Haedong Jang

Model-Based Nonlinear Embedding for Power-Amplifier Design

A fully model-based nonlinear embedding device model including low- and high-frequency dispersion effects is implemented for the Angelov device model and successfully demonstrated for load modulation power-amplifier (PA) applications. Using this nonlinear embedding device model, any desired PA mode of operation at the current source plane can be projected to the external reference planes to synthesize the required multi-harmonic source and load terminations. A 2-D identification of the intrinsic PA operation modes is performed first at the current source reference planes. For intrinsic modes defined without lossy parasitics, most of the required source impedance terminations will exhibit a substantial negative resistance after projection to the external reference planes. These terminations can then be implemented by active harmonic injection at the input. It is verified experimentally for a 15-W GaN HEMT class-AB mode that, using the second harmonic injection synthesized by the embedding device model at the input, yields an improved drain efficiency of up to 5% in agreement with the simulation. A figure-of-merit is also introduced to evaluate the efficacy of the nonlinear embedding PA design methodology in achieving the targeted intrinsic mode operation given the model accuracy.

Asymmetric Doherty Power Amplifier Designed Using Model-Based Nonlinear Embedding

A novel procedure is introduced for designing Doherty amplifiers using the model-based nonlinear-embedding technique. First, the Doherty intrinsic load-matching network is designed at the transistor current-source reference plane with the main and auxiliary devices interconnected. Identical devices with different biasing are used for realizing an asymmetric Doherty implementation with 9-dB back-off. The required multiharmonic impedances at the package planes are then obtained using the embedding device model for both devices, and the complex load impedance at the fundamental is projected back to resistive loads using an offset line. An even-number multisection impedance transformer and a reduced drain voltage of the main amplifier are used to design the asymmetric Doherty load network while providing the necessary loads to the main and auxiliary devices. The optimization of the drain efficiency and gain curves of the asymmetric Doherty operation for the proposed design is further investigated by adjusting the auxiliary gate-bias. An efficiency above 50% over an 11-dB power range is experimentally observed with 41.8-dBm peak output power using continuous wave (CW) at 2 GHz. Using a dual-input implementation of the designed Doherty power amplifier (PA), a systematic dual-input CW characterization of the Doherty operation is performed to establish the relative auxiliary-to-main phase offsets and power offsets yielding a maximum efficiency under constant gain. From this dual-input characterization, it is found that the optimal gate bias for single-input Doherty operation is the one for which the constant-gain maximum efficiency is achieved for a quasi-constant auxiliary-to-main input power ratio corresponding to the one implemented in the input divider in the single-input Doherty PA.

Simulation and measurement-based X-parameter models for power amplifiers with envelope tracking

Static X-parameter (XP) models for RF power amplifiers (PAs), derived from both simulations and nonlinear vector network analyzer (NVNA) measurements, are investigated for the prediction of PA performance under dynamic signal conditions such as in envelope tracking (ET). The instantaneous AM-AM, AM-PM and PAE predictions of XP models extracted from simulation are compared under ET dynamic signal conditions to two types of circuit models using envelope simulation. An XP PA model is extracted for a peak 8W GaN class-F-1 ET PA from NVNA measurements with automated bias control. By applying a constant gain shaping table derived from the XP model to the drain supply voltage, the average PAE is improved from 40% to 57% for 3.84 MHz WCDMA signals at 2.14 GHz compared to fixed drain bias operation.

Adjustable Load-Modulation Asymmetric Doherty Amplifier Design Using Nonlinear Embedding

We developed a new asymmetric Doherty load modulation matching network using identical transistors for the main and auxiliary amplifiers. Asymmetric Doherty power amplifiers (PA) require a larger size transistor for the auxiliary PA than for the main PA to provide the higher power and wider load modulation range. Additional impedance transformers are introduced to alleviate this requirement when using identical devices. The drain bias voltage of the main amplifier is also reduced to achieve a wider back-off. Furthermore, a large-signal model-based nonlinear embedding method is applied to predict the input and output harmonic terminations, removing the need for the multi-harmonic source/load pull characterization. An asymmetric Doherty amplifier was built using two 15 W peak power packaged GaN transistors of the same size. 71 % drain efficiency at the peak power of 41.8 dBm and 62.7 % at the second peak of 32.8 dBm (9 dB back-off) were observed. Above 50% drain efficiency was maintained over an 11 dB power range. 51.86 % average drain efficiency was observed after linearization maintaining -51.46 dBc adjacent channel power ratio excited by 10 MHz bandwidth long term evolution signals with 9.96 dB peak to average power ratio.

New thermometry and trap relaxation characterization techniques for AlGaN/GaN HEMTs using pulsed-RF excitations

This paper presents new microwave characterization techniques to (1) estimate the internal device operating temperature and (2) extract the trap relaxation time-constants in AlGaN/GaN HEMTs. The operating temperature of the device under CW large-signal RF operation is obtained using isothermal pulsed-IV/RF measurements with increasing substrate temperatures. The trap capture and emission times are measured by monitoring the time relaxation of the transient bias drain current in pulsed-IV when a train of large-signal pulsed-RF excitations is applied and removed respectively. Illumination is further verified to accelerate the emission process confirming the trapping activity and identifying trap energy levels. In addition, it is verified that higher RF load impedances for the same biasing, strongly increase the trapping as the instantaneous drain voltage reaches higher peak values.

Pulsed load-pull based optimal load-modulation PA design methodology for average efficiency enhancement

A new load-modulation based power amplifier design scheme is proposed for average power added efficiency (PAE) enhancement. An algorithm for choosing the optimal operating conditions from load-pull data with swept input power is presented. The optimal conditions for the joint control of the inputs power and the tunable output matching network for dynamic load-modulation are extracted and demonstrated using a commercial 15W GaN HEMT transistor. Instead of the PAE peaking only at high output power, greater than 60% PAE over 10 dB output power range is predicted in simulation for class-AB operation. Pulsed-RF real time active load-pull measurements with 1.9μs pulse width and 1.2% duty rate are then performed for swept input power to approximate high peak-to-average power ratio operation. Additional pulsed-RF active load-pull measurements are further used at the selected optimal loads for verification. Greater than 60% PAE over 7 dB are observed using both measurement techniques.

Development of Multiharmonic Verification Artifact for the LSNA and NVNA (MTT-11)

In this article, a unique approach is presented to design and fabricate a circuit, rich in nonlinear contents with reduced sensitivity to bias, temperature, and load variations so that it can be used to validate the performance of LSNAs or NVNAs. This circuit was judged to be a joint winner in the 2012 IEEE MTT-S International Microwave Symposium (IMS2012) Student Design Competition, Design of a Calibration Verification Artifact for Nonlinear VNA, sponsored by MTT-11 Microwave Measurements.

Pulsed RF calibration for NVNA measurements

This paper presents a new calibration theory for pulsed-RF large-signal measurement which relies on a new pulsed HPR and new pulsed power calibration besides the conventional relative linear calibration. The methodology used is based on the multiharmonic characterization of the RF pulser under pulsed-operation. The pulsed-RF calibration technique is facilitated by the fast switching and settling of the pulser which must have reached quasi-steady state when the RF signals are sampled. Experimental results for the new pulsed-RF calibration are presented for an LSNA. A small calibration correction is found to be required due to the dispersion of the buffer stage between the LSNA samplers and ADCs. The technique which is demonstrated for an LSNA operating using the multiple recording technique could be used with other NVNAs.

3-D Fourier Series Based Digital Predistortion Technique for Concurrent Dual-Band Envelope Tracking With Reduced Envelope Bandwidth

In this paper, the design, implementation, and measurement results of a new digital predistortion (DPD) method for a concurrent dual-band envelope tracking (ET) power amplifier (PA) system is presented. The PA gain is represented using a set of 3-D orthogonal Fourier series basis functions, which accounts for the distortions introduced by the two signal bands, as well as the supply modulation voltage. Here, the new Fourier series basis is shown to substantially outperform the traditional memory polynomial approach in terms of its linearization capability for subsequent data for which it was not trained for (prediction) due to its well-defined numerical rank and stability. The new DPD system was implemented using a 10-W peak gallium-nitride (GaN) ET PA operating with a dual-band input based on long-term evolution and WCDMA signals center frequency (1.89 and 2.2 GHz) spaced by 310 MHz. The linearization results are compared to the 3-D memory polynomial in two steps: extraction and prediction. In the DPD coefficient extraction phase, tested under the same signal, the traditional memory polynomial and the Fouries series reach close results in both normalized mean square error (NMSE) and adjacent channel power ratio (ACPR). In the prediction phase, however, the proposed method provides a significant performance improvement. A performance improvement as high as 17 dB in terms of NMSE and 4.6 dB in terms of ACPR in comparison to the conventional dual-band 3-D memory polynomial method implementing ET. Furthermore, the well-defined numerical rank of the Fourier series approach allows for a substantial reduction in coefficients using principal component analysis without detrimenting performance.

New supply modulation optimization methodology for concurrent dual band envelope tracking power amplifier

In this paper, the design, implementation, and measurement results of a concurrent dual-band envelope tracking (ET) system is presented. A GaN power amplifier (PA) is designed for concurrent dual-band operation. The iso-gain shaping functions for envelope tracking are then extracted from the characterization of the PA in each band. To investigate the envelope tracking performance under concurrent dual-band operation and establish the optimal dual-band shaping function, a new supply modulation scheme is proposed. An experimental test bench is also presented. The performances of the new schemes are evaluated with a dual-band signal based on a LTE signal with PAR of 10dB centered at 1892.8MHz and a WCDMA signal with PAR of 5dB centered at 2200MHz. Experimental results show 57 % average drain efficiency for peak output power of 36.6 dBm and 36.5 dBm.