Design and evaluation of a 5-W X-band PA using a low-cost plastic-packaged GaN transistor: Page 4 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.

Input and output matching networks were implemented as distributed 2-section low pass structures. Moving backwards from the input reference plane, the input matching network is essentially: shunt C, series L, shunt C, series L. In a similar way, moving forwards from the output reference plane, the output matching network is essentially series L, shunt C, series L, shunt C. It is worth noting that an unbonded pin was conveniently used as an open-circuit stub to form part of the first input shunt C. This in Figure 3, which shows the layout of the final RF metal work.

Figure 3: RF metal work layout.

A photograph of the final assembled amplifier is shown in Figure 4. The 0.008-inch thick Rogers 4003 PCB is mounted onto an aluminium alloy T-carrier, which in turn is mounted onto a heat-sink. A fast drain switching circuit is provided on the same PCB. This area is coated in green solder resist which is omitted from the RF section.

Figure 4: Final amplifier assembly.

An end plate mounts onto the rear face of the T-carrier through which bias and control is applied. The blue wire is for the gate bias, the red wire for the drain bias, the yellow wire enables the drain switching and the black wire is 0V. The RF input and output have been designed to mate with edge-mount South West Microwave connectors. Note the hole in the front face of the T-carrier – this is to allow a thermocouple to be placed underneath the packaged device. Thermal analysis determined that the thermal resistance between the thermocouple and the package (through the PCB) was 8°C/W, which allows the package temperature to be conveniently calculated for a given power dissipation.

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