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Broadband Power Amplifiers for Unified Base Stations

Release Date:2011-06-17  Author:Pengcheng Jia  Click:


1 Introduction
    It is very challenging for multinational carriers to support multistandard multiband networks. A base station has to support frequencies such as 800 MHz, 900 MHz, 1800 MHz, 1900 MHz, 2100 MHz, and 2600 MHz. It is costly to install independent Radio Frequency (RF) modules that support each frequency. Therefore, a broadband RF module has attracted the attention of carriers because it not only reduces the size of the RF module but is also more energy efficient. It can also lower operation and maintenance cost.

    In an RF module, the most challenging component is the power amplifier. It is very difficult to develop a broadband amplifier that can work beyond 1 GHz with conventional laterally diffused metal oxide semiconductor (LDMOS) transistors. Emerging GaN technology is perfectly suited for this broadband application. A GaN transistor has much higher power density and operational frequency compared with LDMOS, which makes it a good candidate for broadband power amplifiers.

2 Designs of Broadband Power Amplifiers


2.1 Feedback Amplifier
    LDMOS transistors are currently the dominant power transistor candidate for base station power amplifiers. Their output power is high, and their cost is very attractive. However, the peripheral of LDMOS transistors is normally large, and its optimal load is typically in the sub-Ohm range. The high impedance transformation ratio (from sub-Ohm to 50 Ω) limits the bandwidth of the matching network. Typically, it can only cover 100 MHz of bandwidth or less.
Feedback is a classic approach to broadening the bandwidth of an amplifier [1]. As shown in Fig. 1, a feedback can be placed between the drain and gate of a LDMOS transistor. Generally, the gain of the transistor increases monotonically when the frequency drops. With a feedback, the gain curve may be flattened below the corner frequency. The input and output match can both be improved by the feedback when its value is properly selected. The intermodulation components can also be suppressed by the feedback.


    A broadband LDMOS power amplifier with a high power feedback structure is shown in Fig. 2. Two transistors are assembled inside a ceramic package. Feedback resistors are placed on the top and bottom of the package. The feedback resistors are connected to each transistor by lines around the transistor. Capacitors are placed in the feedback path to isolate the DC voltage from drain to gate. The feedback resistor has a metal flange that is directly mounted to the base plate. The feedback resistor can dissipate a large amount of heat, and the length of the feedback path is minimized to avoid oscillation.


    From the output port, a 2:1 balun is first used to split the 50 Ω port impedance into two 25 Ω channels. Then, a 4:1 balun is used to further transform the impedance from 25 Ω to 6.25 Ω. It is much easier to match the optimal load of the transistor to 6.25 Ω. A similar balun is used at the input to improve the matching at the input side. This feedback amplifier can output more than 60 W of power from 20 MHz to 1000 MHz.


2.2 Distributed Amplifier
    Another common approach to design a broadband power amplifier is the distributed amplifier, as shown in Fig. 3. It comprises a series of small transistors. The gates of field-effect transistors (FETs) connect to the input transmission line, and their drains all connect to the output transmission line. The distance between transistors is properly designed so that the output power from each transistor is summed in phase. The characteristic impedance of the input/output transmission lines is higher than 50 Ω. When it is loaded periodically with a series of transistors, its characteristic impedance is lowered to 50 Ω and presents a good
input/output match.


    A distributed amplifier has a very broad bandwidth and can be applied to millimeter wave applications. However, its output power is limited by the breakdown voltage of the last stage transistor, and its efficiency is not good either. Half of the energy is dissipated into the dummy load ZL on the output transmission line. The transistors are typically biased at class A state, and the overall efficiency of the distributed amplifier is only 25% theoretically. It is not a good candidate for base station application because of its poor efficiency and low output power.


2.3 In-Phase Power Combining
    Higher power can be achieved by combining the output power from more than one amplifier cell by broadband combiners [2]. A multisection broadband Wilkinson combiner is shown in Fig. 4. The combiner has four sections and covers a 4:1 (fmax:fmin) frequency range.


    The combiner network has two branches, and each branch transforms impedance from the summing point (100 Ω for a two-way combiner) to the amplifier port impedance (typically
50 Ω ). Each branch has several sections of transmission lines with each section around quarter wavelength at the center frequency. The characteristic impedance of each section changes gradually, and the section closest to the amplifier has the lowest impedance. In combiner design, the network’s Q factor can be defined by the impedance transformation ratio Qr for each transmission line section. The lower the Qr , the broader the bandwidth. To broaden the bandwidth, more sections of transmission lines should be used to reduce the impedance transformation ratio at each section. The relationship between Qr factor and bandwidth is shown in Fig. 5.



2.4 Power Combining with 90 Degree Hybrids
    More output power can also be achieved by combining two amplifier cells with a pair of 90 degree hybrids (Fig. 6). Each of the amplifier cells has the same input/output matching network. The hybrid power combining approach offers the benefit of wide bandwidth. The amplifier is unconditionally stable within the pass-band of the hybrid. It also provides good input/output match. The reflected signals from the inputs of both amplifier cells sum at the input port of the hybrid. One reflected signal goes through the 0 degree coupling port twice, and the other reflected signal goes through the 90 degree coupling port twice. The two reflected signals are out of phase and cancel each other at the input port. This gives the amplifier a very low input and output return loss. When designing a multistage amplifier, this helps reduce the gain ripple, and the driver amplifier’s output power can be fully delivered to the output stage.


3 GaN Broadband Power Amplifier Demonstration
Typically, high power transistors have very low optimum source and load impedance at very high output power level. The GaN transistor can be operated at much higher voltages; for example, 48 V, compared with 28 V for LDMOS. The load impedance is much higher for a GaN transistor when operating at higher voltages compared with the sub-Ohm load impedance for conventional transistors such as GaAs FET or Silicon LDMOS. The peripheral of a GaN transistor is also much smaller than that of a LDMOS transistor with the same output power capability. This leads to a much smaller parasitic capacitance and a simpler matching network design. The GaN transistor also has very low thermal resistance when it is grown on SiC substrate. Its operating frequency can increase to millimeter wave range because of much higher electron mobility. These features make the GaN transistor an ideal candidate for broadband applications.
GaN transistors have much higher gain at low frequency and are very easy to oscillate. A feedback network can help control the gain at lower frequency. However, because of the physical length of the feedback network, it can change into positive feedback at higher frequency. So the feedback network must be carefully selected. It needs to be inductive and become a high impedance component at the high frequency end. Heat dissipation also needs to be taken intr high power amplifier designs. The feedback resistor consumes power and needs to be capable of dissipating heat. With a proper feedback network, a GaN amplifier can operate over an extraordinarily broad bandwidth, ranging from 20 MHz to 2500 MHz with an output power of more than 20 W.

    For very high power applications, amplifiers are typically designed without a feedback network. The bias network and matching network has to be carefully selected to compress the gain at low end and avoid oscillation. Fig.7 shows a GaN amplifier with a broad bandwidth covering 0.5 to 2.5 GHz frequency. The output stage uses two GaN transistors, which can output 90 W at narrow band applications at 32 V. The transistor is currently offered in die format only. A flange-type package is selected, and the die is mounted into the package first. Then, the packaged GaN transistor is dropped into the power amplifier module. The output stage uses a pair of broadband 90 degree hybrids to combine the output power of two transistors. The hybrid is a strip-line-type 90 degree coupler. It has multiple sections of strip line networks stacked between a top and bottom aluminum plate. The hybrid is assembled into the amplifier module through cavities in the printed circuit board (PCB), and the leads of the hybrid are soldered to the microstrip line pads. The matching networks for each transistor are identical. A good input/output match can be achieved for the output stage.


    Measured S21 and S11 are shown in Fig.8. The power amplifier exhibits extremely flat small signal gain over a 5:1 fmax:fmin bandwidth. The average gain is about 57 dB and its ripple is  +/- 3dB over the entire 0.5 G to 2.5 GHz bandwidth. The bandwidth can be further broadened when a hybrid with broader bandwidth is selected.


    As shown in Fig. 9, more than 60 W of output power is achieved from this broadband amplifier. The output power is further increased if more transistors or transistors with higher power capability are used in the output stage. The operating frequency is also extended to 2600 MHz to cover the LTE frequency.


    This GaN amplifier can cover multiple bands with a single amplifier. However, the linearity of the broadband amplifier is compromised to achieve output power covering wide bandwidth.  The GaN amplifier can be used in the back-off mode to meet the linearity requirement of base stations and repeaters. The trade-off leads to low efficiency of the power amplifier. To maintain efficiency, the linearity is improved for a sub-band with proper tuning but not for the entire bandwidth. Therefore, additional linearization techniques such as envelop tracking, are also necessary if both high efficiency and high linearity are required when the amplifier is used for a unified broadband base station.

4 Conclusions
    A broadband GaN high power amplifier is discussed in this paper. Very high output power can be achieved to cover multistandard and multiband applications with new GaN transistors. The GaN amplifier design can be improved to cover the full band from 700 MHz to 2600 MHz. A GaN amplifier can be operated in back-off mode to meet the requirements of linearity. It can also be tailored into several sub-bands to achieve better efficiency for each sub-band. Additional linearization techniques such as envelop tracking or DPD are necessary to further improve linearity and maintain high efficiency for the entire bandwidth [3]. This allows wireless infrastructure vendors to develop a single, highly-efficient multimode, broadband RF front end that can be deployed to meet various transmission standards anywhere in the world.



[Abstract] A broadband power amplifier is required to cover the full range of cellular frequency band—from 700 MHz to 2600 MHz—in a base station that supports multiple frequency bands simultaneously. Conventional laterally diffused metal oxide semiconductor (LDMOS) transistors support narrow band applications up to 3 GHz. However, they cannot operate beyond 1 GHz in broadband applications. GaN transistors have much higher power density and operational frequency compared with LDMOS. Therefore, they are ideal for broadband amplifiers that support multiple bands. Theories for designing broadband amplifiers are introduced in this article, and a 500-2500 MHz 60 W GaN amplifier is discussed.

[Keywords] broadband power amplifier; GaN; LDMOS