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A High Temperature GaN-HEMT Based Power Amplifier for Downhole Communication
Keywords: High Temperature PA, GaN HEMT PA, High Temperature Electronics
To keep up with demands, the oil and gas industry continually explore deeper wells, where well logging equipment experience harsher pressures and temperatures. Electronics operating in this harsh environment must be more robust and reliable, while meeting higher data rate requirements. Because the pressure can be mitigated mechanically, the main challenge for electronics in the downhole environment is the temperature, which can reach an excess of 210℃. Current well logging operations, however, do not exceed 210℃ as the current equipment cannot operate beyond 150℃ and conventional means of thermal management, such as cooling fans or heat sinks, are impractical due to weight and space requirements. A critical component of any RF communication system is the power amplifier (PA) circuit. For use in the intended environment, the PA must operate at temperatures exceeding 210℃, exhibit the linearity and gain required by the RF system, and maintain a high efficiency to prevent excess heat generation. The use of a Gallium Nitride (GaN) high electron mobility transistor (HEMT) from Qorvo (T2G6000528-Q3) as the active device provides a junction temperature of 275℃, due to its wide band gap properties, allowing the PA to operate at very high temperatures without the use of a heat sink. This paper reports the design of a GaN-HEMT based PA intended for use in a downhole GaN based RF modem using frequency division multiple access (FDMA) and quadrature amplitude modulation (QAM) to achieve the desired data rate from several tools. The design makes use of microstrip filters and impedance matching to prevent large impedance variations with temperature. To address temperature concerns, the power dissipation of the transistor was reduced to minimize heat generation and self-heating effects; however, by reducing power dissipation, non-linearity of the system was increased, which is critical for FDMA and QAM systems. The design of the PA balances this trade off to reach the desired operating temperature and to maintain the linearity required by the modulation scheme. The designed PA is of class AB and is capable of operating reliably at ambient temperatures of 25℃ to 230℃ without the use of heat sinks or other conventional thermal management techniques. Additionally, the circuit exhibits a gain of ≥18dB for a frequency range of 230MHz-285MHz, while maintaining the necessary linearity and efficiency.
Brannon Kerrigan,
Virginia Polytechnic Institute and State University
Blacksburg, VA

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