The RF Power Amplifier (RFPA) receives low level RF (approximately 10 mW) from the exciter module and amplifies it to a level of about 5 Watts (PEP).
DC supply input: 10-15 VDC
RF Power Input: 10 mW (approx)
RF Power Output: 5 W PEP
Operating temperature: 0 – 50 C
Operating frequency range: 5.0 - 5.5 MHz
Modes of operation: SSB, CW
PCB dimensions: 3.9 x 6.25”
The RF Power Amplifier (RFPA) module receives low level RF energy from the exciter module, via a bandpass filter on the OSC_Filter board and amplifies it to a level of approximately 5 Watts. The amplified signal is sent back to the OSC_Filter board, where it is lowpass filtered prior being sent to the antenna. The module consists of three stages of amplification, DC power control circuits and metering circuits for monitoring the DC supply voltage and DC current of the final amplifier stage.
The RF amplifier portion of this module was modeled after the RF power amplifier of the Elecraft K2. The reader is referred to the K2 Assembly Manual and Appendix C for further information and detailed construction procedures for the ferrite transformers used in this circuit.
DC power enters the module at J1and passes through P-channel MOSFET Q1, which acts as a reverse polarity protection device. The polarity protected +12V is used to power power amplifier chain, as well as being supplied to another P-channel MOSFET, Q2. Q2 allows +12V to be passed to the SWITCHED_TX+12V bus when its gate is pulled low by the /REGEN_PTT signal coming in via J2. Power from the SWITCHED_TX_+12V is regulated down to +5V and is used to provide bias to transistors in the amplifier chain as well as powering the current metering circuit. This arrangement allows module to draw minimal current while the transceiver is not in the transmit mode.
This sheet also shows transistors Q3 and Q4, which form the first two stages of RF amplification.
This sheet shows the final amplifier stage, consisting of transistors Q5 and Q6, arranged in a push-pull configuration. Also on this sheet is the current sensing resistor for the final amplifier stage, consisting of resistors R25, R27 and R28.
There have been some component value changes that are not documented on the schematic. Resistors R23 and R24 have both been changed to 0 Ohm jumpers. This change was necessary to lower the resistance of the bias supply for the Q5 and Q6.
Component values were changed in the current sense circuit at R25, R27 and R28. R25 is now 0.005 Ohms (5 milliOhms), R28 is now 0 Ohms and R27 is not installed.
The final amplifier stage DC current consumption is monitored by amplifier U2 and its associated circuitry. Current for the final amplifier stage passes through a 5 milliOhm resistor R25, which produces a voltage drop of 5 millivolts per amp of current. The voltage drop across this resistor is amplified by differential amplifier U2, which has a fixed gain of 50, giving an output of 250 millivolts per amp of current. Opamp U4 provides a low impedance voltage reference which is one half of the 5 volt supply voltage, or 2.5 volts. This reference voltage, VREF, is used to reference the (-) side of the meter, as well being supplied to the VREF1 and VREF2 terminals of U2. This causes the output voltage of U2 to be offset from ground by 2.5 volts. The total voltage output of U2 is represented by the following equation:
Vout_u2 = (Ifinalamp * 0.25) + Vref
Since the meter is connected between Vref and U2 output, the Vref term drops out. Therefore, the current through the meter will be:
Imeter = (Ifinalamp * 0.25) / (R26 + Rmeter)
The meter used in this project had a fullscale deflection of 2 mA, with a linear scale reading 0 - 10 units. A section of the scale between 4.5 and 5.5 units was highlighted in red. The value of R26 was chosen such that the meter deflects to within this highlighted zone during normal operation with a power output of 5 watts and a supply voltage of 12.2 volts. The final value of R26 was determined to be165 Ohms. Therefore, a final amplifier current draw of 0.94 Amps results in the meter deflecting to within the red zone.
The Bill of Material (BOM) is provided in three different formats. Excel format is ready for use directly in Microsoft Excel. HTML format can be viewed directly on the screen if you do not have Excel or other spreadsheet software. CSV format can be used by most spreadsheet software, including Microsoft Excel and others. Here is an explanation of the columns:
| Column | Label | Meaning |
| A | NI? | Component Not Installed |
| B | Pattern Name | This is the name of the component pattern used by the layout software |
| C | Ref Des | Component Reference Designator |
| D | Device | Component Type |
| E | Val | Component Value |
| F | Wat | Component Rating in Watts (if applicable) |
| G | PCT | Component Tolerance |
| H | Volt | Component Voltage Rating |
| I | PMFR | Primary Manufacturer |
| J | PMFR P/N | Primary Manufacturer Part Number |
| K | P Vendor | Primary Vendor (Note: digi = Digikey) |
| L | P Vendor P/N | Part Number Used By Primary Vendor |
| M | SMFR | Secondary Manufacturer |
| N | SMFR P/N | Secondary Manufacturer Part Number |
| O | S Vendor | Secondary Vendor |
| P | S Vendor P/N | Part Number Used By Secondary Vendor |
Important note:
The listing of any vendor or manufacturer in this Bill of
Materials does not in any way constitute any endorsement of any vendor or
manufacturer. Vendors,
manufacturers and their part numbers are listed soley for convenience of the
builder. The information presented
in this Bill of Materials may contain errors; the author assumes no liability
for the accuracy of the information contained herein.
The user assumes all liability for the use of any information presented
in this Bill of Materials.
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Copyright 2006 By Paul Alexander wb9ipa