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High-Performance Wideband Power Amplifiers Use GaN Devices

Key Takeaways

  • The growth of semiconductor technology immensely influenced the telecommunication industry by introducing high efficiency, WB power amplifiers built on GaN devices.

  • Switching GaAs devices with GaN helps to achieve the given power level with improved amplifier stage gains and reduced transistor sizes. This helps reduce the number of amplifier stages in cascade WB power amplifiers, resulting in higher efficiency. 

  • The high WB power amplifiers are always accompanied by high temperatures. By using GaN devices in power amplifiers, we can achieve better electrical characteristics at higher temperatures. 

TWT based WB power amplifier

TWT tubes are used as active devices in WB power amplifiers

A successful wireless communication system requires wideband (WB) power amplifiers that cover frequency bands such as GSM, UMTS, WiFi, and Wi-MAX. These amplifiers drive the transmitter antenna with large signals strong enough to travel to the receiving end. 

The growth of semiconductor technology has greatly influenced the telecommunication industry by introducing high efficiency, WB power amplifiers built on Gallium Nitride (GaN) devices. This new GaN technology takes over the monopoly of WB power amplifiers—which were previously dominated by Travelling Wave Tubes (TWTs). 

WB power amplifiers possess many desirable attributes including large frequency bandwidth, high power-added efficiency, large-signal gain, high output power, and reliable operation under a broad range of frequency changes. They are designed to handle large amounts of power and heat, therefore, the transistors in the amplifiers should be selected so that they can withstand high power and temperature. There has been extensive research in this area, and the latest science proposes several new materials that offer higher power efficiency, smaller size, lighter weight, and lower cost. In this article, we will discuss the evolution of WB power amplifiers. 

GaN-Based WB Power Amplifiers

The WB power amplifier shifted from TWT to Ga-based devices, which improved its bandwidth, power, and efficiency capabilities greatly. TWT disadvantages such as low efficiency, high voltage requirements, and poor reliability are no longer an issue with the newer GaN and GaAs devices.  The shorter gate length of GaAs and GaN transistors enables high-performance WB power amplifiers that operate conveniently from millimeter wavelength range. The GaN-based ICs are even capable of constructing WB power amplifiers of 1 W rating, which can cover hundreds of bandwidth.  

GaN-based WB power amplifier ICs follow different topologies and designs to enhance their frequency spectrum, efficiency, and performance. Focusing on WB power amplifier types, there are mainly two configurations—distributed power amplifiers and reactively matched power amplifiers. The merits of distributed WB power amplifiers are high power and wideband capabilities. However, their drawbacks include limited gain and poor return loss. The reactively matched power amplifier offers octave bandwidth with improved gain. A considerable compromise is done in peak gain, peak output, and power efficiency to achieve an improved gain in reactively matched power amplifiers. The GaN-based devices employed in both distributed and reactively matched WB power amplifiers help to achieve SWaP advantages in design. 

Incorporating GaN technology to WB power amplifiers satisfies the  SWaP considerations with wider bandwidth and performance. Switching the Gallium Arsenide (GaAs) devices with GaN helps to achieve the given power level with improved amplifier stage gains and reduced transistor sizes. This helps reduce the number of amplifier stages in cascade WB power amplifiers, resulting in higher efficiency. 

The trend of GaN High Electron Mobility Transistors (HEMT) used in WB power amplifiers in electronic communication systems supports wider signal bandwidth and is effective in processing more signals compared to silicon-based devices. GaN HEMT WB power amplifiers are considered space savers, as these amplifiers offer all the advantages of WB power amplifiers within a compact footprint. They work well in rugged environments and at high temperatures, which make them best suited for RF applications. The long-term reliability of GaN on Silicon carbide (SiC) makes it a constant in WB power amplifier circuits present in electronic warfare and radar communication systems. 

The GaN on SiC devices can deliver high power and high impedances simultaneously. This capability is useful for WB design and there is not much fuss associated with the impedance matching requirements.  Even with broad frequency range applications, the GaN-based WB power amplifier delivers high output power, better drain efficiency across the decade, and low parasitic capacitance. 

Schematic of TRM

The high power amplifier block in the transmit chain is a WB power amplifier

Practical Applications of GaN-Based WB Power Amplifiers

GaN on SiC-based devices are commonly used in transmit/receive modules (TRM) found in radars. The TRM consists of a transmit chain, receive chain, and a connecting common arm. The GaN-based SiC power amplifiers are designed for WB transmit chain applications. In a WB transmit chain, the power amplifier design is challenging, as it requires appropriate device selection, load-pull analysis, and input-output impedance matching designs. Apart from these requirements, electronic warfare and radar applications demand thermal stability. Usually, GaN devices have a stability factor greater than unity which makes them unconditionally stable over WB frequency applications in a harsh environment. The high WB power amplifiers are always accompanied by high temperatures. By using GaN devices in power amplifiers, we can achieve better electrical characteristics at higher temperatures. 

The use of AlGaN materials is commonly seen in Ultra WB (UWB) Monolithic Microwave Integrated Circuits (MMICs), realizing WB power amplifiers for jammer applications. The high breakdown voltage and thermal conductivity of AlGaN provide higher power and efficiency, better thermal properties, and lower DC compared to GaAs counterparts. The GaN-based WB power amplifier is often used in high-frequency bands such as C-, X-, and Ku-bands. 

WB power amplifiers are also used in digital applications, often in direct digital RF transmitters. The WB digital power amplifier can be realized using GaN MMICs. The incorporation of digital circuit designs into GaN technology helps to build switch-mode WB power amplifiers. The GaN-based WB digital power amplifier has excellent interface capabilities, which makes it readily compatible with the CMOS modulators in transmitters. The GaN technology in digital WB power amplifiers introduces a new architecture without the need for mixers and additional filters. The digital transmission with GaN-based devices implements WB power amplifiers with small form factors and reduced component count. 

With all of the new developments in communication systems, the design of WB power amplifiers must be reliable, effective, and efficient. They are expected to showcase best-in-class performance in terms of output power, frequency bandwidth, reliability, thermal stability, electrical response, efficiency, and gain. GaN technology has revolutionized the WB power amplifier architecture and given it reduced occupied volume, more stability, power added efficiency, and interface capabilities. The shift to GaN-based WB power amplifiers in wireless communication is considered to be a great design decision—consider it for your next project.