Kwang Chun Lee

Modeling and Design Methodology of High-Efficiency Class-F and Class-

In this paper, efficiency-limiting physical constraint effects imposed on the knee voltage, along with a variation of the optimum load resistance, are investigated for highly efficient Class-F and Class- amplifiers. First, for an accurate analysis and comparison, new current waveform models are identified, and a realistic approach incorporated using a nonzero knee voltage and voltage-dependent nonlinear capacitance is employed to derive the voltage waveforms of the amplifiers. An analysis is performed to show the efficiency, output power, power gain, and output power compression points for both modes. Using this knowledge, along with a complete performance comparison, we provide a direction for optimizing the amplifier design. The analytic results are further verified based on the measured results of 3.54-GHz Class-F and Class- amplifiers using a commercial 60-W peak-to-envelope power gallium-nitride device. The experimental results show that Class-F and Class- amplifiers operate at drain efficiencies of 69.9% and 69.4% at saturated output powers of 47.4 and 47.2 dBm, respectively. These remarkably similar performances have excellent agreement with the predicted analysis at our operational frequency.

{\hbox{F}}^{{\hbox{-}1}}

This paper introduces the Band Exploration and Channel Sounding System (BECS) as a prototype of 2x8 MIMO wireless channel sounder, which is under developing in ETRI to explore wideband channel and frequency characteristics at 2.6 GHz. Engineering parameters for the BECS implementation are presented and the sounder system response is obtained and analyzed using the hardware channel simulator, Elektrobit PROPsim ® . The impact of imperfect system response on rms delay spread is analyzed using the 3GPP Spatial Channel Model (SCM). Based on Monte-Carlo simulations generating multiple ensembles of the wireless channel, it is found that rms delay spreads measured by the sounder do not depend on the sounder impairment such as the bell-shaped system response induced from RRC filter, and the equalized matched filter technique can improve the delay resolution against the imperfect system response

Power Amplifiers

This paper presents a novel CMOS Doherty power amplifier (PA) with an impedance inverter using a variable balun transformer (VBT) and adaptive bias control of an auxiliary amplifier. Unlike a conventional quarter-wavelength (λ/4) transmission line impedance inverter of a Doherty PA, the proposed VBT impedance inverter can achieve load modulation without any phase delay circuit. As a result, a λ/4 phase compensation circuit at the input path of the auxiliary amplifier can be removed, and the total size of the Doherty PA can be reduced. Additionally, an enhancement of the power efficiency at backed-off power levels can successfully be achieved with an adaptive gate bias in a common gate stage of the auxiliary amplifier. The PA, fabricated with 0.13-μm CMOS technology, achieved a 1-dB compression point (P1 dB) of 31.9 dBm and a power-added efficiency (PAE) at P1 dB of 51%. When the PA is tested with 802.11g WLAN orthogonal frequency division multiplexing (OFDM) signal of 54 Mb/s, a 25-dB error vector magnitude (EVM) compliant output power of 22.8 dBm and a PAE of 30.1% are obtained, respectively.

MIMO channel sounder implementation and effects of sounder impairment on statistics of multipath delay spread

In this paper, a high-efficiency envelope-tracking (ET) transmitter incorporating a novel efficiency-boosting function is proposed and implemented. An inverse Class-F power amplifier is utilized and optimized using the proposed output loading condition, which enhances its efficiency at the high probability region. This matching network can be conveniently implemented by controlling the nonlinear capacitance of the power transistor. For an accurate analysis, the output waveforms are modeled in terms of the nonlinear capacitance, and the efficiency and output power are subsequently analyzed and successfully optimized. For a high-efficiency envelope amplifier (EA), we propose a new EA utilizing a 2-bit switching stage in place of a 1-bit switching stage. This proposed architecture effectively reduces the ripple current, improving the efficiency of the EA. To verify our analysis, we have fully implemented a 3.54-GHz high-efficiency ET transmitter including a digital processing block. The experimental results show that the ET transmitter using a commercial 60-W peak-envelope-power GaN device operates at a drain efficiency of 44% and power-added efficiency of 39.6% with a gain of 10.1 dB at an average output power of 40 dBm for a 10-MHz third-generation long-term evolution signal with 8.5-dB peak-to-average power ratio. Using a digital pre-distortion function, the adjacent channel leakage ratio is less than -47.5 dBc.

CMOS Doherty Amplifier With Variable Balun Transformer and Adaptive Bias Control for Wireless LAN Application

In this paper, we have proposed the new loop circulator structure with high isolation for time division duplexing (TDD) mobile radio system such as the high speed portable internet service (WiBro) in Korea. It is the important factor to isolate between the transmit signal and the receive signal for TDD system, because the white noise generated by the transmitter affect the noise figure of receiver system during the receiving period. Therefore, new technique to increase the isolation of a circulator is presented as adding the two additional circulators to the antenna port and the receiver input port, respectively. Because a proposed structure has twice isolation value than that of the single circulator, it has advantage for a high power TDD system for which a RF switch can not be used. about 20dB, the thermal noise amplified by the high power RF amplifier can be injected to the receiver during the receiving period. Besides, high RF power signal is leaked by a circulator with low isolation during the transmitting period and the receiver can be damaged. Accordingly, an additional device for protecting the receiver is required. As above mentioned, the isolation of the time division duplexing radio system is very important to reduce the noise figure of receiver. In this paper, new technique to increase the isolation of a circulator is presented as adding the two additional circulators to the antenna port and the receiver input port. The strength of transmit signal leaked to a receiver is isolated by the isolation of two circulators, and the transmit signal reduced by the transmission loss of two circulators is radiated through the antenna. Therefore, the function of new loop circulator structure is identical to that of the conventional single circulator, but its isolation is twice than that of a single circulator.

High-Efficiency Envelope-Tracking Transmitter With Optimized Class-

A 1.5-bit 20-MHz bandwidth continuous-time delta-sigma modulator (CT-DSM), which is suitable for envelope pulse-width modulation (EPWM) transmitters, is presented. To compensate the process-voltage-temperature variation, a resistor calibration method is proposed. The proposed calibration scheme improves a signal-to-quantization-noise-ratio (SQNR) by 18%. The switched capacitor digital-to-analog converter (DAC) and tri-level output DAC are adopted to improve non-ideal effects and implement multi-level encoding, respectively. Over 44 dB SQNR has been achieved for all corner simulation condition with an oversampling ratio of 13.056 and 20-MHz of bandwidth. The power consumption of the modulator is 11.6 mW from the 1.08 V supply. 1.74% of error vector magnitude can be obtained for a 20 MHz LTE signal with a 9.7 dB peak-to-average power ratio.

{\hbox{F}}^{-1}

This paper presents a 1.5 bit (3-level) envelope amplifier to improve the overall efficiency of wideband high linearity envelope tracking power amplifier. The proposed envelope amplifier has been devised with two switches providing the different supply voltages. Since the unnecessary current (ripple current) induced by a switching stage can be reduced by the multi-level quantization, the efficiency of a proposed envelope amplifier is 5% higher compared with that of a conventional architecture. With the proposed envelope amplifier, we have designed and implemented the 3.54GHz high efficiency envelope tracking power amplifier using a commercial 60W PEP GaN device. The efficiency boosting effect is further verified by the measured results. The experiment results show that the envelope tracking power amplifier operates at drain efficiency (DE) of 43.6% and power added efficiency (PAE) of 39.2% and a gain of 10.1dB at average output power of 40 dBm for a 10MHz WiMAX signal with 8.5 dB peak-to-average power ratio (PAPR). By using digital predistortion function, the adjacent channel leakage ratio (ACLR) is less than −45.4dBc at 20MHz offset.

Amplifier and 2-bit Envelope Amplifier for 3G LTE Base Station

An advanced Class-S transmitter based on a tri-level delta-sigma modulator (DSM) is presented. The second order DSM shows a signal-to-noise and distortion ratio (SNDR) of 44.7 dB and a spurious-free dynamic range (SFDR) of 63.6 dB with a 3.1-MHz sinusoidal input at 522.24-MS/s using an offset cancellation scheme. The proposed technique improves the SFDR and SNDR over 10 dB and 3 dB, respectively. The proposed scheme can effectively control the input referred offset voltage of the DSM, resulting in the improvement of spectrum efficiency. The power consumption of the modulator is 13.4 mW from a 1.2 V supply. Measurements with a mixer show that the proposed modulator modulates a 10-MHz long-term evolution signal which has an 8.5 dB peak-to-average ratio with 3.30 % error vector magnitude and 38.7 dB adjacent channel leakage ratio at 2.6 GHz.

New circulator structure with high isolation for time division duplexing radio systems

A LC-VCO topology without a varactor device has been proposed. The oscillator includes a variable capacitance block for a frequency tuning. The block consists of two transistors and two capacitors. The LC-VCO has been fabricated in a 130-nm CMOS technology. The measured results show operation frequencies of 3.73GHz to 3.94GHz and a phase noise of 119.1 dBc/Hz at 1-MHz offset frequency. Power consumption is 5.9mA from a 1.2V supply.

20-MHz bandwidth continuous-time delta-sigma modulator for EPWM transmitter

MIMO-OFDM system is one of the promising schemes for achieving high data rate in communication systems. In this paper, channel estimation techniques based on loosely synchronous (LS) codes are proposed for MIMO-OFDM systems. Since LS codes have perfect auto-correlation and cross-correlation functions within certain vicinity of the zero shifts, multiple antenna and multipath signal interference can be reduced substantially. Through simulations, it is shown that the proposed methods outperform both of least square and linear minimum mean square error (LMMSE) estimators.