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

June 06, 2016 // By Stuart Glynn, Tony Richards and Liam Devlin, 
Plextek RFI
Design and evaluation of a 5-W X-band PA using a low-cost plastic-packaged GaN transistor
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.

Measured Performance

Small signal s-parameter measurements were carried out on the fully assembled amplifier at package base temperatures of -33°C, 25°C and 85°C. These measurements were carried out under CW conditions, and the results are shown in Figure 5. The s-parameter data demonstrates that the final amplifier has a small signal gain of around 11 dB across the band, which varies by around ±1.5 dB over temperature. The input return loss is hardly affected by temperature and is better than 15 dB across the band. The output return loss is nominally around 9.2 dB and varies by around ±1 dB over temperature.

Figure 5: Amplifier s-parameter measurements. Click image to enlarge.

Large signal measurements were also carried out over temperature. These measurements were performed under pulsed conditions and utilized the on-board drain switching circuit, which provided a turn-on time of just 20ns. The duty cycle was 10% and the pulse width was 500µs. The RF envelope during the 500µs pulse is shown in Figure 6 for the amplifier running at 3 dB compression at mid-band. Clean, fast edges were evident, and there is very little power drop across the pulse period.

Figure 6: Pulsed power output envelope at 9.4 GHz and 3 dB compression. Click image to enlarge.

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