QRP HF LINEAR AMPLIFIER by Harry Lythall - SM0VPO
This project was a particular surprise for me in that the BC547 (equiv 2N2222)
can be used to build a 500mW linear amplifer covering the entire HF band with
excelent spectral purity and no neutralising at all. Ugly-bug construction was
used but I dare say that the good results are partly to do with the method of
The circuit is fairly straight-forward and does not even need any form of RF
neutralising. Two pairs of BC547 transistors are used in a push-pull type of
output stage, biassed by a single diode and resistor. The driver is also very
conventional using T1 to transform the drive impedance to a very low value
for the output pairs. The amplifier is constructed on a piece of copper-clad
board 45mm long by 17mm wide. Superglue a 44mm long by 3mm wide strip of
copper-clad board along the center. This will become the battery supply rail.
Using a sharp knife, remove some copper to form a 3mm x 3mm pad at one end of
the battery rail to form the RF output terminal. Next fit the 10n and 33n
decoupling capacitors; one pair at either end. These should lay flat on the
board. The rest is easy after you see the photographs.
T1 primary is 14-turns of very thin wire (0.1mm Dia.) and the secondary is
1+1 turn of thin wire (0.2mm Dia.). T1 former is two of the smallest ferrite
beads I could find. You can just see it in the left-hand photograph above. T1
is composed of two grey ferrite beads. The right-hand photograph shows T2 and
the mounting of the two output pairs of BC547 transistors.
T2 is a little special. I found two small ferrite rings in the junk-box and
decided to give them a try. The windings are 11-turns triflar wound using
thin wire (0.2mm Dia.):
- Twist together three 1-metre lengths of thin enamelled wire.
- Wind 11-turns through the ferrite rings (1-turn is passed through
both rings). Do NOT cut off the surplus yet.
- Identify A1-A2, B1-B2, C1-C2 using an ohm-meter.
- Thread each end of C1 and C2 back through the ferrite rings to add
2-1/2 extra turns to each end. Winding C should now have a total of 16-turns.
- Twist together A2 and B1 and connect to the positive battery rail.
- Connect A1 and B2 to the BC547 collectors.
The five transistors are all mounted on their heads using super-glue and with
their legs in the air spread wide apart (Hey! this sounds kinda' pornographic!!).
The finished linear amplifer does not look very pretty but it is very small. It
is less than 10mm high and looks like this. Here you can see it beside my parker
pen for comparison (I thought it would be better than a common 1-crown coin).
Spectral purity was the biggest surprise; nothing to see on the analyser! Not a
single unwanted blip, peak or spurious signal over -30dBm which is about 60dB
down on the fundamental. Not bad for "ugly-bug" construction. The amplifer has
a 30dB gain so the full output can be achieved when driven from a normal signal
generator. Note that Heterodyne generators, such as the Marconi TF995, usually
have a very dirty output signal and are not suitable as a driver as the basis
of a transmitter; just because it cost you money does not mean it is perfect
(as any ham's neighbour owning an Onkyo colour TV will testify). The bandwidth
extends well into the VHF region but the gain is almost unity at 100MHz. Here
is the spectrum analyser displays. The left-hand photograph shows a sweep of
the band from 0-30MHz centered on 15MHz. The input is 17MHz. The right-hand
photograph shows the frequency response, scale is also 3MHz/division and the top
line is +30dBm (1-watt) on both traces. The scales are logarithmic with 10dB
The input level is -4dBm (400uW). As you can see, the gain is reasonably flat
and quite usefull from less than 1MHz to well over 30MHz. Ok, so now you have
all the information you need to build the linear amplifier yourself. Now that
you have got it, what can you do with it? You may want to try:
- Connect a crystal oscillator or VFO into it for a CW transmitter.
- Connect my Phasing-type SSB exciter into it for an SSB transmitter.
- Connect a 29MHz FM signal generator into it for an FM transmitter.
- (Insert another clever use of your own here)
Please do not ask me silly questions, such as what was the relative humidity
and barometric pressure when I wound the transformers or even the density of
the ferrite cores I used. I havn't got a clue. It is as you see it, a rather
slack-handfull of resistors and the rest from the junk-box. If you use a
different type of ferrite then the bandwidth may change a little, maybe not
covering medium-wave (add more turns to T1 and T2) or perhaps a lower output
in the 35MHz radio-control band (remove a few turns from T1 and T2).
Have fun from Harry, SM0VPO.