Design and evaluation of a 5-W X-band PA using a low-cost plastic-packaged GaN transistor

June 06, 2016 // By Stuart Glynn, Tony Richards and Liam Devlin, 
Plextek RFI
This article describes the design of a single stage 5-W X-band GaN Power Amplifier using a low-cost SMT packaged transistor. The amplifier is optimized for the 9.3 to 9.5 GHz band: it has 11 dB small signal gain, and provides more than +37 dBm output power at 3 dB gain compression with a corresponding drain efficiency of greater than 55%. The design is based on a commercially available discrete 0.25µm GaN transistor, housed in an over-moulded SMT plastic package mounted on Rogers 4003 PCB. Fast drain switching circuitry is also included on the same PCB to facilitate pulsed operation with a turn-on time of just 20ns.

Gallium nitride (GaN) discrete transistors suitable for operation at microwave frequencies are now commercially available from a number of vendors. The higher breakdown voltage and maximum junction temperature of GaN transistors make them well suited to the realisation of high power amplifiers.

Most commercially available GaN transistors intended for X-band operation are offered as bare die or in ceramic packages. The use of low cost over-moulded SMT plastic packages for such discrete devices has the advantage of greatly simplifying handling and assembly as well as providing a significant reduction in product cost. There are obviously challenges in developing SMT packaged microwave power transistors: an assembly approach that avoids excessive RF performance degradation must be found; thermal performance must be adequate; the assembly must be very repeatable to provide consistent part to part performance; and accurate device models are required to account for the performance of the packaged part. Here it is demonstrated that excellent performance can be realized at X-band from such an SMT packaged part, in this case the TGF2977-SM from Qorvo, shown in Figure 1.

Figure 1: The Qorvo TGF2977-SM is a plastic-packaged 0.25µm GaN device measuring 3- x 3-mm.

The TGF2977-SM is a 0.25µm GaN device of 1.26mm gate width, housed in an over-moulded plastic package measuring just 3- x 3-mm. The package incorporates a solid copper base to ensure good thermal and electrical contact to the PCB ground.

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