Power Amplifier

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Summary

Two devices were under review for the power amplifier for this project. The project specification calls for a 5W, 50O output over the frequency range of 144-148MHz. One method was to design a power amplifier using discrete RF MOSFETs. The second method was to use a power amplifier module manufactured by Mitsubishi, which integrates input/output matching. I was able to find an application note by STMicroelectronics for the PD54008 and PD84001 devices, covering the frequency range of 135-175MHz.[1] The application note included full schematics, board layout, and test data for the complete amplifier. From the provided schematics, I designed a PCB for the ST amplifier, and did a cost comparison against the Mitsubishi amplifier module.

STMicroelectronics Amplifier

Figure 1: Schematic for the ST Power Amplifier Test Board.
Figure 2: PCB Layout for the ST Power Amplifier Test Board.

The PD54008 is an N-channel RF MOSFET capable of outputting 8W with 11.5dB gain at 500MHz.[2] The PD84001 is an N-channel RF MOSFET capable of outputting 1W with 15dB of gain at 870MHz.[3] The test board provides all necessary matching, DC biasing, and output filtering. The schematic for this test board is shown in Figure 1, and the board is shown in Figure 2.

Bill of Materials

Part Quantity Unit Cost Total Cost Source
PD54008 1 $7.50 $7.50 Digi-Key
PD84001 1 $3.19 $3.19 Digi-Key
SMA Male Connector 2/5 $2.51 $1.00 Dealextreme
0603 Capacitor 23 $0.0096 $0.22 eBay - mib_instruments
0603 Resistor 6 $0.00294 $0.02 eBay - mib_instruments
0603HC-18NX 1 $0.79 $0.79 Coilcraft
1008CS-xxxX 3 $0.79 $2.37 Coilcraft
1812SMS-22N 2 $0.95 $1.90 Coilcraft
A05T 1 $0.91 $0.91 Coilcraft
Male Header 5 $0.01 $0.05 eBay
PCB 2.075"x0.875"/3 $5.00 $3.03 Laen's PCB
Total Unit Cost $28.38

Mitsubishi Amplifier

Figure 3: Schematic for the Mitsubishi Power Amplifier Test Board.
Figure 4: PCB Layout for the Mitsubishi Power Amplifier Test Board.

The RA07M1317M MOSFET amplifier module is a two-stage amplifier capable of outputting 6.5W with approximately 35dB gain, internally matched to 50O over the frequency range of 135-175MHz.[4] The test board provides two SMA connectors and bypassing capacitors for the gate and drain voltage inputs to the module. The schematic for this test board is shown in Figure 3, and the board is shown in Figure 4.

The schematic, board, and Gerber files can be downloaded here: http://rev0proto.com/files/ra07m1317m_v10.zip

Testing

The amplifier was tested using an HP 8922M GSM Test Set, using the Aux. RF Out supplying 0-9dBm at 144-148MHz to the RF IN port of the amplifier, and reading the RF OUT power through the RF In/Out port on the Test Set. VDD was supplied at 6-8.4V through an HP 6284A power supply. VGG was supplied at 0-3.5V using a power supply and potentiometer. Gate voltage was monitored using an HP 3466A multimeter, and current was read using the built-in meter of the 6284A.

Using the data for constant 5W output power at varying drain voltage in conjunction with the measured output powers of the ADF7012 or TH7122 will allow constant RF output power as the battery voltage decreases, as measured by the microcontroller. Varying the gate voltage provides a means of changing output power, at the expense of decreased efficiency at lower powers. At a gate voltage of 0V, the device only draws 7uA at 8.4V, effectively shutting off the device and allowing for a constant connection to the battery.

Bill of Materials

Part Quantity Unit Cost Total Cost Source
RA07M1317M 1 $19.95 $19.95 RFParts
SMA Male Connector 2/5 $2.51 $1.00 Dealextreme
0603 4.7uF Capacitor 2 $0.0096 $0.02 eBay - mib_instruments
3216 22uF Tantalum Capacitor 2 $0.64 $1.28 Digi-Key
Male Header 3 $0.01 $0.03 eBay
PCB 1.35"x0.95"/3 $5.00 $2.14 Laen's PCB
Total Unit Cost $24.42

PCB/Construction Photos

Cost Comparison

From the bills of materials above, you can see the price breakdown for the two amplifiers. While the ST amplifier has a lower cost per module (at a quantity of 3 boards or greater), the price between the two is small, and when factoring in labor, the Mitsubishi amplifier has a lower overall cost. Given that the figure for labor given in this table only includes the labor for designing the PCB, it is clear that the simpler Mitsubishi design would save more time and effort in assembly, test, and debug, given the short timeline of this project.

References

  1. STMicroelectronics, "2 stages RF power amp: PD84001-E + PD54008L-E + LPF N-Channel Enhancement-Mode Lateral MOSFETs," April 2007. [Online]. Available: http://www.rell.com/resources/RellDocuments/SYS_14/ab-54008l-175.pdf. [Accessed 5 February 2012].
  2. STMicroelectronics, "PD54008-E RF power transistor, LdmoST plastic family N-channel enhancement-mode, lateral MOSFETs," May 2011. [Online]. Available: http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/ CD00103331.pdf. [Accessed 5 February 2012].
  3. STMicroelectronics, "PD84001 RF power transistor the LdmoST plastic family," August 2008. [Online]. Available: http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/ CD00147015.pdf. [Accessed 5 February 2012].
  4. Mitsubishi Electric, "RA07M1317M 135-175MHz 6.5W 7.2V, 2 Stage Amp. For Portable Radio," 30 June 2010. [Online]. Available: http://www.mitsubishielectric-mesh.com/products/pdf/ra07m1317m.pdf. [Accessed 5 February 2012].