Design and evaluation of a 5-W X-band PA using a low-cost plastic-packaged GaN transistor: Page 6 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.

During the large signal measurements, the input power was swept from around 10 dB back-off, up to and slightly beyond, 3 dB gain compression. Over the power sweep, the dissipated power in the package increases so the package temperature increases. The gain under linear conditions reduced with temperature from 12.5 dB at low temperature (-33°C) down to 10.1 dB at high temperature ( +85°C), with a 25°C nominal room temperature value of around 11.1 dB. The results were taken at mid-band. 3 dB gain compression occurred at an input power of 28 dBm at low temperature, 29 dBm at nominal temperature and 29.5 dBm at high temperature. A Pout versus Pin plot indicated that the mid-band output power at 3 dB compression is nominally 37.1 dBm, varying by just ±0.2 dB over temperature.

Drain efficiency was also measured, showing that at 3 dB compression, the nominal drain efficiency is 57%, while at low temperature it is around 55% and at high temperature around 54%. Power added efficiency (PAE) at 3 dB compression was 48.5% at nominal temperature, around 48.5% at low temperature also and around 44% at high temperature.

The key performance metrics at 3 dB compression, at nominal temperature across the band of interest, are summarized in the plot in Figure 7.

Figure 7: Summary of key performance metrics at 3 dB compression and 25°C. Click image to enlarge.

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