A Study on Performance Improvement of RF Power Ampliﬁer for Envelope-Tracking and Millimeter-Wave Application
- A Study on Performance Improvement of RF Power Ampliﬁer for Envelope-Tracking and Millimeter-Wave Application
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- The data traffic through the wireless system is growing very fast. The transmitter in mobile devices should guarantee a high quality of the transmission data and high efficiency for a long battery lifetime. Because a power amplifier (PA) takes the major portion of dc power consumption in a handset device, a highly efficient and linear PA is indispensable for a good quality service with seamless connection. A modern mobile communication system (2G/3G/4G) needs a spectrally efficient modulation technique due to the limited spectrum resources. Therefore, the wireless data of each standard are modulated using more complex methods, such as the quadrature phase shift keying (QPSK), multi-level quadrature amplitude modulation (M-QAM) and orthogonal frequency division multiplexing (OFDM). The modulated signals produced by these modulation schemes feature a non-constant envelope characteristic having a wide channel bandwidth and high peak-to-average power ratio (PAPR). Because the signals with the complex modulations have high PAPRs, PAs should operate at the back-off power region to send the signal without distortion. The efficiency of the back-off power region is inherently low. An envelope tracking (ET) technique is a solution for the efficiency improvement in the back-off power region. Beyond the 4G standard, the 5G system will deliver a peak data rate greater than 10 Gbps, cell edge data rate of 100 Mbps and 1 msec end-to-end latency. To realize the data rate, the channel bandwidth should be larger than 1 GHz. The millimeter-wave (mm-wave) band, above 10 GHz, is the realizable solution for the 5G system. Therefore, the evaluation of the mm-wave band PA is needed. This dissertation focuses on the performance improvement of the ET PA and design of the mm-wave band PA. Four major works are explored in this dissertation.
First, a design method of the current bias circuit for the hetrojunction bipolar transistor (HBT) is introduced for high efficiency. The bipolar transistor is very sensitive to temperature variation. An active elements are used in the bias circuit to compensate the variation. This bias circuit generates a 2nd order distortion, inducing an unintended dc voltage at the base node of the PA. By reducing the distortion of the bias circuit with a new design method, a highly efficient PA is developed.
Second, the intermodulation distortion of a HBT ET PA is investigated. The distortion characteristics are simulated using the inter-connection model of the PA and supply modulator. An amplitude modulation to amplitude modulation (AM-AM) characteristic of the HBT ET PA is very linear due to the sweet spot envelope shaping method. It is found that the main distortion source in the HBT ET PA is phase distortion. To reduce the distortion, the phase compensation network is proposed. The amplitude modulation to phase modulation (AM-PM) is linearized and the state-of-the-art performance PA is achieved.
Third, several approaches to improve the performance of the ET PAs are introduced. The efficiency and distortion behaviors of the ET PA are investigated according to the idle current. The optimization process of the idle current considering the overall efficiency and intermodulation distortions are discussed. A highly efficient saturated PA based on a HBT device is designed to maximize the efficiency of an ET PA, also. The output voltage waveform of the saturated PA is shaped as a half-sinusoidal voltage waveform peaked by a second harmonic. The knee voltage effect is minimized in this voltage waveform due to the large fundamental voltage for a given supply voltage, which is very important for an ET PA with a low average supply voltage. Furthermore, a distortion correction circuit for the CMOS ET PA is introduced to maximize the linearity. For the CMOS ET PA, the gain distortion under ET operation is poor than that of the HBT ET PA. The supply dependent AM-AM and AM-PM distortions are linearized by the integrated correction circuit.
Finally, a design method for the mm-wave PA is introduced. For the 5G application, Ka- and V-band PAs are fabricated with a nano-scaled CMOS process. Several techniques to enhance the low gain at the mm-wave band are discussed. An envelope impedance in the multi-input multi-output (MIMO) transmitter module is investigated for the linear operation. The dc feeding inductance and decoupling capacitance at the MIMO module generate the large envelop impedance, generating a memory effect at the drain node of the PA. The memory effect caused by the baseband impedance modulate the drain voltage, generating a large distortion. An ultra-fast low-drop out (LDO) structure is developed to reduce the memory effect by reducing the impedance and the designed mm-wave PA deliver a linear performance across a large video bandwidth.
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