Rev 1.03
4
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.
