Gallium Nitride (GaN) Power Amplifier for 2×2 LTE Test Bed
IIT Madras (Indian Institute of Technology, Madras) had finished the design of a baseband unit which was at the heart of a 2×2 LTE testbed. The baseband carried two 10 Mbps streams of 10 MHz on one side an Gigabyte Ethernet on the other side.
Radio Studio was to deliver the following:
- Hardware enclosure with power supply for the baseband
- Integration of a radio module for baseband to RF conversion. Design of interface circuitry from baseband to module
- Integration of a 2×2 MIMO TDMA Radio Front-end for 5W power delivery across 10 MHz band
- Supply of RF Front Ends (CPE/UE) and Base stations
IIT recommended using an RF module that delivered a maximum of +18dBm at its ports. The requirement for CPE was +24dBm. IIT also recommeded use of an existing low-power amplifier (which they had in stock) which delivered +30 dBm at 40 dB gain.
Shown below is the 2×2 MIMO board that could deliver upto +27 dBm, The design uses 4 high-power, absorbtive high-isolation switches from Mini-Circuits (HSWA2-30DR). High isolation was needed to ensure that there were no oscillations even as the LNA was kept ON all the time. A low-gain, high IP3, Low-NF E-pHEMT device was used was used for the LNA (PGA103+ from Minicircuits).
High Power Amplifier
Cree’s CGH27030 GaN HEMT power transistor was used for the power amplifier. With 30W peak power handling capability, this amplifier provided an optimum solution considering PAE, EVM for an average power delivery of 5W.
CGH27030 is a depletion mode device. Power sequencing is needed to ensure that device is not damaged at zero gate bias. In addition, the device’s package needed electrical bonding between the PCB and the heat sink. This was a problem which was ultimately solved by very careful and calibrated reflow process.