Manoja D. Weiss

Linearity of X-band class-F power amplifiers in high-efficiency transmitters

Modern communication signals have time-varying envelopes with significant peak-to-average ratios, resulting in low average efficiency when amplified by commonly used linear power amplifiers (PAs). For linear amplification with increased average efficiency, the Kahn envelope-elimination-and-restoration method uses a highly efficient saturated PA. In this paper, an 8.4 GHz class-F PA with 55% maximum instantaneous efficiency at 610 mW output power, is experimentally characterized in several different biasing modes. Operated in linear mode with constant drain bias, this PA has 10% average efficiency. The suppression of two-tone intermodulation products is 27 dBc when operated at about 0.7 times the peak output power. For the same PA operated in a modified Kahn mode with drive and bias control, a comparable linearity (27.7 dBc) can be obtained at peak output power. Furthermore, the average efficiency increased to 44%, a factor of 4.4 over the linear fixed bias mode.

A 10 GHz high-efficiency active antenna

This work discusses the use of a microstrip-fed slot antenna to directly provide the necessary output match and harmonic tuning for a 10 GHz class-E power amplifier. There is no matching circuit at the output of the amplifier since the slot is designed to provide the correct impedance at the fundamental frequency and to present an open circuit at the second harmonic. This eliminates losses in the matching circuit and decreases circuit area. Since the class-E amplifier requires a complex output load, the designed slot antenna is not a resonant structure. The device used is an Alpha AFM04P2 MESFET, which has a maximum output power of about 21 dBm. The measured performance of the active antenna shows 74% drain efficiency, 62% power-added-efficiency (PAE), and 20 dBm output power at 10 GHz, at 5 dB gain compression. The PAE is greater than 50% in a 400 MHz bandwidth.

Efficiency of chip-level versus external power combining [microwave power amplifiers]

In this paper, we compare two X-band high-efficiency switched-mode amplifiers designed around two commercially available packaged MESFETs, one having a four times larger gate periphery than the other. The amplifiers using the larger and smaller devices are designed to operate in classes E and F, respectively. The smaller device gives 685 mW output power with 7.4 dB gain and 64% overall efficiency. The larger device gives 1.70 W output power with 5.3 dB gain and 57% overall efficiency. This gives an internal (or chip-level) power-combining efficiency for the larger device of 89% in terms of overall efficiency. This is compared to the combining efficiency of circuit and spatial power combining using high-efficiency amplifiers, with the goal of assessing which architecture is the most efficient in terms of total dissipated power (DC and RF).

Time-domain optical sampling of switched-mode microwave amplifiers and multipliers

Time-domain measurements of the output waveforms of two 8-GHz high-efficiency power amplifiers, a 1-GHz frequency doubler, and a 5-GHz frequency doubler are presented in this paper. A new photoconductive probe has enabled nonintrusive time-domain voltage measurements, which confirm switched-mode class-E and class-F amplifier operation. In order to analyze nonlinear amplifiers designed to deliver a sinusoidal wave to the load, voltages at characteristic points inside the circuit need to be known. In multipliers, waveform measurements track harmonic leakage, expediting the design cycle. The high-impedance probe used here is an optoelectronic sampler, which can sense the charge on an exposed interconnect or the field associated with a buried interconnect. These electric-field data are then converted into voltage.

An X/K-band Class-E High-Efficiency Frequency Doubler

A class-E frequency doubler is presented at 20.8 GHz, the highest frequency reported to date. The circuit has 42.7% drain efficiency and 31.6% overall efficiency for an output power of 7.1dBm and 0.83 dB conversion gain. A maximum gain of 7.18 dB is achievable with 3.4dBm output power and 13.5% overall efficiency. The trade-off between efficiency and conversion gain is optimized by proper biasing and driving to obtain a 5.23 dB gain with an overall efficiency of 23.5%. The rejection of unwanted harmonics is greater than 26 dBm. The MESFET doubler is designed using a simple switching mode theory based on small signal device parameters.

A 10-GHz high-efficiency lens amplifier array

The design of a 36-element quasi-optical amplifier for high-power and high-efficiency transmission at 10 GHz is presented. The following steps are taken in the design procedure: (1) design and characterization of a single high-efficiency CPW amplifier; (2) comparison to a saturated class-A amplifier; (3) design of a driver stage and testing of a two-stage CPW amplifier; (4) design of antennas and passive unit cell; (5) testing of active unit cell; (6) testing of passive lens array; and (7) construction and testing of active lens amplifier array. This systematic procedure enables us to ensure stability, calibrate properly, and measure power-combining efficiency as a function of the number of elements. A high-efficiency amplifier with 275 mW of output power, 48% power-added efficiency, and 6.4 dB saturated gain at 10.1 GHz gives 200 mW output power, 35% power-added efficiency, and 13 dB of saturated power gain in a two-stage amplifier. A single unit cell with second-resonant slot antennas gives the same performance and is the building block for a 36-element focal-point fed array.

Time-domain optical sampling of nonlinear microwave amplifiers

Time domain measurements of the output waveforms of two 8-GHz high-efficiency power amplifiers are presented. A new photoconductive probe has enabled nonintrusive absolute voltage measurements which confirm switched-mode class-E and F operation. In order to analyze nonlinear amplifiers designed to deliver a sinusoidal wave to the load, voltages at characteristic points inside the circuit need to be known. The high-impedance probe used here is an optoelectronic sampler which can sense the charge on an exposed interconnect or the field associated with a buried interconnect. This field data is then converted into voltage.

Overview of applications of optical measurements in microwave circuit and antenna array design

An overview of both photoconductive sampling and optoelectronic mapping measurements (developed at the University of Michigan by the Whitaker and Katehi group) and their benefit in expediting microwave circuit and array design is presented. In particular, photoconductively sampled time-domain waveforms of switched-mode nonlinear power amplifiers and X-band and high-efficiency multipliers at C-band will be discussed. The high-impedance optical sampling probe enables measurements of waveforms within a microwave circuit. This in turn enables us to validate designs of nontraditional nonlinear circuits, as well as to diagnose problems associated with improperly terminated harmonics, leading to new improved circuit designs. We also present near-field measurements using an electro-optic crystal probe. As an example, a 30 GHz (Ka-band) active amplifier antenna array power combiner is examined in detail. The effects of mutual coupling between array elements, and bias line influence on RF mutual coupling between elements are presented. An extensive list of references that contain more details of the overviewed material is also given.