Andrew Zai

X-band MMIC GaN power amplifiers designed for high-efficiency supply-modulated transmitters

The design and measured performance of X-band power amplifier MMICs that utilize 0.15μm GaN on SiC process technology are presented. Under continuous wave operating conditions these single and 2-stage MMICs demonstrate peak power added efficiencies (PAE) from 45% to 69%, output powers from 2.5-13W, and up to 20dB of large signal gain. Designed for drain modulated applications, the power amplifiers maintain good performance at reduced drain bias voltage. The output power of the two stage MMIC can be varied from 2W to 13W when the drain bias is varied between 7.5V and 20V while maintaining a PAE above 54%.

High-efficiency X-band MMIC GaN power amplifiers with supply modulation

This paper presents measurement results on supply-modulated X-band 0.15μm gate width GaN HEMT MMIC power amplifiers for OFDM signals. Two PAs at 10GHz with 4 and 10W output powers show peak CW efficiencies of 69% and 55%, with gains of 8.5 and 20.4 dB respectively. Supply modulation trajectories are designed by static characterization of each MMIC PA, and the drain modulation is performed though both a linear broadband modulator and a high-efficiency 5-MHz switching modulator. The PAs are tested in four modes: (1) with a constant 20-V supply and the 18-MHz OFDM signal input through the drive; (2) pure envelope tracking; (3) signal-split supply modulation; and (4) envelope elimination and restoration (EER) with a 5-MHz switching modulator. Average PAE and composite power added efficiencies are compared, reaching 65.4% and 35%, respectively, with the 4-W PA and a linear modulator.

Resonant Pulse-Shaping Power Supply for Radar Transmitters

The final radiofrequency power amplifier (PA) of a radar transmitter module is a large factor in system efficiency. Typical radar transmitter signals are frequency-modulated with constant-amplitude pulse envelopes in order to optimize efficiency, resulting in spectral broadening and power radiated outside of the radar frequency band. This paper demonstrates a PA with a dynamic power supply which enables high efficiency while reducing the spectral emissions. The resonant pulse-shaping power supply generates a raised-cosine pulse envelope waveform with efficiency greater than 90% and peak envelope power around 6 W. Measured results with a 2.14-GHz GaN power amplifier with an efficiency of 76% at peak power demonstrate over 67% transmitter efficiency.

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.

A 4.2-W 10-GHz GaN MMIC Doherty Power Amplifier

This paper describes the design and performance of an X-band GaN monolithic microwave integrated circuit (MMIC) Doherty power amplifier (DPA) in a 0.15 μm gate length GaN on SiC process. The measured output power is greater than 36dBm at peak PAE of 47% at 10 GHz. Gain flatness of ±0.1 dB around 9.2 dB is obtained at up to 25dBm input power. The PAE at 6 and 10 dB backoff is 41% and 31% respectively. For a 10-Mbps OQPSK signal, the ACPR at 10MHz is >30 dBc at maximum output power and >33 dBc at 10 dB backoff. This combination of linearity and efficiency at backoff represents state of the art for an X-band DPA. Limitations of supply modulation in DPAs to further extend output power back-off are highlighted in the discussion.

RFPA supply modulator using wide-bandwidth linear amplifier with a GaN HEMT output stage

This paper presents an envelope tracking supply modulator for a high efficiency radiofrequency power amplifier (RFPA). A wide bandwidth linear amplifier using a GaN high electron mobility transistor (HEMT) as a source follower output stage is described. A low bandwidth, high efficiency current controlled Buck converter is used to assist the linear amplifier and improve the system efficiency. Steps towards integration of the supply modulator and the RFPA are given through inclusion of the modulator output stage with an RFPA on a microwave monolithic integrated circuit (MMIC). Simulation and experimental results are presented for a 150 MHz bandwidth, 9 V to 27 V output voltage modulator supplying a 10 GHz RFPA.

Supply-Modulated Radar Transmitters With Amplitude-Modulated Pulses

This paper introduces an efficient radar transmitter with improved spectral confinement, enabled by a pulse waveform that contains both amplitude and frequency modulation. The theoretical behavior of the Class-B power amplifier (PA) under a Gaussian envelope is compared to that of a Class-A PA. Experimental validation is performed on a 4-W 10-GHz GaN monolithic microwave integrated circuit PA, biased in Class B with a power-added efficiency (PAE) of 50%. When driven with a Gaussian-like pulse envelope with a 5-MHz linear frequency modulation (LFM), the PA demonstrates a 31% average efficiency over the pulse duration. To improve the efficiency, a simple resonant supply modulator with a peak efficiency of 92% is used for the pulse Gaussian amplitude modulation, supplemented by pre-distortion on the PA input, while the LFM is provided through the PA input. This case results in a five-point improvement in system efficiency with an average PAE=40% over the pulse duration for the PA alone, and with simultaneous 40-dB reduction in spectral emissions relative to a rectangular pulse with the same energy.

Correction of frequency uncertainty in wide field of view interferometric angular velocity measurements

This paper presents a method for correcting the frequency uncertainty encountered in interferometric angular velocity measurements. This new technique of angular velocity measurement allows a wide field of view, where the frequency of the response is proportional to the angular velocity of the object. At wide angles, the measured frequency shift scales with the cosine of the angle, resulting in an uncertainty if the angular location of the object is not known. The correction method presented herein consists of estimating the angle of a signal using a wide-baseline interferometer with three closely spaced frequencies. With frequencies spaced over a fractional bandwidth of 3.5%, the residual uncertainty of the measured frequency shift is shown through simulation to be less than 0.1 Hz over a ±72° field of view with a 6 dB SNR.

Efficient power amplifiers for amplitude-tapered pulses with improved spectral confinement

This paper reviews various techniques that allow efficient amplification of amplitude-modulated radar pulses through some type of supply modulation. Examples at S and X-band with GaN PAs will be shown with efficiencies above 50% for 10-W transmitters and greatly reduced spectral sidelobes.