input power to the PA as the input back-off power P _in,bc . The width of auxiliary transistor IV, is larger than that of the main transistor W _m by a factor of K, thus Its linear power gain and saturated output power level are roughly given by KG _P and KP _sat

This simplified model is useful to provide an understanding on operation of the unbalanced PA.

Using Equation 25 and 29, the output power level at peak-power and back-off can be derived as resulting in the output power back-off (OPBO) level of

It is noted that back-off level is dependent on the transistors' width ratio and the coupling coefficient of the output coupler 20. In figure 16, OPBO Is shown versus the parameters K and C _o . For a 3-dB coupler ( C _o . = the 6-dB back-off requires K = 1, which is the same as in the Doherty PA. For smaller coupling coefficients, e.g., 4.8 dB the transistors' width ratio should be larger (K = 2).

The input power applied to the PA is distributed between the main and auxiliary sub PAs 14, 16. Using the circuit of figure 14(b), assuming that input impedance of the sub PAs 14, 16 Is matched to the input coupler 18 and input source impedance, i.e., Z _l,m = Z _l,a =Z ₀ = R _s , it can be shown that where C _i is the coupling coefficient of the input hybrid coupler 18. The input power delivered to the main and auxiliary sub PAs 14, 16 are given by

Therefore, output power of the main and auxiliary sub PAs 14, 16 are derived in terms of the input power as folows. where P _in,bo and P _in,pp are respectively the input power level at the back-off and peak-power, given by

Using equation 40, the required tum-on power level of the auxiliary sub PA 16 can be derived. The input-output power characteristic of the unbalanced PA can be derived using equations 25, 37, and 38.

The total efficiency of the unbalanced PA can be derived as where respectively denote efficiency of the main and auxiliary sub PAs 14, 16. Using equations 37, 38, and 41 , the efficiency at back-off and peak-power is given by It is noted that the efficiency at back-off and peak-power can be different. Using equations 32, 42, and 43, it can be shown that which is larger than unity for K > (10 ^OPBO/20 -1) η _a /η _m . For the 6-dB back-off level and η _a = η _m , it can be satisfied for Therefore, a smaller output coupling coefficient Is preferred to achieve higher back-off efficiency.

In the output power levels other than the peak-power and back-off, the efficiency described by equation 41 is power-dependent through To obtain in terms of the input power, we first note that the drain current of a short-channel transistor in the saturation is given by where k ₀ is a process-dependent parameter, W Is width of the transistor, V _GS Is the gate-source voltage, and V _T denotes the threshold (pinch-off) voltage of the enhancement (depletion) mode transistor. We assume an RF signal in the form of is applied to the transistor. The drain current waveform is dependent on the bias mode. If the transistor is biased in the class-B mode, i.e., V _GS0 = V _T , as in the main sub PA, the resulting drain current Is an half-wave sinusoid with the peak of k ₀ WV _RF . Both the DC and fundamental components of this waveform, l _D0 and l _D1 , are proportional with V _RF . As a result, the DC and RF power change as leading to For a matched transistor, Therefore, the efficiency of the main sub PA 14 can be expressed as where η _m denotes the maximum efficiency at saturation, e.g., η _m = π /4 = 78.5 % in the class-B mode.

If the transistor is biased in the class-C mode, i.e., V _GS0 < V _T , as in the sub auxiliary PA 16, a different situation should be considered. The conduction angle of the current waveform Is derived using equation 45 and equation 46 as which is dependent on the RF voltage amplitude. Conventionally, the conduction angle is defined at the maximum RF voltage which leads to the PA saturation. It can be shown that the current waveform components I _D0 and I _D1 are derived as

The DC power is given by P _DC = V _DD Ι _D0 (α) . while the RF power is derived as where I _{D1, max} denotes the fundamental drain current components at the maximum RF voltage V _{RF, max} · Therefore, using equations 49 and 50, the efficiency can be expressed as where η _max (α) is the maximum efficiency of class-C PA at saturation

The parameter V _RF / V _{RF, max} can be related to the input power using and noting that the auxiiary sub PA 16 turns on at P _in,bc and reaches the saturation at P, Therefore, the efficiency of the sub auxiliary PA 16 can be expressed as where η _a Is given by (51).

The total efficiency of the unbalanced PA (equation 41) can be derived versus the output power as shown In figure 17. Figure 17(a) is for 6-dB OPBO and figure 17(b) is for 9-dB OPBO. It is assumed that the maximum efficiency of the main and auxiliary sub PAs 14, 16 are respectively η _m = 78.5% (class-B) and η _a = 85.5% (class-C with conduction angle of α = 0.8π), while the coupling coefficient of the input coupler 18 is C _i = - 3 dB. It is noted that by decreasing the coupling coefficient of the output coupler 18 (C _a ), the back-off efficiency %«, improves at the cost of the reduced peak-power efficiency η _pp . For small C _o , η _bo → η _m , as expected from equation 42. It is also possible to achieve a Doherty-like behavior, e.g., for C _o . ~ 6 dB, but higher efficiency at back-off is preferred for modulated signals with large PAPR which their probability density function (PDF) is mostly concentrated around the back-off.

In figure 18 (figure 18(a) for e-dB OPBO, and figure for 18(b) 9-dB OPBO), gain of the unbalanced PA 21 Is shown versus the output power. It is assumed that gain of the main and auxiliary sub PAs 14, 18 are respectively G _p = 10 and KG _p , where K is determined based on OPBO and C _o . using equation 32. Also, C _i = -3 dB is considered in simulations. The gain at back-off, using equations 25 and 36, is given by (1 - C ₂ ) while it increases at higher output power levels as the auxiliary sub PA 16 turns on. In practice, the gain of auxiliary sub PA 16, biased at the class-C mode, is lower than KG _P and gradually increases with the output power, resulting in lower gain enhancement from the back-off to peak-power range.

The effects of the coupling coefficient of the input coupler 18 on the efficiency and gain of the unbalanced PA are Illustrated In figure 19(a) and (b), respectively. The efficiency Is very slightly affected by C _i , only between the back-off and peak-power levels, while a larger C _i is preferred to achieve higher gain. However, based on the input power requirements of the main and auxiliary sub PAs 14, 16, equations 35 and 36, C _i can be derived as C _i = Theoretically, it is assumed that saturated output power and gain of the auxiliary sub PA are K times those of the main sub PA 14, thus both need the same output power levels and C _i = - 3 dB is derived. In practice, the larger auxiliary transistor, due to nonlinearity and loss effects, requires higher input power drive, leading to smaller C _i . The C _i should therefore be chosen based on this trade-off.

Linear Operation

We derive small-signal scattering parameters of the unbalanced PA 21 in terms of the sub PAs' scattering parameters and hybrid couplers' 18, 20 coupling coefficients. It is assumed that the sub PAs 14, 16 are unilateral, i.e., S«= 0, to simplify the analysis. Using the circuit of figure 20 and equation 22, the voltage waves V ₁ ⁺ and V ₂ ⁺ are derived as

Since the amplifiers 14, 16 are assumed to be unilateral, reflected voltage waves at their Input ports are given by

The input reflected wave is given by The input reflection coefficient of the unbalanced PA 21 can be derived using equations 54-58 and S _11,UPA =

Similarly, it can be shown that the output reflection coefficient Is derived as Moreover, voltage waves at output ports of the sub PAs 14, 16 are given by Therefore, the output voltage wave is derived as

Using equation 54-63, gain of the unbalanced is derived as follows

If the two sub PAs are designed such that S _11,m = S _11,a and S _22,m = S _22,a equations 58 and 60 are simplified to indicating that the input and output reflection coefficients of the unbalanced PA 21 are smaller than that of the constituent sub PAs 14, 16 by the factors respectively. It is noted that the unbalanced PA still partially inherits the impedance matching improvement feature of the conventional balanced PA. This feature alleviates the design of output and input matching networks of the sub PAs 14, 16. In the case of balanced PA with identical sub PAs 14, 16 and 3-dB couplers, these results are simplified to as expected. PA CIRCUIT DESIGN

A broadband fully integrated unbalanced PA 21 prototype is presented with peak efficiency at 6 dB back-off for 4-7 GHz bandwidth. The PA is implemented using a 0.25 μm GaN-on-SiC process. The circuit schematic is shown in figure 21 The application of the developed theory and practical considerations in the circuit design are presented in the following.

A. Output and Input Lange Coupllrs The output and input couplers 18, 20 are realized using meandered Lange couplers 22, 24 to achieve broadband performance and save chip area. The coupling coefficient of the output coupler is determined based on the 6-dB OPBO requirement (figure 16) and the efficiency versus output power profile (figure 17). A coupling coefficient of - 8 dB is chosen in this design. The Lange coupler 22, 24 layout structure, e.g., width and spacing of the constituent transmission lines, is optimized using extensive EM simulations. The resulting performance of the output coupler 20 is shown in figure 22(a). The transmission coefficients of two output ports are - 7.9 dB (= C _o ). and -12 dB at 5.5 GHz. The amplitude imbalance, ΔΑ, and the deviation of the phase difference from - 90° , ΔΦ, respectively read as 0.6-1.3 dB and 3.2° - 6.3°, across 4.0 - 7.0 GHz. The insertion loss is 0.3-0.5 dB and the reflection coefficients of three ports are < -27 dB. The coupling coefficient of the input coupler 18 is chosen as -5 dB based on the gain and P _i,a considerations. The coupler performance shown in figure

22(b) indicate transmission coefficients of -5.1 dB (= C _i ) and -2.2 dB at 5.5 GHz. Furthermore, ΔΑ of 0.8-1 .0 dB, ΔΦ οf 2.4° - 5.1°, the insertion loss of 0.3-0.5 dB, and < -27 dB are achieved across 4.0 - 7.0 GHz.

B. Main and Auxiliary Sub PAs

The width of transistors in the main and auxiliary sub PAs 14, 16 can be determined based on the output power target for the unbalanced PA 21 and the coupling coefficients of the couplers derived in Section lll-A. From the required peak output power and OPBO level, the back-off output power is given by P _out,bo = P _out,bo

OPBO. Using equation 31 and estimated loss of the output matching network of the main sub PA 14 L _omn,m . the required output power of the main transistor can be derived as P _tr , _m = P _out,bo - 10log ₁₀ (1 - + L _omn,m .

This can be used in load-pull simulations to derive the width of the main transistor Wm. In this design, to achieve 36 dBm peak output power and 6-dB OPBO, with C _o = - 8 dB and assuming 1dB, the required power of the main transistor is 31.8 dBm. This is satisfied using a transistor width of 4 x125 μm.

Furthermore, using figure 16 with C _o . chosen previously, the parameter K is obtained, giving to the width of the auxiliary transistor as W _a = KW _m . In practice, larger transistors have higher loss and thus lower efficiency which can degrade efficiency of the unbalanced PA 21. Moreover, larger parasitic capacitors of this transistor limits bandwidth of the PA. The optimum width of the auxiliary transistor should therefore be much smaller than the theoretical value. In this design, a transistor width of 8 x 125 μm results in the best efficiency and bandwidth performance. This reduces the OPBO level to roughly 5 dB.

The output matching networks of the main and auxiliary sub PAs 14, 16 should provide the optimum load resistances R _opt,m ~ 150Ω and 70Ω in the fundamental frequency band. In this design, we realize output matching networks such that enable operation of the sub PAs in the continuous class-F mode. This improves efficiency of the PA over a broad bandwidth through providing optimum load impedances in the fundamental, second, and third harmonic bands. The networks are implemented using stacked metal microstrip transmission lines and metal-insulator-metal MIM) capacitors. Simulation results indicate the main sub PA provides 32 dBm output power, 56% efficiency, and 10 dB power gain, at 22 dBm input power and 5.5 GHz. For the auxiliary sub PA 16, these are respectively 36 dBm, 49%, and 10 dB, at 26 dBm input power. The unbalanced PA 21 achieves peak output power of 34 dBm, efficiency of 35% at peak power and 41 % at 5 dB back-off. The efficiency reads 27%-38% at peak power and 28%-42% at back-off, across 4.5-6.S GHz. To improve efficiency of the PA 1 , the output matching networks 10 of the main 14 and auxiliary 16 amplifiers are designed such that they present optimum load impedances to the transistors at fundamental and harmonic frequencies. A broadband operation is achieved thanks to the use of amplifiers with broadband impedance matching networks and hybrid couplers implemented as broadband Lange couplers 22, 24.

Proof-of-concept PA circuit using the proposed structure Is designed and Implemented in a Gallium Nitride monolithic microwave integrated circuit (MMIC) process. The circuit simulation results are provided in figure 24. Small-signal S-parameters of the PA indicate higher than 8 dB gain over 4.3-6.6 GHz bandwidth. In the folowing plots, of figures 25 the drain efficiency and large-signal gain of the PA are shown versus output power at 5.0 GHz. Considering 35.5 dBm as the peak power, the drain efficiency is derived as 39% ate peak power and 35% at 6-dB back-off. In the following plots of figure 26, the output power and drain efficiency are plotted versus frequency at several input power levels. The difference between efficiency at peak power and back-off Is less than 10% over the bandwidth 4.5-6.S GHz. The PA architecture 13 of figure 13 can maintain efficiency in back-off power over a broad bandwidth, which is a desired component In wireless communications. The conventional PA architectures that provide enhanced back-off efficiency, normally have a narrow bandwidth. The proposed architecture is a possible solution for these issues. In conventional PAs, the efficiency degrades when the PA is operated in back-off from the peak power. The PA architecture 13, shown in figure 27, composed of two amplifiers operating 14, 16 In parallel, where the auxiliary amplifier 16 itself is composed of several unit cells 17, 19. These unit cells 17, 19 have separate gate bias controls, and depending on the output power, are turned on and off to achieve high peak power and Improve the efficiency at back-off. The number of unit cells 17, 19 is n = 2^k-1 where k is the number of control bits. All transistors have the same gate width.

Furthermore, to achieve a high efficiency, a harmonic output matching network 10 is used for this PA 1 to provide harmonic load Impedances for the class-F operation. This network 10 provides the optimum load resistance at the fundamental frequency, a short-circuit impedance at the second harmonic, and an open- circuit at the third harmonic. The network features low loss and compact chip area that are essential for integrated circuit implementation of the PA 1. The network 10 absorbs the parasitic drain-source capacitance of the transistors 4 and the drain bias feed as its constituent elements. These features enable fully integrated implementation of the PA 1. The input power divider network exploits voltage-dependency of the transistors' 4 gates-source capacitance to adaptively divide the input power between the main 17 and auxiliary 19 cells. For the Gallium Nitride (GaN) monolithic microwave Integrated circuit (MMIC) process used for implementation of the PA circuit, the gates-source capacitance of the transistors decreases approximately by a factor of two when their gate bias voltage reduces from ON to OFF state. At low input power levels, all auxiliary cells 19 are OFF, and their input capacitance is smaller than that of the main amplifier 14 (C _aux < C _main ) The input impedance of the main amplifier 14 would be smaller, and more power is delivered to the input of the main amplifier 14. This improves back-off gain and efficiency of the PA 1. At high input power levels, the main 17 and auxiliary cells 19 have the same gate bias voltage, and hence the same gate-source capacitance (C _aux = C _main ). Thus, the input power Is divided equally between the main 14 and auxiliary 16 amplifiers. A proof-of-concept PA 1 based on the proposed reconfigurable architecture with three auxilary cells has been designed and fabricated in a GaN monolithic microwave Integrated circuit (MMIC) process. The PA small-signal characteristics simulated using a nonlinear model provided by the foundry are shown in the following figures 28 to 30. In the state 00, all the auxiliary cells 19 are turned off. By Increasing the Input power level, more number of auxiliary cells 19 are turned on, from one cell at the state 01 , to two cells at the state 10, and finally all three cells at the state 11. The PA maintains its performance in all states.

The large-signal simulations performed at 4.8 GHz are shown in the following figures 31 and 32. Considering the peak power at 34 dBm, the drain efficiency (DE) at 6-dB back-off, 28 dBm output power, can be Improved from 14% to 23% (1 ,6X) by turning all the auxiliary cells 19 off. Gain is also reduced when more auxiliary cells 19 are turned off.

By reconfiguring the PA 1 , its output power level can be controlled with an improved efficiency at back-off. The output matching network 10 of the PA 1 also provides harmonic load impedances for the class-F operation to improve efficiency of the PA 1. The proposed technique at least mitigates the bandwidth limitation by using conventional Impedance matching networks, Instead of the Impedance Inverter network In the Doherty PA or the load modulation network of the Outphasing PA that can be designed to have a broad bandwidth. Furthermore, the adaptive Input power division can Improve gain of the PA at back-off.

From reading the present disclosure, other variations and modifications wil be apparent to the skilled person. Such variations and modifications may involve equivalent and other features which are already known in the art of receivers and which may be used instead of, or in addition to, features already described herein.

Although the appended claims are directed to particular combinations of features, it should be understood that the scope of the disclosure of the present Invention also Includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention.

Features which are described In the context of separate embodiments may also be provided in combination In a single embodiment. Conversely, various features which are, for brevity, described in the context of a single embodiment, may also be provided separately or In any suitable subcombination. The applicant hereby gives notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

For the sake of completeness it is also stated that the term "comprising" does not exclude other elements or steps, the term "a" or "an" does not exclude a plurality, a single processor or other unit may fulfil the functions of several means recited in the claims and reference signs in the claims shall not be construed as limiting the scope of the claims.