A doherty power amplifier based on dual-mode impedance converter

Abstract

The application relates to a Doherty power amplifier based on a dual-mode impedance converter. The application relates to the technical field of mobile communication and discloses a Doherty power amplifier with small circuit size, high impedance conversion and harmonic injection, and performance comparable to that of a conventional post-matching wideband power amplifier, which comprises a wideband coupler for separating two input signals, a stepped impedance line connected with the wideband coupler and serving as an input matching network, a main path connected with the input matching network and comprising a main power amplifier tube, an auxiliary path comprising an auxiliary power amplifier tube, an output matching network connected with the main path and the auxiliary path, wherein the output matching network comprises a drain bias connected with the main power amplifier tube and the auxiliary power amplifier tube and a dual-mode impedance converter connected with the drain bias, the drain bias comprises a parallel branch connected with the main path and the auxiliary path and having a length less than a quarter of a wavelength of a working frequency; a direct-current power supply and a bypass capacitor are connected with the far end of the parallel branch; and a terminal short-circuit branch of the dual-mode impedance converter establishes an anti-phase second-harmonic injection path between the main path and the auxiliary path.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of mobile communication technology, in particular to a Doherty power amplifier based on a dual-mode impedance converter. BACKGROUND

[0002] High-order modulation format signals with multiple sub-channels are widely used in modern wireless communication fields. When they are used in power amplifiers, their high peak-to-average power ratio (PAPR) still poses challenges, such as how to maximize the efficiency at the output power back-off (OBO) while maintaining saturated output power and efficiency. In response to this, researchers have developed various technologies such as outphasing PA, Doherty PA, envelope tracking PA, envelope elimination and restoration PA, and sequential PA to meet the above needs. Among them, the Doherty PA is widely used in modern base stations due to its simple and durable characteristics. The ever-increasing bandwidth design requirements of mobile communication standards gradually increase the bandwidth of the Doherty PA, which is limited by factors such as drain-source capacitance C ds , offset line, and impedance converter. At the same time, the classic Doherty PA requires an output matching network (OMN) with high conversion, but the conversion rate of the output matching network is inversely proportional to the bandwidth.

[0003] Traditional wideband Doherty PA is mainly designed based on post matching network (PMN), and its output impedance conversion is shifted to the back of the convergence point of the main path and auxiliary path of the Doherty PA. However, the post matching network based on low-pass impedance matching network is usually composed of multiple sections of stepped impedance lines or parallel branches, which will inevitably occupy a large area and cause higher loss, which is particularly evident in high-frequency or on-chip designs. Currently, there are ways to absorb C ds into the impedance converter to increase the bandwidth of the MMIC Doherty PA; or by designing the output matching network, without the impedance converter and the offset line, to eliminate the bandwidth limitation; or placing an LC resonant circuit in parallel at the convergence point of the main path and the auxiliary path to form a new combiner with expanded bandwidth to achieve the purpose of expanding the bandwidth of the Doherty PA without using the post matching network. However, the bandwidth of the above-mentioned power amplifier is significantly smaller than that of the post matching network-based scheme, making it difficult to meet the bandwidth requirements in mobile communication. SUMMARY

[0004] Therefore, it is necessary to provide a Doherty power amplifier based on a dual-mode impedance converter that can significantly reduce the circuit size, achieve high impedance conversion and harmonic injection, and have performance comparable to that of traditional post matching wideband power amplifiers.

[0005] A Doherty power amplifier based on dual-mode impedance transformer, comprising a wideband coupler for splitting two input signals, a stepped-impedance line connected with the output of the wideband coupler and serving as an input matching network, a main path and an auxiliary path connected with the input matching network respectively, and an output matching network connected with the output of the main path and the output of the auxiliary path, the main path comprising a main power amplifier tube, the auxiliary path comprising an auxiliary power amplifier tube, the output matching network comprising a drain bias connected with the main power amplifier tube and the auxiliary power amplifier tube, and two dual-mode impedance transformers connected with the drain bias and arranged on the main path and the auxiliary path respectively, the drain bias comprising a parallel branch connecting the main path and the auxiliary path, the distal end of the parallel branch being connected with a DC power supply and a bypass capacitor; the dual-mode impedance transformer comprising a terminal short-circuit branch, the terminal short-circuit branch of the dual-mode impedance transformer on the main path and the terminal short-circuit branch of the dual-mode impedance transformer on the auxiliary path together forming a coupled-line bandpass filter for establishing an anti-phase second harmonic injection path between the main path and the auxiliary path.

[0006] In one of the embodiments, the length of the bias line of the drain bias is less than one quarter of the wavelength of the operating frequency.

[0007] In one of the embodiments, the phase difference of the two input signals split by the wideband coupler at the drain is 90°.

[0008] In one of the embodiments, a phase shift line is arranged on the auxiliary path for compensating the 90° phase difference of the two input signals at the drain.

[0009] In one of the embodiments, the length of the terminal short-circuit branch of the dual-mode impedance transformer is one quarter of the upper side frequency of the operating frequency.

[0010] In one of the embodiments, the dual-mode impedance transformer is also used for transforming complex impedance to real part at two frequency points.

[0011] The present invention implements a Doherty power amplifier based on a dual-mode impedance converter. The Doherty power amplifier, based on the dual-mode impedance converter, can simultaneously transform complex impedance into real impedance at two frequencies. After drain biasing is coordinated, the conversion ratio of the dual-mode impedance converter can be significantly increased and fluctuations can be reduced, eliminating the need for a post-matching network and significantly reducing circuit size. The inherent structure of the dual-mode impedance converter can establish an inverted second harmonic injection path between the main path and the auxiliary path to improve the saturation efficiency and output power of the high-frequency sideband without the need for an additional bridge, further reducing circuit size. The Doherty power amplifier operates in the 2.1-3.1 GHz frequency range, exhibits a saturated gain greater than 7 dB, a maximum output power of 43.3 dBm, and maximum 6-dB back-off and saturation power-added efficiency (PAE) of 53% and 61%, respectively. Its circuit performance is comparable to that of a conventional post-matched broadband Doherty power amplifier. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 A schematic structural diagram of a Doherty power amplifier according to an embodiment of the present invention;

[0013] Figure 2 is a simplified schematic diagram of a Doherty power amplifier according to one embodiment of the present invention;

[0014] Figure 3 is an LC equivalent circuit of a dual-mode impedance converter in one embodiment of the present invention;

[0015] Figure 4 is the real part conversion characteristic of the LC equivalent circuit of the dual-mode impedance converter in one embodiment of the present invention;

[0016] Figure 5 is the imaginary part conversion characteristic of the LC equivalent circuit of the dual-mode impedance converter in one embodiment of the present invention;

[0017] Figure 6 A microstrip form of a dual-mode impedance converter in one embodiment of the present invention and a second harmonic stopband generated by a terminal short-circuit branch therein;

[0018] Figure 7 The impedance conversion characteristics of a microstrip dual-mode impedance converter in one embodiment of the present invention are shown;

[0019] Figure 8 A simplified structural diagram of a second harmonic injection path in an output matching network in one embodiment of the present invention;

[0020] Figure 9 for Figure 8 The second harmonic phase curve in the embodiment shown;

[0021] Figure 10 The simulation internal voltage and current curve when the output power is 2.6 GHz in the saturation state of an embodiment of the present application;

[0022] Figure 11 The simulation internal voltage and current curve when the output power is 3 GHz in the saturation state of an embodiment of the present application;

[0023] Figure 12 The simulation small signal performance curve of the Doherty power amplifier of an embodiment of the present application;

[0024] Figure 13 The simulation large signal performance curve of the Doherty power amplifier of an embodiment of the present application;

[0025] Figure 14 The simulation performance comparison whether to use harmonic injection in the output matching network of an embodiment of the present application. DETAILED DESCRIPTION

[0026] In order to make the above objectives, features and advantages of the present application more apparent, specific embodiments of the present application will be described in detail below with reference to the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present application. However, the present application can be practiced in a number of ways other than those specifically described herein, and the present application is not limited to the embodiments described herein as long as they do not depart from the spirit of the present application. It is to be understood that other implementations can be employed and the embodiments disclosed herein are only exemplary and do not limit the scope of the application defined by the appended claims.

[0027] The present application discloses a Doherty power amplifier based on a dual-mode impedance converter, which can significantly reduce the circuit size, realize high impedance conversion and harmonic injection, and has performance comparable to that of a conventional post-matching wideband power amplifier. The Doherty power amplifier is actually a compact wideband Doherty power amplifier, which reduces the circuit size and the difficulty of circuit design by canceling the post-matching network, and forms an anti-phase second harmonic injection path by matching the drain bias and utilizing the structural characteristics of the dual-mode impedance converter, so as to improve the saturation efficiency and output power of the high-frequency sideband. The final design is comparable to the performance of the conventional Doherty power amplifier using a post-matching network, so as to meet the needs of mobile communication.

[0028] Specifically, please refer to Figures 1 to 3The Doherty power amplifier based on the dual-mode impedance converter in the embodiment comprises a wideband coupler for separating two input signals, a stepped impedance line connected with the output end of the wideband coupler and serving as an input matching network, a main path and an auxiliary path respectively connected with the input matching network, and an output matching network connecting the output end of the main path and the output end of the auxiliary path. The wideband coupler is used to divide the input radio frequency signal into two paths, and the two paths of signals have a phase difference of 90°, in other words, the phase difference of the two input signals separated by the wideband coupler at the drain is 90°. In the embodiment, the wideband coupler with the model number of 11306-3S of the Anaren Company is used, and the selection of the wideband coupler is not limited to this, and a coupler with similar performance to the product can be selected. The stepped impedance line serves as the input matching network. The main path comprises a main power amplifier tube, and the auxiliary path comprises an auxiliary power amplifier tube. In the embodiment, the main power amplifier tube and the auxiliary power amplifier tube are the same, and both are selected to be the transistor with the model number of CGH40010F GaN HEMT of the Cree Company, and the selection of the transistor is not limited to this. Further, the load-pulling impedance Z lp of the main transistor (main power amplifier tube) after software simulation is selected to be 20+3j. In addition, in the embodiment, the output matching network comprises a drain bias connected with the main power amplifier tube and the auxiliary power amplifier tube, and two dual-mode impedance converters connected with the drain bias and respectively arranged on the main path and the auxiliary path, the drain bias comprises a parallel branch connecting the main path and the auxiliary path, and the far end of the parallel branch is connected with a direct current power supply and a bypass capacitor. The dual-mode impedance converter comprises a terminal short-circuit branch, and the terminal short-circuit branches of the dual-mode impedance converter on the main path and the dual-mode impedance converter on the auxiliary path jointly form a coupled line band-pass filter, which is used to establish an anti-phase second harmonic injection path between the main path and the auxiliary path. In the embodiment, the input matching network adopts a two-stage cascaded structure. In the embodiment, the dual-mode impedance converter serves as the output matching of the Doherty power amplifier, realizes the transformation of the complex impedance to the real part at two frequency points, provides a guarantee for the wideband design, and meanwhile, a part of it also builds the anti-phase harmonic injection path. In addition, in the embodiment, a phase shift line for compensating the 90° phase difference of the two input signals at the drain is arranged on the auxiliary path.

[0029] Further, in the embodiment, a first input matching circuit connecting the wideband coupler and the main power amplifier tube is arranged on the main path, and a second input matching circuit connecting the wideband coupler and the auxiliary power amplifier tube is arranged on the auxiliary path, and the main path gate bias is connected in parallel on the first input matching circuit, and the auxiliary path gate bias is connected in parallel on the second input matching circuit.

[0030] Figure 2 A simplified structure diagram of the Doherty power amplifier in the embodiment is shown, wherein Z lprepresenting the load-pull impedance, the output matching network converts Z lp to R L , in the saturation state lp to 2R L , when R L = 50 Ω, the post matching network is no longer needed. Figure 3 The LC equivalent circuit of the dual-mode impedance converter is shown in Fig. 1, in combination with Figure 2 and Figure 3 , the input impedance of the dual-mode impedance converter, i.e. the load-pull impedance Z lp is calculated as follows:

[0031]

[0032] In this embodiment, L3 can be neglected, thus the real and imaginary parts of Z lp can be simplified as:

[0033]

[0034]

[0035] In this embodiment, L1 can be adjusted independently to X lp , while R lp can be converted to R L and 2R L in the back-off and saturation states, respectively, i.e.:

[0036]

[0037] Solving equation (4), two positive solutions can be obtained:

[0038]

[0039] In this embodiment, in order to realize the conversion of the dual-mode impedance converter from complex impedance to real part between two frequency points, R lp needs to be converted at the two frequency points (i.e. in the back-off and saturation states). Therefore, substituting equation (5) into (2), the conversion ratio is 1.5, that is, 1.5R lp = R L , and after normalizing R L to 1 and normalizing the frequency to w1+w2=2, the relationship between normalized and can be obtained as:

[0040]

[0041] Based on equation (6), two sets of and The normalized Z lp As Figure 4 and 5 shown, in Figure 5 , L1=0. Combining Figure 4 it can be seen that the solid and dashed lines intersect at two points in the figure, verifying equation (5). In other words, in the frequency range near the two intersection points, the dual-mode impedance transformer can be used as an output matching network. For an inductive LC tank, the two intersection points can be brought closer to each other to form a broadband design with a bandwidth of about Δw. However, the design has a limitation, that is, there is a peak at , where This means that the conversion ratio decreases and eventually deteriorates the efficiency and output power of the power amplifier. Figure 5 It is shown that within the range of Δw decreases monotonically, which is consistent with the characteristics of power amplifier design, because there are a large number of parasitic capacitors in the transistor, which become more and more significant as the frequency increases. At the same time, for an inductive LC tank, the fluctuation of the imaginary part is smaller, which meets the design requirements of a broadband power amplifier.

[0042] In addition, in the present embodiment, the LC equivalent circuit can also be implemented by a microstrip line. In the present embodiment, two cases of setting a short stub drain bias and not containing a drain bias for a Doherty power amplifier are analyzed to determine the influence on the conversion ratio of the circuit. Specifically, please refer to Figure 6 , the first step of the analysis assumes that there is no bias stub, and the cascade matrix of the microstrip dual-mode impedance transformer is:

[0043]

[0044] According to equation (7), the S 21 of the output matching network can be solved. At the same time, the input impedance Z lpt can also be calculated. Since L3 can be ignored, and Z c3 and θ c3 are small values. The inductive LC tank can be realized by a parallel terminal short stub θ c2 with a large Z c2 . Z c1 and θ c1 can be used to adjust impedance matching and subsequent harmonic injection. In addition, the parallel stub can produce a notch at frequency 2f n , so if the power amplifier based on the dual-mode impedance transformer works at frequency f n , the second harmonic generated by the nonlinearity of the transistor will be suppressed.

[0045] Figure 7 shows the characteristics of the input impedance Z lpt , which is consistent withFigure 4 With Figure 5 in ( and ) are similar. In this embodiment, Figure 6 With Figure 7 Z c1 = 32, Z c2 = 70, Z c3 = 24 Ω, θ c1 = 20°, θ c2 = 70°, θ c3 = 13°, θ b = 30°, R L = 50 Ω. Wherein, Figure 7 The dashed line in Figure 4 represents a conversion ratio of 1.5. Referring to the analysis of Figure 6 , the designed Doherty power amplifier is expected to operate in the range of Δf, where Z lpt has a real part R lpt that exhibits a parabolic shape and X lpt continues to decrease. For the solid line, R lpt ≈ R L at the peak, meaning that the conversion ratio becomes smaller at the center frequency f0, while the loss in the range of Δf is large, reducing the efficiency of the power amplifier.

[0046] In one embodiment, the length of the bias line of the drain bias is less than one quarter of the wavelength of the operating frequency. Further, the length of the terminal short stub in the dual-mode impedance transformer is one quarter of the wavelength of the upper sideband of the operating frequency to form a second harmonic stopband to prevent its transmission to the radio frequency output (RFout). In this embodiment, the Z b can be improved by adding a short bias line θ lpt to the output matching network. As shown by the dashed line, decreasing θ b can significantly increase the conversion ratio, and its real part R lpt and imaginary part X lpt have smaller fluctuations, which helps the design of a wideband PA. In particular, when R lpt decreases from about 50 Ω to 24 Ω at the peak, the conversion ratio at the peak doubles, thus improving the inherent shortcomings of the dual-mode impedance transformer. Figure 7

[0047] ​Traditional Doherty power amplifiers use additional signal sources and complex circuits to inject harmonics from the outside to achieve fundamental wave shaping of the power amplifier to improve its performance, which makes the circuit structure complex and the circuit size large. Since the Doherty power amplifier itself has two paths, which can be used for harmonic mutual reflection, it requires the output matching network to suppress the harmonics and create a bridge between the two paths to guide the harmonics to pass through, and the bridge can block the fundamental wave. In this embodiment, due to the parallel branch θ c2 Will be at 2f n Therefore, if the Doherty power amplifier based on the dual-mode impedance converter operates at f n , its second harmonic will be suppressed. Figure 8 The two parallel branches of the main path and the auxiliary path shown can form a coupled line bandpass filter, which can suppress f n (S 21 <-13dB), and let 2f n This embodiment uses a broadband coupler at the input end to separate the input signal, so that the phase shift of the two signals at the drain is within f n is 90°, and at 2f n is 180°. Figure 9 , by adjusting θ c1 and θ p1 Make the phase of the injection path at 2f n The angle of the phase angle is close to 0°, so the designed harmonic injection is inverted. This path is marked with a double arrow in the figure, indicating that the dual-mode impedance converter establishes an inverted second harmonic injection path between the main path and the auxiliary path. It is worth noting that due to the strict phase constraint, the operation of injecting the inverted second harmonic through the dual-mode impedance converter is a narrowband operation. In this embodiment, the auxiliary power amplifier tube generates harmonics in the saturation state, so this harmonic injection only improves the saturation performance and only works at the upper sideband of the operating frequency.

[0048] Figure 10 and Figure 11 The intrinsic voltage and current waveforms in the main path are shown, where Figure 10 The bifurcated currents shown are caused by harmonics. After the introduction of anti-phase harmonic injection, as shown in Figure 11 The bifurcation disappears and the voltage rises, indicating higher output power. The current and voltage waveforms exhibit a standard half-sine wave form, similar to a Class B power amplifier. Simultaneously, the overlap of the shaded areas decreases, improving efficiency.

[0049] Figure 12The simulation small signal performance of the Doherty power amplifier of the embodiment is shown, and it can be seen that the small signal gain is greater than 10 dB in the range of 2.1-3.1 GHz, and the return loss is less than -14 dB. Figure 13 The simulation large signal performance of the Doherty power amplifier of the embodiment is shown, and it can be seen that the maximum PAE in the backoff and saturation states is 53% and 61% respectively, and the small signal gain is greater than 9.5 dB at the output power of 36 dBm, wherein the maximum output power is 43.3 dBm. In order to facilitate the display of the simulation large signal performance of the Doherty power amplifier, in the embodiment, the output power is divided into two parts, i.e. the backoff part and the saturation part, and the simulation large signal performance of the Doherty power amplifier is shown in two parts. Figure 13 The simulation large signal performance of the Doherty power amplifier of the embodiment is shown, and it can be seen that the maximum PAE in the backoff and saturation states is 53% and 61% respectively, and the small signal gain is greater than 9.5 dB at the output power of 36 dBm, wherein the maximum output power is 43.3 dBm. In order to facilitate the display of the simulation large signal performance of the Doherty power amplifier, in the embodiment, the output power is divided into two parts, i.e. the backoff part and the saturation part, and the simulation large signal performance of the Doherty power amplifier is shown in two parts. Figure 14 , Figure 14 The performance with and without the use of the anti-phase harmonic injection is compared, and it can be seen that the backoff efficiency does not change obviously, but the saturation efficiency and the output power at high frequency are significantly improved, and the performance at low frequency is slightly deteriorated due to the phase mismatch. With the anti-phase harmonic injection, the backoff PAE is from 43.3% to 53%, the saturation PAE is from 50.5% to 61%, and the output power is from 42.6 to 43.3 dBm in the frequency range of 2.1 to 3.1 GHz.

[0050] The Doherty power amplifier based on the dual-mode impedance converter of the embodiment is different from the traditional post-matching power amplifier, and can realize high impedance conversion and harmonic injection in the design of the Doherty power amplifier. The compact wideband Doherty power amplifier related to the embodiment is based on the dual-mode impedance converter, and the performance is comparable to that of the traditional post-matching wideband power amplifier. Since the post-matching network is removed, the designed wideband Doherty power amplifier occupies a very small circuit area, and the size of the entire output matching network is only 19x37 mm 2 , which is suitable for realizing wideband and multi-standard small cell base stations, and is easy to integrate into a chip-on-design.

[0051] The technical features of the above-described embodiments can be combined arbitrarily, and in order to make the description simple, all possible combinations of the technical features in the above-described embodiments are not described, however, as long as the combinations of the technical features do not exist contradictory, they should be considered as the scope of the description.

[0052] The above-described embodiments only express several implementation manners of the application, and the description is relatively specific and detailed, but it should not be understood as the limitation of the patent scope of the application. It should be pointed out that for ordinary skilled persons in the art, some modifications and improvements can be made without departing from the concept of the application, and these all belong to the protection scope of the application. Therefore, the protection scope of the patent of the application should be subject to the appended claims.

Claims

1. A Doherty power amplifier based on a dual-mode impedance converter, characterized in that, The application relates to a wideband coupler for separating two input signals, a stepped impedance line connected with the output of the wideband coupler and serving as an input matching network, a main path and an auxiliary path connected with the input matching network respectively, and an output matching network connecting the output of the main path and the output of the auxiliary path, wherein the main path comprises a main power amplifier tube, the auxiliary path comprises an auxiliary power amplifier tube, the output matching network comprises a drain bias connected with the main power amplifier tube and the auxiliary power amplifier tube, and two dual-mode impedance transformers connected with the drain bias and arranged on the main path and the auxiliary path respectively, the drain bias comprises a parallel branch connecting the main path and the auxiliary path, and the far end of the parallel branch is connected with a direct current power supply and a bypass capacitor; the dual-mode impedance transformer comprises a terminal short-circuit branch, and the terminal short-circuit branch of the dual-mode impedance transformer on the main path and the terminal short-circuit branch of the dual-mode impedance transformer on the auxiliary path jointly form a coupled line band-pass filter for establishing an anti-phase second harmonic injection path between the main path and the auxiliary path.

2. The Doherty power amplifier of claim 1, wherein, The length of the bias line of the drain bias is less than one quarter of the wavelength of the working frequency.

3. The Doherty power amplifier of claim 2, wherein, The phase difference of the two input signals separated by the wideband coupler at the drain is 90 degrees.

4. The Doherty power amplifier of claim 3, wherein, The auxiliary path is provided with a phase shift line for compensating the 90-degree phase difference of the two input signals at the drain.

5. The Doherty power amplifier of claim 4, wherein, The length of the terminal short-circuit branch of the dual-mode impedance transformer is one quarter of the upper side frequency of the working frequency.

6. The Doherty power amplifier of claim 5, wherein, The dual-mode impedance transformer is also used for realizing the transformation of complex impedance to real part at two frequency points.

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