Microwave Journal

A High Efficiency and Gain Doherty Amplifier for Wireless Mobile Base Stations

April 15, 2008

This article presents a high gain and efficiency Doherty amplifier for WLAN mobile base stations with three advanced methods. First, a Doherty amplifier is developed with uneven power drive, which provides more input power to the peak amplifier for full power operation and appropriate load modulation. Second, a special inverted Doherty topology is proposed in order to optimize the average efficiency of the Doherty amplifier. Finally, a three-stage structure is adopted to increase the total gain. These methods are applied to implement a Doherty power amplifier using Freescale MRF6S21050L devices. The power-added efficiency (PAE) and adjacent channel leakage ratio (ACLR) are 33 percent and –42 dBc, which represents an improvement of 3.2 percent and 2 dB, respectively, compared with a conventional Doherty power amplifier and its gain reaches 45 dB by adopting a three-stage structure.


Wireless communications have recently progressed to require an increase in bandwidth and the number of carriers for high data rate capability. In addition to the wide bandwidth, the instantaneous transmit power of the wireless communication systems, such as wideband code division multiple access (WCDMA) or orthogonal frequency division multiplex (OFDM), vary widely and rapidly, carrying high peak-to-average ratio (PAR) signals.1 The mobile wireless base station power amplifiers for the systems require a high linearity to amplify the high PAR signal source without distortion.2,3 Another requirement of power amplifiers for mobile wireless communications systems is high efficiency.4 The communications systems are reduced in both size and cost but require a power amplifier with high efficiency.5-7

In order to accomplish these requirements, the techniques that can improve the linearity and efficiency of the mobile base station power amplifier and overcome the wideband effect are hot issues in the research community. As for conventional techniques, the simplest method is to back-off signals from the saturation region to the linear region at the cost of reduced efficiency of the power amplifier. Class-A, -B, or -AB may be used with a low efficiency from 12 to 20 percent, due to the output back-off.8 Other methods involve predistortion or elimination and restoration techniques or feedforward.9-11 However, these techniques need additional components that result in an increase in cost, size and power dissipation.12 In order to solve these problems, a Doherty amplifier is the most promising candidate for the application, as shown in Figure 1.

The fundamental operation theory has been well described in previous literature. It has high linearity and efficiency for wideband signals and has been studied extensively for the application, as depicted in Figure 2. However, the conventional Doherty power amplifier has its limitation. Because of its lower bias point, the current level of the peaking cell is always lower than that of the carrier. The load impedances of both cells cannot be fully modulated at the value of the optimum impedance for a high power match. Thus, neither cell can generate full output power. In this article, two advanced methods that represent good approaches to solve these problems are presented. First, when the magnitude of the instantaneous input signal increases, the gate bias voltage of the peak cell increases according to the signal’s envelope. Therefore, uneven power drive is possible to achieve a good Doherty operation. Second, the inverted Doherty topology is another efficient method to improve the carrier cell efficiency at low levels when the peak cell is off. The implementation of the amplifier is simple and results show excellent performance.

Advance Design Methods of A Doherty Power Amplifier

Uneven Power Drive

Figure 3 depicts the different fundamental current characteristics of the Doherty power amplifier with even drive and uneven drive. Usually, in even drive, the load impedances of the carrier amplifier and the peak amplifier cannot be completely modulated at the optimum impedance and the peak amplifier cannot generate full power because it is biased in class-C, and its current Ip remains at a low level.13 The linearity of the even drive amplifier is more complex than that of a class-AB amplifier.

Figure 4 shows the load impedance of both amplifiers versus input voltage.

where

ZL = load impedance of the Doherty amplifier

IC = fundamental currents of the carrier amplifiers

IP = fundamental currents of the peak amplifiers

ZC = output load impedances of the carrier amplifiers

ZP = output load impedances of the peak amplifiers

In the low power region, the linearity of the amplifier is entirely determined by the carrier cell. Therefore, the carrier cell should be highly linear for its optimized load impedance. In the high power region, the current level of the peaking cell plays an important role in determining the load modulation of the amplifier. For the asymmetric amplifier with even power drive, the fundamental current of the peaking cell is insufficient to achieve the full load modulation. The load impedances of both cells are larger than the optimum values in the high-power region. As a result, the carrier and peaking cells are driven into saturation without producing full power. Thus, the amplifier’s linearity is seriously affected, as is its power level.

In order to enhance the output power from the peak amplifier, a Doherty amplifier with uneven power drive is proposed, applying more power to the peak cell. Since the amplifier now has an uneven power drive, the linearity of the amplifier is improved due to proper power operation without severe saturation. The linearity is further enhanced by the harmonic cancellation of the two cells at appropriate gate biases. The carrier cell, which is biased in class-AB, has gain compression at high output power levels, while the class-C biased peak cell has gain expansion. Hence, the gain expansion of the peak cell can compensate for the gain compression of the carrier cell. Specifically, the third-order intermodulation (IM3) level from the carrier cell increases and the phase of IM3 is decreased because the gain of the carrier cell is compressed. On the other hand, when the gain of the peak cell is expanded with uneven drive, both the IM3 level and phase increase. To cancel out the IM3s from the two cells, the components must be 180° out-of-phase with the same amplitudes.

Inverted Doherty Topology

The carrier amplifier operational theories indicated that the best efficiency at average envelope power actually occurred with a load impedance closer to 25 than 100 Ω. In order to achieve maximum efficiency at 100 Ω, approximately one-quarter wavelength of a 50 Ω transmission line will be introduced in the carrier amplifier’s output matching network (OMNc). Similarly, the off-state impedance presented by the peak amplifier is so low that this also suggests appending a λ/4 wavelength 50 Ω line to the peak amplifier’s output matching network (OMNp) to guarantee high impedance at the combining node. Size and loss constraints make this approach undesirable. By reversing the Doherty combining point, a 25 Ω maximum efficiency load is provided for the carrier amplifier at the average envelope power. The impedance inversion previously accomplished with the 50 Ω, λ/4 line is incorporated into OMNc, which constrains θS21 = –90°. In the peak cell, a 50 Ω, λ/4 line becomes the off-state impedance rotation appended to OMNp. The output is then taken from the carrier amplifier side of the combining node, as shown in Figure 5. This configuration is called an “inverted Doherty” power amplifier.

This “inverted Doherty” will guarantee a high efficiency at low drive level. However, the biggest challenge of the Doherty design is the carrier amplifier output match. In addition to the –90° phase requirement mentioned above, the gain of the carrier amplifier must decrease by 3 dB as its output power transitions between average envelope power and half of the peak envelope power. This can be understood by noting that the carrier amplifier’s input power ranges from average envelope power to peak envelope power, while the required output power range is from average envelope power to half of the peak envelope power. The gain reduction is necessary to accommodate half of the peak envelope power of the peaking amplifier. With uneven drive, more power will be delivered to the peak cell. This creates a constant gain for the composite Doherty amplifier at lower power gain, which is an important linearity consideration. To optimize the Doherty amplifier’s average efficiency, the carrier amplifier’s output match must be designed for best efficiency performance at average envelope power. For the inverted topology, this occurs at 25 Ω. The maximum carrier amplifier efficiency is limited by the linearity as a result of its operation, together with the class-C peaking amplifier. The carrier amplifier design is thus constrained by the gain, phase, efficiency, linearity and absolute power requirements.

Operating in class-C, its transfer characteristic must be smooth, without evidence of discontinuities. The adjacent channel leakage and IMD problems are made obvious with a two-tone test. The design of the bypass and decoupling networks, as well as the bias circuit, are crucial to avoid the problem of bypass capacitors. The output contribution of the peaking amplifier is expected to range from zero to half of peak envelope power for the same drive range, which causes the carrier amplifier to deliver average envelope power to half of peak envelope power. Finally, the phase matching network, Φ is 83.88°, or an electrical length of 0.223λ, determined initially from individual measurements of the carrier and peaking amplifier, when delivering half of the peak envelope power. It is set to provide equal phase lengths in both signal paths. The final phase length is optimized for best linearity and gain flatness.

Three-stage Structure

In this article, a Doherty power amplifier with three stages is proposed because the conventional Doherty power amplifier always has a single stage and its gain is not high enough, from 8 to 15 dB. Figure 6 is a schematic of the proposed Doherty power amplifier. The novel Doherty amplifier consists of three stages: pre-driver, driver and final stage. The pre-driver and driver are used to obtain a higher output and power gain. They both work in class-A, since this method makes the two stages work in the small-signal regime. This can maintain a low noise figure for the entire amplifier, while it also makes the total PAE a little less. The PAE of the entire amplifier depends mainly on the efficiency of the final stage.

Amplifier Implementation and Results

A photograph of the power amplifier is shown in Figure 7. The Doherty power amplifier performance not only considers the linearity, but also its efficiency. To satisfy these demands, a Freescale MFR6S21050L device was used. A conventional Doherty amplifier was also fabricated for comparison. Figure 8 shows the gain and PAE of the proposed Doherty power amplifier and of an ordinary Doherty power amplifier with even drive. As with an uneven power drive (1:2.5), the carrier cell is compressed early and the peaking cell is expanded early and the region is wider than for the usual Doherty amplifier. Therefore, the amplifier with an uneven drive generates a more linear power because the early gain expansion of the peaking cell compensates for the gain compression of the carrier cell over a wide power range. The power gain of the uneven case keeps a better linearity compared to the even drive case. The PAE of the uneven drive amplifier achieves 33 percent, a 3.2 percent improvement from the inverted structure with optimum load impedance modulation.

Figure 9 shows the measured ACLR of the two types of Doherty amplifier. Compared with the even case, the uneven case, with an inverted output structure, delivers a improved ACLR performance by 2 dB and 3.1 dB at the average output power of 45 dBm for a two-tone test at –5 and +5 MHz tone spacing, respectively. These results confirm that the proposed bypass and decoupling networks in OMNp provides a better adjacent channel leakage performance.

Figure 10 shows the measured IMD3 of the Doherty amplifier with both even and uneven power drives for a two-tone signal. A two-tone signal is used with –5 and +5 MHz tone spacing, respectively. The IMD3 of the amplifier with uneven drive is improved by 10 W compared to the even case.

Conclusion

A high linearity and efficiency Doherty amplifier is proposed and fabricated with uneven power drive and inverted topology. Its PAE achieves 33 percent, an improvement of 3.2 percent over the even drive case. With its inverted structure and offset line in the output-matching network, the amplifier offers a better linearity and efficiency than for the even case. The uneven power drive delivers more linear power, which is improved by a better load impedance modulation. For a ±5 MHz two-tone test, the ACLR is –42 dBc at 40 dBm output power, which is a 2 dB improvement. The IMD3 of the proposed amplifier gives a 10 W improvement over the even case, with appropriate cancellation of the carrier cell harmonics. These experimental results clearly demonstrate the superior performance of the proposed Doherty power amplifier, compared to the conventional Doherty power amplifier. The proposed design methods are suited for the design of the Doherty amplifier for high efficiency and high linearity operation.

Acknowledgment

This work is supported by the National Natural Science Foundation of China under Contract No. 60571057.

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