Patent Application
20130178168
This invention relates to power amplifiers, and more particularly to radio frequency (RF) power amplifiers with multi-band matching for wireless transceivers used in cellular and other wireless networks.
The advancement of wireless communication networks has necessitated the need for mobile transceivers (terminals or user equipment (UE)) capable of multi-band operation. For example, the Global System for Mobile Communications (GSM), which accounts for 60% of the global mobile telephone market in 2010, had only the 900 MHz band available when introduced. A few years later, the Digital Cellular Service (DCS) band (1.8 GHz) was added. These bands are used in Asia and Europe. In the U.S., the Personal Communications Service (PCS) band (1.9 GHz) and the 850 MHz band were adopted. As a result, the GSM system currently spans four bands.
The 3rd Generation Partnership Project (3GPP), Long Term Evolution (LTE) is the next advance in mobile communication to cope the overwhelming increase in the data traffic resulted from applications such as online gaming, mobile TV, and multimedia streaming. The main targets for LTE are an enhanced data rate, an increased spectral efficiency, and a reduced latency. There are more than 30 bands defined for LTE according to 3GPP Rel.10. The main bands of interest for North America are bands 13 and 14 (700 MHz bands) and band 4 (1710 to 1755 MHz). In Europe, band 7 is expected to be widely used, with operation from 2500 to 2570 MHz. In Japan, it is likely that band 1 (1920 to 1980 MHz) will be used first for LTE.
To achieve seamless operation among the networks worldwide, mobile terminals with multi-band operation capability are required. A radio frequency (RF) power amplifier (PA) is one of the key components in mobile terminals. It is difficult for the PA to achieve high output power and high power efficiency concurrently over multiple bands.
Several approaches that address this issue are known. One approach is based on a parallel line-up of single-band PA. The PA corresponding to the frequency band is selected via an array of switches. That approach heeds as many PAs as the number of the operation frequency bands, which increases the size and cost of the terminal.
Another approach uses a multi-band matching network (MN). Several MN configurations are available. Broadband MN can achieve a wide frequency operation range. However, it is difficult to achieve high power efficiency over a broad frequency range because the output characteristics of the PA vary over frequency. Reconfigurable MNs use RF switches. Variable devices can also address this issue. However, the addition of RF switches or variable devices degrades the system performance and/or reliability. RF switches suffer from insertion loss and limited isolation. Variable devices such as varactor have a limited quality factor, and may require high tuning voltage.
It is therefore desirable to have a PA with multi-band operation capability with a MN composed of only passive devices.
The embodiments of the invention provide a multi-band matching network for RF power amplifiers that utilize multiple impedance transformer branches connected in parallel. Each transformer branch achieves matching at one frequency band.
The core of each transformer branch is connected between frequency blocking networks, which reject out-of-band signals. The resulting matching network can achieve an optimal impedance match between the load and the output of the amplifier simultaneously at different frequency bands. Tuning or switching elements, which are inevitably lossy as in the prior art, are not used.
Specifically, a multi-band matching network (MN) includes a set of impedance transformer branches connected in parallel. Each transformer branch includes an L-shaped LC MN that is optimized for one of the required operation frequency bands. Frequency blocking networks are added before and after each LC MN core to prevent interference between each other. The MN offers optimal impedance matching for the PA at multiple frequency bands simultaneously, without using any active tuning or switching elements.
The MN for a triple-band PA works at LTE bands of 700 MHz, 1.7 GHz, and 2.6 GHz. The MN is designed and to achieve over 40% maximal power added efficiency (PAE) with a peak output power exceeding 28 dBm, and can be used in a last RF PA stage of multi-band terminals.
The multiband MN includes a set of N branches 101 of impedance transformers connected in parallel, where N denotes the number of operation frequency band signals. At an input port 102 of the MN 100, an impedance at an output of the multi-band PS amplifier output is Zn for band n, and at an output port 103, an impedance of the load for band n is ZLn.
For each nth branch, there is a first N−1 frequency blocking element 111 before each nth MN core 120, and a second N−1 frequency blocking element 111 after each nth MN core 120. That is, the MN core is connected in series between the first and second frequency blocking elements.
The frequency blocking elements reject out-of-band frequencies from both the amplifier output and the load to avoid impedance deviation caused by other parallel transformer branches. For the nth frequency band, the output of the amplifier is for the nth branch of the MN, while all the other parallel branches appear as an open circuit at the output port 103. The same holds for the input.
As a result, all the other branches can be neglected at the frequency band under analysis. The frequency blocking elements before the core transform the impedance Zn to Zn′, while the frequency blocking elements after the core transform the load impedance from ZLn to ZLn′.
The MN core n conjugately matches Zn to Zn′. Therefore, an optimal matching is achieved at the nth frequency band. The same analysis applies to other branches. The overall MN simultaneously achieves optimal matching at the set of N frequency bands.
Example Circuit
The frequency blocking element includes two LC networks 111 connected to either side of the MN core 120. The center frequencies of the three operations bands are denoted as f1, f2, and f3 respectively. L1 and Cn resonates at a frequency fn. At this resonance frequency, the serial connected LC network appears as an open circuit. For simplicity reason, the load impedance is selected to be the same impedance Z0 for all operation frequency bands. At frequency f1, the optimal output impedance of the amplifier is Z1. The frequency components of band 1 are blocked by L1C1 network. As a result, the second and the third transformer branches appear as an open circuit to both the amplifier output and the load. The frequency components of band 1 are able to pass through the L2C2 network and the L3C3 network. However, L2C2 and L3C3 transforms the output impedance of the amplifier from Z1 to impedance Z1′, whereas the load impedance is transformed from Z0 to Z01′.
The LC networks 111 can be followed, or preceded by LC circuits 112. The circuits suppresses out-of-band frequencies, e.g., Z(f1)=0, and Z(f≠fi)=∞.
The MN core 1 between the LC networks, which is an L-shaped LC matching network, conjugately matches Z1′ to Zo1′ to achieve maximal power transfer between the output and the load. The same analysis applies to the other branches, which operate at frequencies f2 and f3, respectively.
The parallel connection of the three transformer branches results in a triple band MN that optimally matches the load impedance to the amplifier output at three operation bands simultaneously without any tuning or switching elements.
Design of Triple-Band PA
A triple-band PA operates at 700 MHz, 1.7 GHz, and 2.6 GHz bands. The field effect transistor (FET) at the input port 102 is a high electron mobility transistor (HEMT). The HEMT can deliver 30 dBm output power with 14.8 dB gain at 2 GHz and a supply voltage of 4.5 V. The PA is designed to operate in class AB mode as known in the art. That is two active elements conduct more than half of the time as a means to reduce the cross-over distortions of class-B amplifiers.
After setting the DC bias, load pull and source pull simulation is performed at each of the three frequency bands to find the optimal load and source impedance.
In load pull, a variable AC load is connected to the output of the FET directly. The load impedance is swept over the whole Smith chart. The corresponding output power and power added efficiency (PAE) are measured at each point and corresponding contours are generated. The optimal load impedance at each frequency band is determined based on the output power and PAE simulation results, as shown in Table I (normalized to Z0).
The source side is less sensitive to frequency variation according to source pull simulation. The source impedance is set to 0.11-0.11j for all the three frequency bands. This has minimum impact on the amplifier's power transfer characteristics.
The next step is to determine the LC value of the LC circuits that are used as frequency blocking elements. The LC circuits affect the bandwidth of the overall system. The following equation dictates the relationship among fr, L and C
When L1, L1 and L3 are all set to 2 nH, the value of C1, C2 and C3 are 1.87 pF, 4.38 pF and 25.9 pF, respectively. After the LC value of the LC circuits is determined, we can determine Zn′ and Zon′ based on Zn and Zon using the Smith chart.
Example L-shaped MN core are shown in
Triple-Band PA Simulation
Large signal and small signal frequency response based simulation are shown in
The three peaks at 0.7 GHz, 1.7 GHz and 2.6 GHz exist in both the power gain and S21. This PA achieves 13.4 dB, 11.2 dB, and 8.7 dB power gain at operations bands of 0.7 GHz, 1.7 GHz, and 2.6 GHz, respectively. The decrease of power gain as frequency increases is due to the intrinsic S21 degradation of the FET at higher frequencies. The maximal power gain achievable at each frequency bands shown in
The invention provides simultaneous multi-band matching for RF power amplifier (PA) without the integration of any tuning or switching elements. The PA shows a peak output power greater than 28 dBm and a maximal PAE greater than 40% at three operation frequency bands. This circuit can amplify signals from multi-bands simultaneously.
Although the invention has been described by way of examples of preferred embodiments, it is to be understood that various other adaptations and modifications may be made within the spirit and scope of the invention. Therefore, it is the object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the invention.
