QRP Labs PA User manual

Rev 1.03 1
PA
5W HF PA kit SIMPLIFIED assembly instructions
1. Introduction
This is a Power Amplifier kit (PA) for CW or FSK modes. The output power is approximately 5W in
the middle of the HF frequency range, using a 13.8V supply. A single inexpensive IRF510
MOSFET is the amplification device. An unusual feature of this PA is the built-in facility for
envelope shaping, which can create a precise raised-cosine keying envelope shape. This greatly
reduces key-clicks when using on/off keyed modes such as CW or at the
commencement/completion of an FSK mode transmission.
I highly recommend building the full kit as described in the assembly instructions. The kit has a
discrete component power modulator (voltage regulator) which is controlled by an 8-bit R-2R
Digital Analogue Converter, and allows an external microcontroller to control the amplitude of the
output by loading the on-board 8-bit shift register using three I/O signals, and thereby create the
raised cosine shape.
These instructions are for a simplified assembly of the PA kit, which omits raised cosine
envelope shaping, or the power modulator with fold-back current limiting. It is easier to
build this way, but the functionality is less.
The kit has a PCB of size 80 x 37mm, which is the same size as the other QRP Labs modules
such as the Ultimate3S QRSS/WSPR transmitter kit, the VFO kit and the Relay-switched LPF kit.
It is therefore physically (as well as electrically) compatible with the other QRP Labs kits and can
be bolted behind them in the familiar PCB sandwich. Alternatively this PA can be used with your
own homebrew projects. The PCB has space for SMA connectors (not supplied by default with the
kit), if you wish to use SMA cables.
The PA kit must always be followed by a low pass filter to attenuate unwanted transmitter
harmonics, as usual for RF power amplifiers.
Carry out an inventory of the components according to the parts list to make sure all the
components required are present. Remember that there will be many surplus components that are
required only for the full build and will not be used here. Read the assembly instructions first
before starting the assembly, understand everything, and then finally start the assembly, following
all the steps carefully!

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The following diagram shows the simplified circuit diagram of this HF PA.
This diagram includes a 10-ohm resistor modification which is highly recommended, and will be
described later in this document.

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2. Parts list
Unused parts are coloured grey –these are used only in the full build.
Resistors
R27 0.33-ohm, 2 watt resistor
(none) 10-ohm resistor
R2, 19 15K resistor (2 pieces)
R4..10, 28 1.1K resistor (8 pieces)
R3, 11..18, 24 2.2K resistor (10 pieces)
R20, 22 3.3K resistor (1 pieces used, 1 piece not used)
R21, 30 330-ohm resistor (2 pieces)
R23 8.2K resistor
R29 47K resistor
R1 4.7K trimmer potentiometer resistor
R26 3.3K resistor (not supplied)
R25 4.7K trimmer potentiometer resistor (not supplied)
Capacitors
C1, 2, 3, 7, 8 1uF capacitor (5 pieces)
C5, 6 1uF capacitor (2 pieces)
C4 1nF capacitor
Semiconductors
IC1 74HC595 8-bit shift register, 16-pin DIP package
IC2 7805L 5V regulator, TO92 package
Q3, 4 2N3904 NPN transistor, TO92 package
Q1, 2, 6, 8 2N3906 PNP transistor, TO92 package
Q5 2SC4242 NPN power transistor, TO220 package
Q7 IRF510 N-channel power MOSFET, TO220 package
Q9 BS170 N-channel MOSFET, TO92 package
Miscellaneous
16-pin socket 16-pin DIP socket for IC1
10-pin header 10-pin 0.1-inch header
Heatsink 65mm long PCB-mounting heatsink and pins
Insulation sets 2 pieces silicone rubber insulation pads + washers for the TO220 transistors
Nut and bolt 12mm long M3-size bolt and matching nut
FT50-43 FT50-43 toroid
Wire 100cm 0.33mm diameter wire
PCB 80 x 37mm Printed Circuit Board (PCB)

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Theoretical circuit explanation
The chosen amplification device is the IRF510
MOSFET. It is inexpensive, robust, easily able
to handle the power requirements, and in a
convenient TO220 package. On the other
hand, the IRF510 does have high gate
capacitance. It is originally intended as a
switch for switched-mode power supplies and
other such industrial switching requirements.
Using it as an RF amplifier wasn’t the
manufacturer’s original intention! But it works
well on HF, nevertheless! The power output
does drop off at the higher frequencies.
For this reason the PCB has been designed to
also accept the RD15HVF1 transistor (and
similar devices) which are intended and
designed for RF applications. They perform
very well right up into VHF, 2m band etc. So if
you wish to use this PA kit for 6m and 2m you
may want to replace the supplied IRF510 with
a RD15HVF1 or one of its siblings. The pinout
of the RD15HVF1 is different to that of the
IRF510. The PCB has a second set of pads to suit the RD15HVF1.
The incoming RF is coupled to the IRF510 gate via a 1uF capacitor. The largish 1uF value is
intended to help the PA work well on LF and MF. A DC bias voltage must be applied to the IRF510
gate, this is supplied by 4.7K preset potentiometer R1 via the 3.3K resistor R20.
A trifilar output transformer wound on an FT50-43 toroid provides matching to the 50-ohm output.
There are pads/holes on the PCB for an optional SMA connector. This is not supplied in the kit but
is available in the QRP Labs shop. You can also fit an SMA connector at the PA kit’s RF input
instead of the 2-pin header. The RF output is also available at the 2-pin header JP5, and at a 2 x
5-pin header JP1 (not supplied). The 2 x 5-pin header matches the headers on the Ultimate3S
QRSS/WSPR Transmitter PCB and the 6-band relay-switched filter kit PCB. This option may be
used when using this PA kit with Ultimate3S, and is described in the application note AN004.
The PA kit also contains a TO92-package 5V regulator IC type 78L05, IC2 (also equivalently
called 7805L by some manufacturers). Note that this is shown on the main circuit diagram but in
not the sub-block above. This 5V is used for the PA bias. Providing the regulator on board
simplifies the use of this PA kit module because you only need to provide a single PA supply e.g.
13.8V. However do not be tempted to power other circuits from this 5V regulator output, such as
the Ultimate3S kit or other. The 78L05 is only a low power voltage regulator and has no heatsink.
The 78L05 is rated for 100mA maximum supply current.

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3. Assembly
Assembly of this kit is quite straightforward. The usual kit-building recommendations apply: work in
a well-lit area, with peace and quiet to concentrate. Some of the semiconductors in the kit are
sensitive to static discharge. Therefore observe Electrostatic discharge (ESD) precautions.
And FOLLOW THE INSTRUCTIONS!!
A jeweller’s loupe is really useful for inspecting small components and soldered joints. You’ll need
a fine-tipped soldering iron too. It is good to get into the habit of inspecting every joint with the
magnifying glass or jeweller’s loupe (like this one I use),
right after soldering. This way you can easily identify any
dry joints or solder bridges, before they become a
problem later on when you are trying to test the project.
It is always best to detect and correct any mistakes as
early as possible (immediately after soldering the
incorrect component). The board is quite compact, to fit
the required 80 x 37mm PCB dimensions. Removing a
component and re-installing it later is often very difficult!
The resistors in the kit are installed vertically, because
PCB space is limited. It was important to design it to fit
on an 80 x 37mm PCB for potential use with other QRP Labs kits such as the Ultimate3S
QRSS/WSPR transmitter. You will need to bend the resistor wires as shown in the picture below.
On the parts layout diagram there is a small circle around the hole where the body end of the
resistor should be installed. This is
not critical (resistors don’t care
which way around they are) but it is
recommended, just to try to ensure
that the folded over wire-end of the
resistor doesn’t touch any other
wire or metal part.
Please refer to the layout diagram and PCB tracks diagrams below, and follow the steps carefully.

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PCB track diagram. Tracks shown in BLUE are on the bottom layer. Tracks shown in RED are on
the top layer. There are only two layers (nothing is hidden in the middle). Not shown in this
diagram are the extensive ground-planes. Everything on the bottom layer that isn’t a RED track, is
ground-plane! Large areas of the top side are also ground-plane, connected to the bottom ground-
plane layer at frequent intervals by vias.
Note that the heatsink is not electrically connected to ground or anything else. The area right
under the heatsink has no ground-plane on the top layer. This is to prevent the heatsink potentially
scratching the soldermask and connecting to a ground-plane if there was one. For the same
reason there are no tracks on the top layer under the heatsink. The metal tabs of the TO220
transistor is NOT connected to ground, and is insulated from the heatsink, so long as all the
insulating hardware is properly installed. The heatsink itself is therefore connected to nothing at
all.

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3.1 Inventory parts
This is the full list of parts. Many are not used in this simplified build. Please refer to the parts list.

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3.2 Install required components
Install and solder the components indicated in this diagram and described as follows.
YELLOW: Capacitors C1, C2, C3, C7 and C8 are 1uF capacitors. The writing on the capacitor is
“105”. Be careful – the kit also contains one 1nF capacitor labelled “102” – do not mix them up!
RED: Resistor R20, 3.3K, with colour code with colour code orange-orange-black-brown-brown.
BLUE: 78L05 voltage regulator IC2. Be careful to install this with the same orientation as the white
silkscreen printing on the PCB. Also check the label on the component –there are many TO92-
style transistors which look similar –do not mix them up!
GREEN: The 4.7K preset bias adjustment potentiometer, with label “472”.
PURPLE: Optionally you can install header pins. Many people prefer to solder wires to header
pins, rather than in holes. Or you may wish to use header socket connectors. In this case snap off
a 2-pin section and a 4-pin section from the supplied 10-pin header, and install them as shown.
3.3 Install jumper wires
The jumper wires are necessary to bypass the power modulator circuit, and to bypass the leakage gate
circuit. These parts of the full PA kit assembly are not undertaken in this simplified build.
Four jumpers need to be installed as per the following diagram. This diagram shows the PCB holes/pads in
green to make it clear where the jumper wires need to be installed. These jumpers can be made using wire
offcuts from the installed components or surplus components.

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3.4 Install trifilar transformer
The trifilar transformer is installed in the position coloured red in this diagram. The photo (right)
shows the board after installing the trifilar transformer. But it is a tricky part of the assembly, so
please read and follow these steps carefully.
Firstly, the wire. The best way to un-wind it, without tangling it
up, is to think of what the kit-packing person that wound it up
did. Then reverse his steps. So, first unwind the tightly wrapped
part in the middle where the end of the wire has been secured.
Then, open out the spool of wire so that it is a circle. Then
unwind the spool, around your fingers, reversing the process of
winding it in the first place.

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When you have unwound the wire and straightened it, cut it into three approximately equal pieces.
These three pieces now need to be tightly twisted together to make the trifilar wire. My method for
this is to tie one end in a knot around a small screwdriver shaft. Similarly tie the other end around
another small screwdriver. Now clamp one end somehow to something solid. You could use a
vice, if you have one. If you don’t, then you have to get creative and think of something. Here I
taped it to the edge of the desk. Now you can twist the screwdriver at the free end, repeatedly until
you twist the three wires together thoroughly. You need to keep the wire under a little tension to
keep the twists evenly spaced.
I put about 70-90 twists into a 25cm
length of wire. The end result is
something like the photo (right). The
measurement scale is in cm.
Now cut off the untidy ends, and this is
the piece of wire that will be used to
wind the FT50-43 toroidal core as a
trifilar transformer.