Abstract Application Note RFG1M20180, 2110MHz to 2170MHz, 48V, 300Wpk Doherty Reference Design This application note is intended to provide a reference point for an amplifier circuit design using RFMD s RFG1M20180 in a Symmetrical Doherty configuration using two RFG1M20180 devices. The frequency of operation is optimized for 2110 to 2170MHz. The operating drain voltage is 48VDC. At an output power of +48.3dBm, the gain >13dB and the drain efficiency is >45%. The RFG1M20180 is optimized for commercial infrastructure applications in the 1800 to 2200 MHz frequency band, ideal for WCDMA and LTE applications. Using an advanced 48V high power density Gallium Nitride (GaN) semiconductor process optimized for high peak to average ratio applications, these high-performance amplifiers achieve high efficiency and flat gain over a broad frequency range in a single amplifier design. The RFG1M20180 is an input pre-matched GaN transistor packaged in an air cavity ceramic package which provides excellent thermal stability. Ease of integration is accomplished through the incorporation of simple optimized matching networks external to the package that provide wideband gain, efficiency, and linearizable performance in a single amplifier. Introduction The reference design circuit described in this document was designed to achieve a maximum back-off efficiency and linearity. A trade off of output power, gain, distortion and efficiency was made. All recommended components are standard values available from well-known manufacturers. Components specified in the bill of materials (BOM) have known parasitics, which may affect the circuit s performance. Deviating from the recommended BOM or design layout may result in a performance shift due to different parasitics, line impedances, and line lengths. Component placement, line impedances, and line lengths are critical to each circuit s performance. Circuit Details The circuit recommended for this application note was designed using the following PCB material: Material: Taconic, RF-35 Core thickness: 0.020 inch Copper cladding: 1.0oz with plating Dielectric constant: 3.5 at 1.9GHz Dissipation Factor: 0.0018 at 1.9GHz http://www.taconic-add.com/en--index.php http://www.taconic-add.com/pdf/rf35.pdf A 0.25 thick copper plate interface was used between device flange and aluminum heat sink. Aluminum heat sink is Cool Innovations dense pin configuration #3-505017R http://www.coolinnovations.com/ http://www.coolinnovations.com/includes/pdf/heatsinks/3-5050xxr.pdf RF MICRO DEVICES and RFMD are trademarks of RFMD, LLC. BLUETOOTH is a trademark owned by Bluetooth SIG, Inc., U.S.A. and licensed for use by RFMD. All other trade names, trademarks, and registered trademarks are the property of their respective owners. 2013, RF Micro Devices, Inc. Support, contact RFMD at (+1) 336-678-5570 or customerservice@rfmd.com 1 of 10
Design Background Information As systems move to complex modulations, amplifier designs are required to handle the peak power requirements in order to keep nonlinearity levels at a minimum. One traditional straight forward method to combat nonlinearity is to design an amplifier utilizing the largest power transistor available. As the power transistor size reaches a maximum, adding additional transistors in parallel (in a balanced configuration) will increase the peak power capability. The issue becomes averagee power (back-off) efficiency. Even though the balanced amplifier is designed and tuned for peak power and efficiency, the efficiency at the average power levels in complex waveforms is much less. The Doherty amplifier design improves the average power back-off efficiency without a major degradation in RF performance. The Doherty amplifier transistors are biased differently than their balanced amplifier counterparts. Typically, balanced amplifiers are biased equally, so they turn on and operate in unison. The Doherty amplifier will have one transistor that operates as the carrier (or main) amplifier and one or more transistors that operate as the peaking amplifier. For this referencee design, the carrier amplifier transistor is biased in class AB, so it can amplify the incoming signal under all levels of power. The peaking amplifier iss biased in class C, therefore, will only amplify signals when they reach a large enough level (thee peaks) to turn on the gate. Also, the design configuration of the carrier amplifier while the peaking amplifierr is turned off incorporates quarterwavee lines and matching structuress effectively shifting the load impedance to a state thatt increases the efficiency at lower power levels. In a Doherty amplifier, the efficiency and linearity trade-off can be shifted easily by changing the amount of bias voltage applied to the gate of the carrier and peaking amplifier. The following information will describe the fundamental differences of the traditional balanced amplifier configuration to the Doherty configuration. Additional informationn on Doherty amplifier design can be found at http://www.rfmd.com/cs/ /documents/commdruntonpasymposium11.pdf http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=5616135& &url=http%3a%2f%2fieeexplore.ieee.org%2fi el5%2f5606055%2f5614756%2f05616135.pdf%3farnumber% %3D5616135 http://www.home.agilent.com/upload/cmc_upload/all/7march2013webcast.pdf?&cc=us&lc= =eng TRADITIONAL AMPLIFIER: Good for High power operation Both amplifier A1 and A2 contribute equally to Pout Both have standard Efficiency vs. Pout characteristics SYMMETRICAL DOHERTY AMPLIFIER: Good for 3 to 7dB Back-off operation A1 and A2 operate independently when needed A1 operates most of the time - handles average signal A2 operates only when peak power is needed 2 of 10
Load Modulation dropped carrier power by 3dB This moves the efficiency enhancement point back Even more suitable for CDMA type signals ASYMMETRICAL DOHERTY AMPLIFIER: Good for 7 to 10dB Back-off operation Change the ratio of the carrier/peaking amp power Equal power creates maximum efficiency at -6dB Size the carrier amp smaller to effectively move the max efficiency point lower in average power, suitable for higher peak to average ratio signals 3 of 10
Typical Performance RFG1M20180, 2110MHz to 2170MHz, 48V, 300Wpk Symmetrical Doherty Reference Design 4 of 10
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Schematic RFG1M20180, 2110MHz to 2170MHz, 48V, 300Wpk Symmetrical Doherty Reference Design Bill of Materials RFG1M20180, 2110MHz to 2170MHz, 48V, 300Wpk Symmetrical Doherty Reference Design Component Value Manufacturer Part Number C1 0.1pF ATC ATC100A0R1BT C17 0.3pF ATC ATC100A0R3BT C16 0.4pF ATC ATC100A0R4BT C14, C15 0.5pF ATC ATC100A0R5BT C4, C5 1.2pF ATC ATC100A1R2BT C18, C19 8.2pF ATC ATC100B8R2CT C2, C3, C6, C7, C20, C21 10pF ATC ATC100B100JT C24, C25 0.1uF Murata GRM32NR72A104KA01L C10, C11, C22, C23 1.0uF Murata GRM32ER72A105KA01L C8, C9, C26, C27 4.7uF Murata GRM55ER72A475KA01L C12. C13 100uF Panasonic ECE-V1HA101UP C28, C29 330uF Panasonic EEU-FC2A331 R1 100 ohms Panasonic ERJ-1TYJ101U R2, R3, R4, R5 10 ohms Panasonic ERJ-8GEYJ100V RF MICRO DEVICES and RFMD are trademarks of RFMD, LLC. BLUETOOTH is a trademark owned by Bluetooth SIG, Inc., U.S.A. and licensed for use by RFMD. All other trade names, trademarks, and registered trademarks are the property of their respective owners. 2013, RF Micro Devices, Inc. 6 of 10 Support, contact RFMD at (+1) 336-678-5570 or customerservice@rfmd.com
Photo RFG1M20180, 2110MHz to 2170MHz, 48V, 300Wpk Symmetrical Doherty Reference Design 7 of 10
Parts Layout RFG1M20180, 2110MHz to 2170MHz, 48V, 300Wpk Symmetrical Doherty Reference Design RF MICRO DEVICES and RFMD are trademarks of RFMD, LLC. BLUETOOTH is a trademark owned by Bluetooth SIG, Inc., U.S.A. and licensed for use by RFMD. All other trade names, trademarks, and registered trademarks are the property of their respective owners. 2013, RF Micro Devices, Inc. 8 of 10 Support, contact RFMD at (+1) 336-678-5570 or customerservice@rfmd.com
Thermal Management As with most power amplifiers, the circuit must have adequate thermal management in order to operatee in an effective and reliable fashion. An external fan is recommended. Thermal compound is recommended between flange of the device and heat sink. Mounting Instructions STEPS FOR MOUNTING A FLANGED DEVICE 1. Heat-sink surface flatness control. a. Surface finish = average deviation of the mean value of thee surface height. b. Surface roughness (Ra) = 0.8um ( 0.03mils) 2. A clean interface surface on both the heat-sink and flange. 3. Device mounting holes need to be clean and flat (no burrs). 4. Apply a thin and even layer of thermal compound to the surface of the flange. 5. Place the device, flange-side-down n, into the recesss of the PCB. 6. Attach the device to the PBC/Heat-sink.178 O.D. X.123 ID X.01 Thick) with the specified the screw and washer assembly. and RND Shim-bearing. (SS #4-40 X ¾ captive SHCS 7. Use a Two Step torque sequence: a. 1st step torque the screw and washer on each side = 0.5kg.cm. b. 2nd step torque the screw and washer on each side = 6kg.cm (+- 1kg.cm). Caution: excessive torque may damage the flanged device. 8. Solder the device leads to the PCB. a. One industry standard is to use a Pb-free alloy (typically SAC305; 96.5%Sn, 3%Ag, 0.5%Cu) with a liquidus temperature of 221C. b. Temperature at the device lead interface should be <400C (750F) for <10 seconds. c. Pre-tin the leads to reduce any effects of gold embrittlement. 9 of 10
Biasing instructions for the RFG1M20180 Symmetrical Doherty 1. Connect RF cables at RFin and RFout 2. Connect ground to the ground supply terminal, and ensure that both the VG and VD grounds are also connected to this ground terminal 3. Apply -6.5V to Vg_peak 4. Apply -5.5V to Vg_carrier 5. Apply 48V to Vd 6. Increase Vg_carrierr until drain current reaches 600mA or desired biass point. 7. Turn on the RF input power 8. Re-adjust Vg_peak for desired linearity versus efficiency List Paragraphh 10 of 10