Design and evaluation of a 5-W X-band PA using a low-cost plastic-packaged GaN transistor: Page 2 of 7

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.

Design of PA

The performance of the transistor, mounted on the chosen PCB material, was measured; drain bias was 32V with a quiescent bias current of just 25mA. The chosen PCB material was Rogers 4003, 0.008-inch, with 1oz (25g) final metallization, and copper-filled through-PCB vias were used for transistor grounding to provide improved thermal performance.

The measured maximum available gain of the transistor showed breakpoints occurring at 4.8 GHz and 12.5 GHz, indicating that the band of interest lies well within a region of unconditional stability and that a maximum gain of around 14 dB is available. The final amplifier gain will obviously be less than this, due both to real losses associated with adding the matching and bias networks and to the fact that the amplifier output will be power-matched rather than matched in a conjugate fashion.

Although the device is unconditionally stable across the whole of X-band, at frequencies above 12.5 GHz and below 4.8 GHz it is potentially unstable, so steps must be taken to ensure stability in these regions. There is also a significant amount of low frequency gain and it is good design practice to suppress this to avoid potential low frequency stability issues in the final amplifier.

Large-signal load-pull data for the mounted device demonstrated that at 9.4 GHz the device is capable of delivering around 37.5 dBm output power at 3 dB compression. The target load impedance to present at the output reference plane was found to be 12.98Ω - j9.39Ω, and the target source impedance to present at the input reference plane was found to be 11.64Ω - j55.75Ω. The reference planes are set at the edges of the SMT pads required for attachment of the transistor to the PCB.

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