1. Feedforward Power Amplifier with a New Adaptive Control Method

 

 

Feedforward amplifier is one of the most popular linearization techniques for base station power amplifiers. The feedforward amplifiers ideally have the capability of perfect cancellation of unwanted in-band IM products. However, a high level cancellation is not easily achieved due to premature saturation of an error amplifier, as well as the gain and phase mismatches. The distortion generated by the error amplifier reduces the error cancellation capability of the feedforward amplifier. The perfect signal cancellation in the 1st loop of the feedforward amplifier is not an optimum for minimizing the output error level since the error amplifier with an imperfectly cancelled input generates less distortion signals and the linearity of the amplifier can be improved.

 

 

 

Figure 1

 

Fig. 1 shows the input and output signals of the error amplifier for a perfect signal cancellation and an optimum signal cancellation cases. The pure error signal after a perfect signal cancellation has a high peak-to-average ratio (around 20dB) and the error amplifier with the input generates significant distortions even at a very low average output power. This distortion terms degrade the cancellation performance of the feedforward amplifier. However, the error amplifier with an imperfectly cancelled input signal generates less harmonic distortions due to cancellation of the distortion by the cross modulation term of the error and residual main signals. The cancellation level can be optimized by the new adaptive control method. Therefore, we adopt a new merged control method to adjust the signal cancellation level for the best linearity of the feedforward amplifier.

Fig. 2 shows a feedforward system to apply the new adaptation Method. The new adaptive method detects error term of the final output and then adjusts the main signal cancellation level (1st loop) and the error signal cancellation level (2nd loop), simultaneously. By this operation, the error signal becomes imperfect cancellation signal and the distortion generated by the error amplifier is reduced.

  

 

 

Figure 2

 

 

 

 

Figure 3

 

To validate the proposed adaptive control scheme, a 2.14 GHz adaptive feedforward amplifier for down-link 16-channel WCDMA signal has been implemented as shown at fig. 2. Fig. 3 shows signal cancellation levels of the error signals and final outputs with two-carrier WCDMA signal.

The signal cancellation level of the new case is 20 dB, which is 8 dB lower than that of the conventional case (see fig. 3(a)). The IMSR at 5MHz offset of the new method is about -50 dBc, but the conventional case is -44 dBc (see fig. 3(b)). Therefore linearity of the new case is improved by 7 dB.

 

 

2. Predistortion Power Amplifier for Base-Station using an Feedforward Loop Linearizer

 For linear amplification of the error signal in the feedforward system, the peak envelope power(PEP) of the error amplifier should be high because the harmonics from the error amplifier reduce the cancellation level [10]. Besides, the delay compensation line and coupler losses are burdens of the amplifier in terms of an average output power and efficiency of the system. Due to the reasons, the feedforward amplifier has a low efficiency contrary to the high linearity performance. Therefore, the feedforward loop has been used  for the predistortion signal generator. Since the feedforward loop is placed in front of the main amplifier, the PEP and gain of the error amplifier is reduced significantly compared to the conventional feedforward system. Also the delay line and coupler losses are not significant factors affecting the amplifier performance. The main requirement of the new amplifier is the same IM amplitude and phase characteristics between the predistorter and main amplifier.

 

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Fig. 1. The proposed linearization circuit using feedforward predistortion(FFP).

 

Fig. 1 shows the proposed amplifier diagram. Five identical Doherty amplifiers, one for the error generation amplifier(EGA) and four for the main amplifier, are built using 90W PEP LDMOSFETs. Therefore, the main amplifier can handle a 720W PEP and has been tested at an average output power of 47.8 dBm, 10.8 dB backed-off from the peak power. The error amplifier stage with the gain of 51 dB has been configured in the second loop. The vector modulator for the amplitude and phase controls is fabricated with the series connection of a reflection type attenuator and phase shifter using 3 dB hybrid couplers, PIN diodes and varactor diodes. Both the first and second loop delays have been compensated using coaxial cables of 9.9 ns and 16 ns, respectively.

 

 For the verification, the experiments have been conducted using a forward-link WCDMA 4-carrier signal at 2:35 GHz. Fig. 2(a) and (b) show the power spectral density of the output at an average output power of 47.8 dBm with the spans of 40 MHz and 100 MHz. The ACLRs at 5 MHz and 10 MHz offsets are -46.3 dBc and -48.7 dBc, respectively, enhanced by about 7.1 dB and 5.4 dB, respectively, by the cancellation. This cancellation is a little lower than that of the single DPA case due to the nonuniform DPAs, power divider and combiner, which can be solved in the well controlled manufacturing environment. As shown in Fig. 2(b), the power spectral density of the canceled output has a symmetrical shape, illustrating the strong immunity to the serious memory effect. The total efficiency of the amplifier with the proposed topology is about 12.7 %, which is enhanced siginificantly compared to the general feedforward type LPA for basestation.

 

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Fig. 2. The power spectral density of the amplifier with four Doherty main amplifier at average output power of 47.8 dBm : 40 MHz and 100 MHz span.

 

 

  Reference

[1] Y.Y Woo, Y. Yang, J. Yi, J. Nam, J.H. Cha and B. Kim, "Feedforward Amplifier for WCDMA Base Stations with a New Adaptive Control Method," IEEE MTT-S Int. Microwave Symp. Dig., Vol. 2, pp. 769-772, June 2002.

[2] I. Kim, J. Cha, S. Hong, Y. Y. Woo, J. Kim, and B. Kim, "Predistortion power amplifier for base-station using an feedforward loop linearizer," in Proc. IEEE European Microw. Conf., Manchester, UK, Sep. 2006, pp. 141-144.

[3] Y. Y. Woo, Y. Yang, J. Yi, J. Nam, J. Cha, and B. Kim, "A new adaptive feedforward amplifier for WCDMA base stations using imperfect signal cancellation,"  Microwave J., vol. 46, no. 4, pp. 22-44, Apr. 2003.

[4] Y. Yang, Y. Y. Woo, and B. Kim, "Optimization for error-canceling loop of the feedforward amplifier using a new system-level mathematical model," IEEE Trans. Microw. Theory Tech., vol. 51, no. 2, pp. 475-482, Feb. 2003.

[5] B. Kim, Y. Y. Woo, J. Yi, J. Nam, and J. Cha, ¡°A new adaptive feedforward amplifier using imperfect signal cancellation,¡± 3rd International Conference on Microwave and Millimeter Wave Technology (ICMMT2002), Beijing, China, Aug., 2002.

[6] Y. Y. Woo, Y. Yang, J. Yi, J. Nam, J. Cha, and B. Kim, "Feedforward amplifier for WCDMA base stations with a new adaptive control method," IEEE MTT-S Int. Microwave Sympo., Seattle, Vol. 2 , pp.769 -772, June, 2002.