Junghwan Son

Push the Envelope: Design Concepts for Envelope-Tracking Power Amplifiers

As mobile communication systems evolve to handle higher data rates, their modulation schemes only become more complicated, generating signals with large bandwidth and high peak-to-averagepower ratio (PAPR). To amplify such signals with high efficiency, the power amplifier (PA) should have high efficiency not only at the peak power level but also at low power, especially over the maximum power generation region. To realize these PA characteristics, the drain bias voltage of the transistor can be modulated on the basis of the input envelope power to minimize the dc supply power. Thus, the drain bias voltage should follow the envelope of the modulated signal, and this is called envelope tracking (ET). Usually, the envelope is shaped to realize the optimum performance from the ET PA. The PA is biased close to class B, and the dc current is automatically adjusted to the power level. The resulting PA has high efficiency for all power levels, comparable to the maximum efficiency of the PA at high power. In practice, the efficiency is degraded somewhat at low voltage because of the knee effect, lower transconductance, and mismatch effect for different drain bias voltages.

A Multimode/Multiband Envelope Tracking Transmitter With Broadband Saturated Amplifier

A multimode/multiband envelope tracking (ET) transmitter consisting of a hybrid switching amplifier (HSA) and a broadband saturated power amplifier (PA) is developed across 1.3 to 2.7 GHz. For the various standard signals with different bandwidth and peak-to-average power ratio, the HAS efficiently provides a supply signal to the PA by changing the reference value of the hysteresis comparator. The saturated amplifier employs the nonlinear output capacitor to shape the voltage waveform, resulting in the half-sinusoidal or rectangular waveform. Since the nonlinear capacitor generates harmonic component, the voltage shaping is mainly carried out by the capacitor and slightly supported by the harmonic loading circuit. Thus, with the harmonic load higher than the output capacitor, the saturated amplifier can operate with high efficiency. This characteristic enables the saturated PA to operate with broadband characteristic and high efficiency because the design is mainly focused on the fundamental matching problem. The broadband saturated PA is implemented based on load/source-pull methodology. Broadband matching networks for the high efficiency are synthesized by simplified real frequency technique. For the bandwidth from 1.3 to 2.7 GHz (70% fractional bandwidth), the measured output power, drain efficiency, and power-added efficiency (PAE) performances are between 39.8–42.0 dBm, 55.8–69.7%, and 51.2–65.3%, respectively. The ET transmitter is demonstrated at 1.8425-GHz long-term evolution (LTE), 2.14-GHz wideband code division multiple access (WCDMA), and 2.6-GHz mobile worldwide interoperability for microwave access (m-WiMAX) applications. It delivers the PAE of 32.16, 37.24, and 28.75% for LTE, WCDMA, and m-WiMAX applications, which are improved by 3.1, 4.2, and 1.7%, respectively.

A highly efficient asymmetric Doherty Power Amplifier with a new output combining circuit

An asymmetric Doherty Power Amplifier (ADPA) is introduced using a new output combining circuit for easy of implementation with a large matching tolerance. The proposed APDA has been implemented using GaN HEMT devices at 2.6 GHz for WiMAX signal with 5MHz bandwidth and 8.3 dB peak to average power ratio. This ADPA delivers a saturated output power of 51.7 dBm and a drain efficiency of 60.4% at an average output power of 43.6 dBm. After linearization using digital feedback predistortion technique, the ADPA satisfies the linearity specification with −51.98 dBc of adjacent channel leakage ratio at 10MHz offset. To the best of our knowledge, the drain efficiency of 60.4% is the highest efficiency at 2.6GHz frequency for a WiMAX signal with 8.3dB PAPR.

Doherty amplifier with envelope tracking for high efficiency

A Doherty amplifier assisted by a supply modulator is presented using 2.14 GHz GaN HEMT saturated power amplifier (PA). A novel envelope shaping method is applied for high power-added efficiency (PAE) over a broad output power range. Experimental comparison with the Doherty and saturated PAs with the supply modulator is carried out. For the 8 dB crest factor WCDMA 1FA signal, the Doherty PA supported by the modulator presents the improved PAE over the broad output power region compared to the standalone Doherty PA. In addition, it achieves better PAE than the saturated PA with the supply modulator due to the lower crest factor envelope signal applied to the Doherty PA. At the maximum average output power, back-off by 8 dB from the peak power, the Doherty amplifier employing bias adaptation shows the PAE of 50.9%, while the comparable saturated PA with supply modulator and standalone saturated Doherty amplifier and saturated PA provide the PAEs of 42.3%, 49.7%, and 35.0%, respectively.

A Saturated Doherty Power Amplifier Based On Saturated Amplifier

A saturated Doherty power amplifier (PA) based on the saturated PA tuned by the self-generated harmonic currents is presented. When driven with mobile WiMAX 1FA signal, this Doherty PA demonstrates high efficiency performance (average 57%) over the WiMAX band from 2.5 GHz to 2.7 GHz. Simulated and experimental results allow the evaluation for the load modulation behavior of the saturated PA and bandwidth correlation between the Doherty and saturated unit PAs. The linearity requirement of the WiMAX signal is met using the digital feedback predistortion linearization technique.

Switching Behavior of Class-E Power Amplifier and Its Operation Above Maximum Frequency

The switching behavior of Class-E power amplifiers (PAs) is described. Although the zero voltage switching can be performed properly, the Cout charging process at the switch-off transition cannot be abrupt and the waveform deviates from the ideal shape, degrading the efficiency. For the operation above maximum frequency, the charging process should be even faster but it cannot follow. Moreover, the discharging process is not sufficiently fast and further degrades the efficiency. The discharging process is assisted by the bifurcated current at saturation. The performance of the Class-E PA above the maximum frequency is enhanced by the nonlinear Cout, which helps to shape the voltage waveform. The bifurcated current itself cannot generate enough of a second-harmonic voltage component to shape the required voltage waveform. The performance of the Class-E PA can be further improved by a second-harmonic tuning and a conjugate matched output load, leading to the saturated PA. Compared with the Class-E PA, the saturated amplifier delivers higher output power and efficiency. A highly efficient saturated amplifier is designed using a Cree GaN HEMT CGH40010 device at 3.5 GHz. It provides a drain efficiency of 75.8% at a saturated power of 40.2 dBm (10.5 W).

A multimode/multiband envelope tracking transmitter with broadband saturated power amplifier

A multimode/multiband envelope tracking (ET) transmitter consisting of a hybrid switching amplifier (HSA) and a broadband saturated power amplifier (PA) is developed across 1.3 to 2.7 GHz. For the various standard signals with different bandwidth and peak-to-average power ratio, the HSA efficiently provides a supply signal to the PA by changing the reference value of the hysteresis comparator. The broadband saturated PA, taking advantage of a nonlinear output capacitor to shape the voltage waveform, is implemented based on load/source-pull methodology. Broadband matching networks for the high efficiency are synthesized by simplified real frequency technique. For the bandwidth from 1.3 to 2.7 GHz (70% fractional bandwidth), the measured output power, drain efficiency, and power-added efficiency (PAE) performances are between 39.8–42.0 dBm, 55.8–69.7%, and 51.2–65.3%, respectively. The ET transmitter is demonstrated at 1.8425-GHz long-term evolution (LTE), 2.14-GHz wideband code division multiple access (WCDMA), and 2.6-GHz mobile world wide interoperability for microwave access (m-WiMAX) applications. It delivers the PAE of 32.16, 37.24, and 28.75% for LTE, WCDMA, and m-WiMAX applications, which are improved by 3.1, 4.2, and 1.7%, respectively.

Sequential Digital Predistortion for Two-stage Envelope Tracking Power Amplifier

To improve the efficiency of the main power amplifier, the envelope tracking technique is widely used. However, due to the reduced gain of the main amplifier by the envelope tracking, the driving power of the drive amplifier requires to be larger and the amplifier should be optimized accordingly for an efficiency operation. In this letter, a two stage envelope tracking power amplifier is studied for the purpose. The envelope tracking technique is applied to both the drive and main power amplifiers to improve efficiency of the drive amplifier, which is usually operated at a low efficiency. The experimental results show that the efficiency of the total system is improved by 2.1% while the efficiency of the drive amplifier is increased by 8%. In addition, to overcome a serious non-linearity due to the dual envelope tracking operation, a new sequential digital predistortion architecture is proposed and using this architecture, the linearity specification of the long term evolution signal has been successfully satisfied.

Broadband Saturated Power Amplifier With Harmonic Control Circuits

This letter presents a broadband saturated power amplifier (PA) using the harmonic control circuits for base station application. The saturated PA has advantages for broadband operation with high efficiency due to the large tolerance of the second harmonic tuning. However, it is difficult to simultaneously achieve the fundamental and second harmonic impedance matching across a wide bandwidth. To solve the problem, the harmonic control circuits are placed at the input and output of the devices die. These harmonic control circuits play a role of leading the second harmonic impedances to the optimum regions but are specially designed to be insensitive to the following fundamental matching circuit. The saturated PA with the harmonic control circuit is designed using a 120 W GaN device, achieving a high efficiency and wide bandwidth characteristics simultaneously. The measured output power, drain efficiency, and gain are at least 51.0 dBm, 71.0%, and 8.22 dB at the saturation across the 1.75 to 2.17 GHz (21% relative bandwidth) under pulse test (10% duty). This saturated PA also delivers good performances for long term evolution (LTE) and wideband code division multiple access (WCDMA) modulated signals at 1.85 and 2.14 GHz, respectively. These results show that the broadband saturated PA with the harmonic control circuit is suitable to wide bandwidth multimode/multiband applications.

Effect of input second harmonic control for saturated amplifier

Effect of the input second harmonic control on saturated amplifier is investigated. With only nonlinear input capacitor, the proper phase relationship between fundamental and second harmonic currents for the half-sinusoidal voltage waveform can be achieved at the expense of the enlarged conduction angle. However, with both input and output nonlinear capacitors, the output capacitor, which generates a lot of second harmonic with small higher harmonics, can be employed for the half-sinusoidal voltage shaping. Thus, the input second harmonic can be terminated to reduce the conduction angle for high efficiency. The effect of the input second harmonic control is validated using simplified and real device models. Based on the strategy for the harmonic control, a high-efficiency saturated amplifier is implemented at 3.5 GHz, delivering a power-added efficiency of 78.5% at a saturated output power of 38.2 dBm.