Date of Award

Spring 1-1-2018

Document Type

Thesis

Degree Name

Master of Science (MS)

First Advisor

Taylor W. Barton

Second Advisor

Dimitra Psychogiou

Third Advisor

Gregor Lasser

Abstract

In RF and Microwave communication and RADAR systems it is critical that the transmitter generates sufficient RF output power. In order to achieve that output power amplifiers capable of reaching that output power with linearity such that the signal can be received without significant distortion. In addition to the output power and linearity constraints this amplifier must have high power efficiency to enable their use in power-constrained systems and to prevent thermal issues that can be present in the device. These conflicting requirements form the basis for power amplifier design. The design of RF and microwave power amplifiers (PAs) at RF and microwave frequencies is well understood, but effectively supplying dc bias points to the transistor at the heart of a PA while achieving high output power, linearity and efficiency is poorly documented.

In this thesis the practical effects associated with PA bias line design will be investigated with the CGH27015F transistor, a Gallium Nitride (GaN) based device. This theses will focus on three conflicting design parameters present in bias line design: the RF loss induced in the bias, nonlinear effects due to the impedance presented at low frequencies, and self-modulation effects.

To understand these parameters four PAs were designed that were biased with either quater-wave or shorted stub transmission lines of 50 Ω or 30 Ω characteristic impedance. These lines were tested under two-tone excitation of 5 MHz, 10 MHz, and 20 MHz tone spacing. The presence of a IMD3 "sweet spots" was linked to the nonlinear behavior of the baseband impedance in all four amplifiers. Between the amplifiers the 50 Ω shorted stub design exhibited the lowest IMD3 tone powers of the four designs with the 30 Ω quarter wave line showing the next lowest IMD3 tone powers. The 50 Ω lines exhibited worse self-modulation of the current waveform over the same range of input powers and tone spacings compared to the 30 Ω lines.

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