Plextek RFI has published a whitepaper on the design of a 25W X-band Gallium Nitride (GaN) PA MMIC (Monolithic Microwave Integrated Circuit). This case study describes the design and performance of a 25W X-band GaN PA MMIC designed on WIN Semiconductor's 0.25μm GaN on SiC process.
The design has a small signal gain of over 25dB from 10-12GHz with a gentle positive gain slope. The saturated output power is over 25W from 10-11.5GHz and PAE is over 33%.
GaN MMIC processes suitable for operation at microwave frequencies are now commercially available from a number of vendors worldwide. The high breakdown voltage of GaN transistors and their ability to operate reliably at high junction temperatures make them well suited to the realisation of high power amplifiers. The design presented here uses the 0.25μm GaN on SiC process from WIN Semiconductor.
As the total gate periphery of a transistor increases (unit gate width and/or number of gate fingers) the associated parasitics increase and the high frequency performance degrades. Microwave PA MMICs are therefore best realised by combining multiple transistors of an appropriate geometry. The first stage in the design process is to select the transistor size and bias.
An 8x120μm device was chosen, biased at +28V drain-source voltage and a drain-source current of 100mA per mm of gate width. The Gmax plot (and stability factor, K) are shown above. It can be seen that over the entire X-band this particular device is unconditionally stable, which is an attractive feature. The transistor has a maximum available gain of more than 17dB up to 12GHz. Series or shunt resistors can be added to the device input to further improve or broaden in-band stability. As the full PA design develops, these resistors can be reduced (or even removed) as practical implementation losses are introduced.
Download and read the full whitepaper, below.