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Linearity improvement of a dual-band Doherty power amplifier using E-CRLH transmission line

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Abstract

A dual-band Doherty power amplifier (DB-DPA) using the extended composite right and left handed transmission lines (E-CRLH TL) is proposed. In this structure the λ/4 transmission lines are replaced with open-ended E-CRLH TL. The power amplifier exhibits dual-band property at f1=2.5GHz, f2=3.4GHz input frequencies and suppresses the second harmonics (2f1, 2f2) at the amplifier output. The simulation studies show that the second harmonic and second order intermodulation (IM2) of the proposed DB-DPA are improved compare to the conventional DPA. This improvement is obtained because of the quad band property of E-CRLH transmission lines. Then, the proposed DB-DPA is examined using various multi tones signals. The better ACPR and efficiency are achieved compare to the conventional Doherty power amplifiers. The measurement results confirm the accuracy of the simulation results.

Introduction

THE USE of high linearity and high efficiency multi-band systems in modern wireless technologies such as third generation (3G) and fourth generation (4G) are necessary. Furthermore, the multiband system can use different standards simultaneously, such as GSM, CDMA, WCDMA, WiMax and LTE, which operate in different frequency bands [1], [2].

Power amplifiers (PAs) are the core modules in wireless communication systems and have important effects on the linearity of the whole system [3]. Meanwhile, the system power consumption can be reduced by using high efficiency amplifiers in communication systems [4], [5]. On the other hand, the high linearity amplifiers can decrease spur components and enhance the quality of communications services. The PAs have its high efficiency when used at high power level [1]. This is why most of the PA used at a compression level of 1 dB (P1dB), however this point is extremely nonlinear. PAs should be used at a assumed back-off (below 1 dB compression point) to confirm high linearity. In this case, the efficiency drops sharply [4]. Using digital modulation techniques to produce signals that have a large peak to average power ratio (PAPR) is another issue. These conditions require PAs that have high linearity and high efficiency [5].

The Doherty Amplifier technique generally uses parallel Class-A or AB and Class-C amplifiers and thus an improvement in efficiency arises particularly in back-off regions [6].

P1dB point is enhanced by using parallel structure [7]. In the Doherty method an input signal is split into two signals with a phase difference of 90. The λ/4 transmission line is utilized to perform matching, to perform impedance inversion, and to compensate phase delay [4].

So far, the use of composite right and left handed (CRLH) transmission lines is explored for the realization of a Doherty PA [8], [9]. An expanded composite right / left handed transmission line (E-CRLH TL) is proposed in this paper to construct a dual-band Doherty PA [10], [11]. The concept of E-CRLH TL, presented in [12] and the quad-band metamaterial devices is considered in [13], [14], [15]. In the proposed Doherty PA, the E-CRLH provides two pass bands at f1, f2 frequencies and also suppresses the second harmonics (2f1, 2f2) of the amplifier at output. This performance is achieved by means of an E-CRLH TL which allows the manipulation of phase slope and phase offset at the specified frequencies.

This paper is organized as follows. First, the principles of the design a Doherty power amplifier is presented and a conventional Doherty amplifier is realized. Then, the proposed dual-band Doherty power amplifier based on E-CRLH TL is introduced and its simulation results are demonstrated by using an electromagnetic simulator. At last, the simulation results are confirmed with measurement results.

Section snippets

Conventional Doherty Power Amplifier (DPA)

A DPA contains carrier amplifier, peaking amplifier, power divider and λ/4 transmission lines is shown in Fig. 1. The carrier amplifier works at low power levels in the linear region, and it saturates at high power levels (nonlinear region). Carrier amplifiers are used as Class-A or Class-AB. The efficiency at low power level is defined by the efficiency of this amplifier [4]. The peaking amplifier doesn't really operate at low power levels and the transistor is cut off. At high power levels,

Dual-band Doherty PA using E-CRLH TL

In the proposed dual-band DPA, the open-ended E-CRLH TLs are used in place of TL#1 and TL#2 in Fig. 1. The schematic of the proposed DPA is presented in Fig. 3. The dispersive property of E-CRLH structure can be used to modify its nonlinear phase response to obtain different electric lengths (phase shifts) at different frequencies [17]. E-CRLH TL is designed to be able to exhibit as a quad-band configuration and is extracted from eight equations for quad-band transmission line synthesis with

Measurement and simulation results

The proposed DB-DPA is fabricated on the RO4003 substrate with 20 mil thickness, loss tangent equal to 0.0021 and dielectric constant of 3.38, as shown in Fig. 5. This structure is simulated by ADS software.

Fig. 6 shows simulation and measurement results of the output power of both DB-DPA and conventional DPA vs. input power. In the linear region, the operation of the proposed DB-DPA is nearly similar to the conventional DPA. However, near the saturation region, the manner of the proposed

Conclusion

In this paper a traditional DB-DPA based on the E-CRLH TL is presented. This configuration can be used in LTE-advanced applications at two frequencies 2.5 GHz (LTE band 7) and 3.4 GHz (LTE band 22). The drain efficiency of the proposed DB-DPA is 75%. The second harmonic and IM2 of the proposed structure is 30 dB and 20 dB, respectively at the first band and in the other band 20 dB and 10 dB. Accordingly, the proposed technique can provide an efficient method to design dual-band microwave

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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