RMO3D-1
`
`High Efficiency and Wideband Envelope Tracking Power Amplifier
`with Sweet Spot Tracking
`Dongsu Kim, Jinsung Choi, Daehyun Kang, and Bumman Kim
`Department of Electrical Engineering, Pohang University of Science and Technology,
`Pohang, Gyeongbuk, 790-784, Republic of Korea
`
`Abstract — This paper describes the implementation of a
`high efficiency and wideband envelope tracking power
`amplifier with sweet spot tracking. By modulating supply
`voltage of power amplifier (PA), efficiency can be increased
`significantly. And linearity is improved by envelope shaping
`and sweet spot tracking. The supply modulator has a
`combined structure of a switching amplifier and a linear
`amplifier to achieve high efficiency as well as wide
`bandwidth. The measurement results show efficiencies of
`36.4/34.1 % for 10/20 MHz long term evolution (LTE) signals
`with peak to average power ratio (PAPR) of 7.5/7.42 dB.
`Index Terms — Boost converter, envelope tracking, linear
`amplifier, long term evolution (LTE), power amplifier (PA),
`sweet spot, switching amplifier.
`
`3.4V to 5V, the output voltage of the supply modulator is
`increased up to 4.5V and the power amplifier shows
`higher gain, efficiency, output power and wider bandwidth.
`In [8], they analyze nonlinear distortion of envelope
`tracking PA. Because of knee voltage and nonlinear
`capacitance, AM-AM and AM-PM distortion are
`generated. By adopting envelope shaping and sweet spot
`tracking, linearity can be improved.
`In this paper, we implement a high efficiency and
`wideband envelope tracking PA for LTE applications
`using a hybrid switching supply modulator, HBT PA,
`envelope shaping, and sweet spot tracking.
`
`I. INTRODUCTION
`
`As wireless communication systems provide high data
`rate services, the channel bandwidth and PAPR of the
`signals are increased and the efficiency for the power
`amplifier is decreased. In the case of conventional PA
`with fixed supply voltage (Fig. 1a), the PA should be
`operated in the back off power region to linearly amplify
`the modulated signal with high PAPR and its efficiency is
`much lower than its peak value as shown in Fig. 2. On the
`other hand, the envelope tracking PA (Fig. 1b) operates
`under modulated supply voltage according to its output
`power level and its efficiency is degraded slightly.
`Because overall efficiency of the envelope tracking PA
`is proportional to efficiency of the supply modulator and
`its linearity is affected by linearity of the supply
`modulator, a realization of the supply modulator with a
`high efficiency and good linearity is very important. In [1],
`they have designed a low dropout (LDO) regulator as a
`supply modulator, but its efficiency is very low for high
`PAPR signals. In [2], a switching amplifier is used as a
`supply modulator. Although it achieves high efficiency, it
`requires high order passive filter and its bandwidth is too
`narrow to use for wide bandwidth signals such as LTE and
`WiMAX. To achieve high efficiency and wide bandwidth,
`we use hybrid switching supply modulator combining the
`advantages of two supply modulators [3]-[7]. To improve
`the performance of power amplifier, a boost converter is
`added to the supply modulator as shown in Fig. 3. By
`boosting the supply voltage of the linear amplifier from
`
`(a) Conventional PA with fixed supply voltage. (b)
`Fig. 1.
`Envelope tracking PA with modulated supply voltage.
`
`PA’s efficiency curves with fixed supply voltage and
`Fig. 2.
`modulated supply voltage.
`
`978-1-4244-6241-4/978-1-4244-6242-1/
`978-1-4244-6243-8/10/$26.00 © 2010 IEEE
`
`255
`
`2010 IEEE Radio Frequency Integrated Circuits Symposium
`
`INTEL 1113
`
`

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`switching frequency, and duty ratio. For the protection,
`high efficiency, and low switching noise of the switches,
`anti-shoot-through circuit and divided switches with
`current control technique are employed (Fig. 5) [9]. Gate
`driver for the divided switches, which is shown in Fig. 6,
`turns on / off the 4 switches with a little delay. It can be
`designed easily using 4 MUXs and inverter chains.
`
`
`
`Fig. 4 Wideband linear amplifier.
`
`
`
`Fig. 5 High efficiency switching amplifier.
`
`
`
`
`
`
`
`Fig. 6. Gate driver for divide switches with current control
`technique.
`
`
`
`
`Fig. 3. Block diagram of
`modulator with boost converter.
`
`
`
`the hybrid switching supply
`
`II. DESIGN OF HIGH EFFICIENCY AND WIDEBAND
`SUPPLY MODULATOR
`
`The proposed supply modulator consists of a wideband
`linear amplifier, a high efficiency low speed switching
`amplifier, and a boost converter. Usually the switching
`amplifier supplies low frequency component of the
`envelope signal with high efficiency and the linear
`amplifier supplies other high frequency component with
`high speed. Because most of the power of the envelope
`signal is located at a low frequency, this structure is
`suitable for an operation with high efficiency and
`wideband.
`In Fig. 3, the wideband linear amplifier operates as a
`voltage-controlled voltage source (VCVS). It means the
`output voltage of the linear amplifier is the same with its
`input voltage up to tens of MHz due to its high gain, wide
`bandwidth, and negative feedback. As shown in Fig. 4, we
`use folded-cascode OTA as a gain stage to achieve a large
`bandwidth and high DC gain. For large current driving
`capability and rail-to-rail operation, the output buffer has a
`common source configuration and it is biased as class-AB
`for linearity and efficiency.
`The high efficiency, low speed switching amplifier
`operates as a dependent current source. It senses the
`direction of the linear amplifier’s current and controls the
`switching amplifier using a hysteretic comparator.
`Generally, the average switching frequency is dependent
`on the hysteresis width, inductor value, and some other
`parameters for a narrow-band signal. For a wideband
`signal,
`the average switching frequency
`is mainly
`determined by its bandwidth. The sizes of the power
`switches are determined by considering the conduction
`loss and switching loss at the specific load resistance,
`
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`III. ENVELOPE SHAPING AND SWEET SPOT TRACKING
`
`The modulated PA operates differently according to the
`supply voltage level. Especially at a low supply voltage, a
`power amplifier shows severe nonlinear characteristics
`such as AM-AM and AM-PM distortions because of knee
`voltage effect and nonlinear capacitance. To compensate
`these effects, an envelope shaping method should be used
`[7].
`In addition to this basic method, a sweet spot tracking is
`proposed in this work. Fig. 7 is third-order and fifth-order
`intermodulation distortions (IMD) of PA in two-tone
`analysis. In this figure, there are sweet spots which are
`local minimums of IMD and are occurred by cancellation
`of the harmonics. As supply voltage decreases, the sweet
`spot also moves to lower power. By adjusting the supply
`voltage to minimize the distortions at each power level,
`the linearity of the envelope tracking PA can be improved
`significantly.
`
`
`from 3 MHz to 6 MHz according to the bandwidth of an
`input signal.
`To implement the envelope tracking PA, 2.535GHz
`class-AB PA, which is fabricated using InGaP/GaAs 2um
`HBT process, is used. It has about 30 dBm peak output
`power at 3 V supply voltage. By boosting the supply
`voltage of the linear amplifier, PA’s supply voltage
`increases up to 4.5 V and peak output power of the PA
`also increases to 33.4 dBm. Performance of the envelope
`tracking PA is measured using 10/20 MHz LTE signals
`with 7.5/7.42 dB of PAPR.
`Fig. 9 shows the measured efficiency and gain of the
`envelope tracking PA. For the 10 MHz LTE signal, the
`envelope tracking PA has efficiency of 36.4 % at output
`power of 27.2 dBm. For the 20 MHz LTE signal, its
`efficiency is 34.1 % at output power of 26.1 dBm.
`Estimated efficiencies of the supply modulator are about
`75/71 % for 10/20 MHz LTE signals. These values can be
`calculated from PA’s efficiency curve at each supply
`voltage. Fig. 10 is measured output spectra of the
`envelope tracking PA at the peak output powers without
`any linearization technique.
`
`
`Fabricated chip photograph of the supply modulator.
`
`
`
`
`Fig. 8.
`
`
`
`Fig. 9. Measured efficiency and gain of the envelope tracking
`PA.
`
`
`
`
`Simulated third-order and fifth-order intermodulation
`Fig. 7.
`distortions of PA in two-tone analysis.
`
`IV. MEASUREMENT RESULTS
`
`The designed supply modulator is fabricated using
`65nm CMOS process and it uses thick oxide I/O devices
`for a high voltage operation. Chip photograph is shown in
`Fig. 8 and its size is 2.6 mm (cid:153) 1.7 mm. The supply voltage
`for the supply modulator is 3.4 V (the battery voltage) and
`the boost converter generates 5 V for supply of the linear
`amplifier. In this configuration, output voltage range of
`the supply modulator is 0.5 to 4.5 V regardless of the
`battery voltage
`fluctuation,
`replacing
`the DC-DC
`converter [10]. The linear amplifier shows over 100 MHz
`bandwidth and over 55 dB DC gain. The average
`switching frequency of the switching amplifier is varied
`
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`

`

`ACKNOWLEDGEMENT
`
`This work was supported by WCU (World Class
`University) program through the Korea Science and
`Engineering Foundation funded by
`the Ministry of
`Education, Science and Technology (Project No. R31-
`2008-000-10100-0), and by the MKE (The Ministry of
`Knowledge Economy), Korea, under
`the
`ITRC
`(Information Technology Research Center) support
`program supervised by the NIPA (National IT Industry
`Promotion Agency) (NIPA-2009-C1090-0902-0037).
`
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`
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`
`
`
`
`Fig. 10. Measured output spectra of the envelope tracking PA
`at peak output power.
`
`
`TABLE I
`PERFORMANCE SUMMARY OF ENVELOPE TRACKING
`POWER AMPLIFIER FOR LTE APPLICATIONS
`
`
`
`Signal bandwidth
`
`PAPR
`
`Supply voltage
`
`10 MHz
`
`7.5 dB
`
`20 MHz
`
`7.42 dB
`
`3.4 V
`
`Peak output power
`
`27.2 dBm
`
`26.1 dBm
`
`Peak efficiency
`
`36.4 %
`
`34.1 %
`
`Estimated efficiency of
`supply modulator
`@ peak output power
`
`75 %
`
`71 %
`
`V. CONCLUSIONS
`
`A high efficiency and wideband envelope tracking PA
`with sweet spot tracking technique is proposed and
`implemented for LTE applications. For
`the supply
`modulation, a hybrid switching supply modulator with
`boost converter is fabricated using 65nm CMOS process.
`An envelope shaping with sweet spot tracking is adopted
`to compensate AM-AM and AM-PM distortions.
`Efficiencies of the implemented envelope tracking PA are
`36.4/34.1 % at output power of 27.2/26.1 dBm for 10/20
`MHz LTE signals, respectively. Measured results show
`the proposed supply modulator with sweet spot tracking is
`a suitable structure to achieve a high efficiency and
`wideband envelope tracking PA.
`
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