Transmitter PA Introduction


This stage provides the Class A PA based on Mitsubishi RF MOSFET RD16HHF1.

It is always a good idea to double check the bill of materials in this set of documentation with the official BOM on the Mobo site. Discrepancies may be introduced inadvertently, as the changes are sometimes not communicated or discovered, making timely update of this site a problem.

Theory of Operation

When biased to Class A (the biasing is done via software control and user customisable), a very linear output for SSB and digital modes is possible. When driven hard (beyond CLASS A limits), the linearity will decline. Measured IMD Ratios (a measure of the whole Tx chain's linearity) on the ALPHA build are:>/p>

(go directly to build notes)

Transmitter PA Schematic

(Click for Full Schematic)
Transmitter PAschematic


(above schematic has clickable areas that can be used for navigation)

(go directly to build notes)

Transmitter PA Bill of Materials

Stage Bill of Materials

(resistor images and color codes courtesy of WIlfried, DL5SWB's R-Color Code program)

1BN43-202 Binocular core
1BN-43-2402 (no markings!)none noneBinocular core
1BN-43-2402 (no markings!)none noneBinocular core
1220 uF/16Vdc Electrolytic
1header, 2-pin Header
1TO-220 heatskinkMisc
1shunt, 2-pin Shunt
40.1 uFSMT 0805
10.1 uFSMT 0805
10 ohm 5%SMT 0805 1/8W
11 k ohm 1%SMT 0805 1/8W
12.2 ohm 1%SMT 0805 1/8W
1249 ohm 1%SMT 0805 1/8W
133 ohm 1%SMT 0805 1/8W
149.9 ohm 1%SMT 0805 1/8W
2499 ohm 1%SMT 0805 1/8W
1750 ohm 1%SMT 0805 1/8W
20.1 uF(smt) black stripe (smt) black stripeSMT 1206
10.2 ohm 5%SMT 1206 1/4W
10.47 uH 20%SMT 1210
1ZXCT1021 low sense voltage monitorZXCT1021 ZXCT1021SOT-23-5
12N3866 NPN Transistor TO-39

Transmitter PA Summary Build Notes

Transmitter PA Detailed Build Notes

Bottom of the Board

Transmitter PA Bottom View

Install Bottomside Rs and Cs

C470.1 uFSMT 0805
R300 ohm 5%SMT 0805 1/8W
R322.2 ohm 1%SMT 0805 1/8W
R430.2 ohm 5%SMT 1206 1/4W
R5949.9 ohm 1%SMT 0805 1/8W
R33249 ohm 1%SMT 0805 1/8W

Top of the Board

Transmitter PA Top View

Install U14

U14 - the voltage monitor - would normally be associated electronically with the next phase. However, due to the close quarters around this tiny IC, it is installed during this phase. Take great care to avoid solder bridges, since, once this phase is completed, getting access to the IC will be quite difficult.

Install U14 photo
U14ZXCT1021 low sense voltage monitorZXCT1021 ZXCT1021SOT-23-5

Install Topside Rs, Cs, and Ferrite

FB2 is an SMT RF choke - no winding required :>}. The parts supplied include an extra BN43-2402 for those who would prefer to wind this choke: 6T #30 for 56uH.

SMT (optional:6T (8") #30 on BN43-2402 for 56uH)
C180.1 uFSMT 0805
C260.1 uF(smt) black stripe (smt) black stripeSMT 1206
C480.1 uFSMT 0805
Install C48 before installing C28
C290.1 uF(smt) black stripe (smt) black stripeSMT 1206
C440.1 uFSMT 0805
C490.1 uFSMT 0805
R31750 ohm 1%SMT 0805 1/8W
R57499 ohm 1%SMT 0805 1/8W
R58499 ohm 1%SMT 0805 1/8W
R6033 ohm 1%SMT 0805 1/8W
R611 k ohm 1%SMT 0805 1/8W
L010.47 uH 20%SMT 1210
C28220 uF/16Vdc Electrolytic

Wind and Install Inductors

Wind and install the transformers and chokes (refer to the preceding stage for transformer winding instructions).



T2 is wound on the larger BN43-202, using 4 turns of a a bifilar length (16" total) of #26 (or heavier) wire.

Note that T2 is erroneously shown on the PCB as being installed to the bottom (Cu) side; this is an error. T2 must be installed to the top (Cmp) side in the 4 holes provided.

Use a continuity checker to separate out the 2 pairs of windings. One winding goes to the left 2 holes, and the other winding goes to the right 2 holes. This way, the flat surfaces of the BN core faces east-west.


T5 is wound on a BN43-2402 binocular core using 5 turns of a bifilar length of #30 wire (11" total wire length)

One winding goes to the left two holes and one winding goes to the right 2 holes.

The core is oriented so that its two holes are in a vertical (north-south) row.

Windings and placment of T2 and T5


FB1 is wound on a BN 43-2402 binocular core with a 4 inch length #26 (or thicker) magnet wire. The number of turns Is 3. The inductance (for turns verification purposes only) should be about 11.7 uH

Install Connectors

P20header, 2-pin Header
TP1 - Drain Current Test Point 1
Jumper 1-2 for normal Op. Unjumper to measure drain current with external Ammeter
P20_Shuntshunt, 2-pin Shunt
Install shunt for normal operation; remove to measure drain current

Install Transistors and Heatsinks

In mounting the large heatsink, use only the nylon spacer for the pcb bottom on the side closer to the USB connector. This is to avoid shorting to the nearby traces. On the other side and for the pcb top, try to replace the nylon spacers with metal spacers. We recommended putting a "star" metal spacer because it conducts RF but has fair thermal isolation. Therefore you will have good RF grounding of the heatsink via 3 of the pcb surfaces.

Make sure that the Heatsink has a good ground to the Mobo PCB through the mounting screws, preferably using a locking washer. If the Heatsink is not grounded, you WILL get VHF oscillation in the PA.

Q062N3866 NPN Transistor TO-39
Q04_heatsinkTO-220 heatskinkMisc
Heatsink mounting hardware to be supplied by kit buyer. Suggested that external star washers in conjunction with metallic flat washers be used. Heatsink must be at ground potential for best amplifier stability.

Transmitter PA Completed Stage

Top of the Board

View of Completed Top

Bottom of the Board

View of Completed Bottom

Transmitter PA Testing

Test Program Requirements

Test Setup

Go to Temperature Control Stage Go to Band-switching/Ptt control stage