A Doherty Amplifier with Envelope Tracking Technique and DSP for High Efficiency

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					International Journal of Electronics and Computer Science Engineering                                        147

                      Available Online at                                  ISSN-2277-1956

 A Doherty Amplifier with Envelope Tracking
   Technique and DSP for High Efficiency
                              Ambar Saxena 1, Rakesh Kumar 2, Sahab Ram 3
                             Deptt. of Electronics & Communication Engineering,
                        Bhagwati Institute of Technology and Science, Ghaziabad, U.P.

Abstract- A Doherty amplifier employing input signal envelope tracking technique. In the amplifier, gate bias of
peaking amplifier is controlled according to the magnitude of the envelope. The performance of the microwave
Doherty amplifier has been compared with class AB amplifier. DSP is used to dynamically adjust the gate bias of the
auxiliary (peaking) amplifier at the rate of the signal envelope to obtain gain flatness. DSP is used as a digital
predistortor to improve the overall linearity and efficiency.
  Keywords —Adjacent channel leakage ratio (ACLR),Adjacent channel power ratio(ACPR), envelope tracking,
microwave Doherty amplifier, power added efficiency (PAE), wideband code-division multiple access (WCDMA).

                                              I. INTRODUCTION

The Doherty amplifier has gained renewed interest recently since it is able to achieve high efficiency over a
wide range of output power [1-2]. This feature is highly advantageous in mobile phone applications. However,
a common problem with the Doherty amplifier is nonconstant gain and phase behavior as a function of power,
which is detrimental to ACPR. An invaluable technique to improve the performance of the power amplifiers is
to incorporate digital signal processing (DSP). DSP has been widely applied to base station amplifiers to
significantly improve linearity [3-4].

     For the power amplifiers of code-division multiple access (CDMA) base stations, linearity and efficiency is
the most important figure of merit. As the power level of amplifier increases and the size becomes more
compact, the lower efficiency causes severe thermal problems. Hence, the efficiency of the high power amplifier
has become an important issue. The previously reported microwave Doherty amplifiers without envelop
tracking circuits deliver more output power compared with their counterparts of class AB amplifiers at the same
linearity [6], [7]. However, the drain current of the peaking amplifier is slightly lower than that of the carrier
amplifier at a high power level because of the fixed lower bias of the peaking amplifier. Thus, the load
impedance of this microwave Doherty amplifier cannot be fully modulated to the value for a high power match.
As a result, the carrier and peaking amplifiers cannot generate their respective full powers. The reduced output
powers directly affect the improvement of efficiency as well as cost. The Doherty amplifier is a promising
solution for high efficiency with good linearity.

    In this paper, we compare the gain and phase characteristic of DSP gate bias control combined with digital
predistortion and envelope tracking technique for the improving of the linearity and efficiency of the Doherty

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   The Doherty amplifier (Fig.1) consists of main and auxiliary amplifiers with an impedance inverter (usually
implemented as a quarter-wave transformer) connecting the outputs. At low power, the main amplifier is the
only device operating, and the auxiliary amplifier, biased Class C, turns on when the main amplifier becomes
saturated. The role of the auxiliary amplifier is to supply an appropriate amount of current to decrease the
effective load seen at the output of the main amplifier. By doing so, the main amplifier is able to deliver current
to the load while remaining in saturation. A significant consequence of this is that the efficiency of the total
amplifier system remains high while the main amplifier operates in saturation.

                                            Fig. 1 Doherty amplifier topology

                                 Fig. 2 System implementation of the DSP with the Doherty
    DSP can be used to obtain significant improvement in amplifier performance. With the Doherty amplifier,
DSP can be used to dynamically change a variety of parameters according to the signal input. In fig. 2, baseband
signal is generated in the form of I & Q, and then up-converted to RF. After being amplified by a pre-amplifier,
the signal is split by a 1:2 ratio Wilkinson power divider into the main and auxiliary amplifier paths. DSP
generates another voltage signal Vgg2, which is applied to the gate bias of the auxiliary amplifier. This system is
used for both gate bias control of the auxiliary amplifier and baseband phase predistortion. These two DSP
applications will solve the problem of compressed gain and nonconstant phase of the Doherty amplifier,
respectively. By doing so, relatively flat gain and phase can be achieved, resulting in improved linearity.


ISSN- 2277-1956/V1N1-147-152
A Doherty Amplifier with Envelope Tracking Technique and DSP for High

   Fig. 3 shows the Doherty amplifier consists of two-part circuits; a fully matched microwave Doherty
amplifier and an adaptive gate bias control circuits for the peaking amplifier using an envelope tracking
technique. Linearity as well as efficiency can be improved by optimizing the shape of gate voltage of the
peaking amplifier. To determine the optimum shape of gate control voltage according to the input signal
envelope, we have constructed the envelope detector and shaping circuits which is shown in Fig. 4.

                    Fig. 3. Structure of a microwave Doherty amplifier employing envelope tracking technique.

                                     Fig.4. Optimized envelope detector and shaping circuits

                                           IV. EXPERIMENTAL RESULTS
   The performance of the Doherty amplifier with DSP has been compared with the Doherty amplifier with
envelope tracking technique.
  To demonstrate the effectiveness of the DSP control strategy to improve efficiency and linearity, the same Doherty
amplifier described previously was used in the system in Fig. 2. DSP-control also improves the PAE in the high power
region as seen in Fig. 5 (a). This is because the dynamic bias control compensates the gain, allowing higher output
power and thereby increasing PAE.
   The microwave Doherty amplifier has better linearity at high output power level and at low power level the
signal is degraded. This degradation at a low power level is not a problem because our target has been focused
on ACLR level for all output power level and the amplifier is optimized for it.

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                               Fig 5 (a) Gain Vs. Average power (output) of
                                 uncorrected and DSP corrected amplifier

                       Fig. 5. (b) Gain Vs. ACLR for a forward-link WCDMA signal.

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A Doherty Amplifier with Envelope Tracking Technique and DSP for High

                                 Fig 5. (c) ACPR improvement in CDMA spectrum at 23dBm
                                                      average power

                Fig. 5. (d) Power spectral densities at 32.7 dBm output power for a forward-link WCDMA signal.

  Finally, the test results for a forward-link WCDMA signal have been represented in Fig. 5 (b) and 5 (d). Fig.
5 (c) shows a spectrum of a CDMA signal at 23dBm average power with and without DSP-control. There is
more than l0dB improvement of ACPR (right shoulder) by utilizing the proposed DSP techniques. [6-7]

                                                   V. CONCLUSION

   DSP techniques were applied in the Doherty amplifier to improve both the linearity and efficiency. AM-AM
distortion was corrected by dynamically adjusting the bias of the auxiliary amplifier at the rate of the signal
envelope. AM-PM distortion was improved by adjusting the phase of the signal at the baseband frequency. In a
microwave Doherty amplifier, the gate voltage of the peaking amplifier is controlled according to the input
signal envelope. The most important advantage of the microwave Doherty amplifier relative to the other version

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of the Doherty amplifier is that the peaking amplifier generates full power at a high input power level and a full
load modulation can be achieved. Thus, a reduction of output power at a high power level can be prevented.
   For a forward-link WCDMA signal, the microwave Doherty amplifier has an improvement of 15.2% in
efficiency and 1.5 dB of output power relative to the DSP technique Doherty amplifier.


[1] Yu Zhao, Masaya Iwamoto, Lawrence E. Larson and Peter M. Asbeck, “Doherty Amplifier with DSP Control to Improve Performance
    in CDMA operation.”IEEE Trans. Vol. WE3A-1, pp. 687-690, 2003
[2] Y.Yang, J. Cha, B. Shin and B. Kim, “A microwave Doherty amplifier employing envelope tracking technique for high efficiency and
    linearity”,IEEE Trans. Microwave And Wireless Components Letters, vol.13, No. 9, pp. 370-372, September 2003.
[3] W.H. Doherty, "A New High Efficiency Power Amplifier for Modulated Waves," Proc. IRE vol. 24, no. 9, p1163-1I82, 1936.
[4] F.H. Raab, "Efficiency of Doherty RF Power-Amplifier Systems", IEEE Trans. on Broadcasting BC-33, p77-83, 1987.
[5] A. Andrea, et al, "RF power amplifier linearization through amplitude and phase predistortion," IEEE Trans. Commun, vol. 44, pp.
    1477-1484, Nov. 1996.
[6] Y.Yang, J. Yi,Y.Y.Woo, and B. Kim, “Optimum design for linearity and efficiency of microwave Doherty amplifier using a new load
    matching technique,” Microw. J., vol. 44, no. 12, pp. 20–36, Dec. 2001.
[7] B. Kim, Y. Yang, J. Yi, J. Nam, Y. Y. Woo, and J. Cha, “Efficiency enhancement of linear power amplifier using load modulation
    technique,” Int. Symp. Microwave Optical Technology Dig., pp. 505–508, June 2001.

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